RA (AY4398) Investigation into the EMC emissions from ... · RA (AY4398) Investigation into the EMC...

Chris Clegg

RA (AY4398) Investigation into the EMC emissions from SMPSs and SELCs 8219CR1

York EMC Services Ltd. Page 2 of 57 Issue 1

EXECUTIVE SUMMARY A study was undertaken to consider the EMC emission from Switched Mode Power Supplies (SMPSs) and similar Switched Electronic Load Controller (SELC) devices tested under different load conditions.

Various products based on the SMPS/SELC technology were purchased for this investigation. Specific loads were designed for each of the equipment under tests (EUTs) to enable the EMC testing to be performed for the 25%, 50% and 100% load condition. EMC testing was performed on three items (two based on SMPS technology and one on SELC technology) for harmonic emission, conducted emission and radiated magnetic and electric field emissions.

Results of the testing showed that substantial levels of emissions were measured in the harmonic and conducted emission tests. Few emissions were measured for the radiated type emission (magnetic and electric). In the case of SMPS based devices, the 100% load condition was found to be the most appropriate load to measure the maximum emission, whereas in the case of the SELC based devices, lower load values (25% and 50%) were found to lead to higher emission levels.

Numerical modelling was performed on the three items to determine the feasibility of using SPICE models to predict harmonic emission and conducted interference between 9kHz and 30MHz. Comparison with harmonic test results showed that it is possible to accurately model the amount of harmonic emission in the case of both SMPS and SELC based devices.

The modelling of the conducted emission was found to be more difficult however, overall, the general emission trend was predicted but only with poor accuracy

A Code of Practice was written as part of this investigation for the design SMPSs and SELCs giving guidance on how to design such devices to ensure good EMC characteristics. The Code of Practice is aimed at engineers who have experience of general electronic design but need to gain knowledge of EMI problems and solutions peculiar to SMPS/SELC.



CONTENTS

EXECUTIVE SUMMARY .......................................................................................................2

List of Tables .............................................................................................................................6

List of Figures ............................................................................................................................7

List of Terms and Abbreviations ...............................................................................................9

Acknowledgements ..................................................................................................................10

1 Introduction......................................................................................................................11

2 Overview of SMPSs and SELCs......................................................................................13

2.1.1 Basic Switched Mode Power Supply ............................................................13

2.1.2 Non Isolated Topology..................................................................................14

2.1.2.1 The Buck Converter...............................................................................14

2.1.2.2 The Buck-Boost Converter ....................................................................15

2.1.2.3 The Boost Converter ..............................................................................16

2.1.2.4 Summary of the basic converter in the non- isolated topology ..............17

2.1.3 Isolated Topology .........................................................................................18

2.1.3.1 The Flyback Converter...........................................................................19

2.1.3.2 The Forward Converter ..........................................................................20

2.1.3.3 The Push-Pull Converter ........................................................................21

2.1.3.4 The Half-bridge Converter.....................................................................23

2.1.3.5 The Full-bridge Converter .....................................................................23

2.2.1 Silicon Controlled Rectifier (SCR)...............................................................24

2.2.2 Triac ..............................................................................................................25

3 Testing of the SMPSs and SELCs ...................................................................................27

3.1.2 Products based on the Switched Electronic Load Controller Technology ...27

3.2.1 EMC Test Plan..............................................................................................28

3.2.2 Load Guidelines Booklet ..............................................................................28

3.2.3 EMC Test Reports.........................................................................................28

2.1 The Switched Mode Power Supplies (SMPSs)........................................................13

2.2 The Switched Electronic Load Controllers (SELCs) ...............................................24

3.1 Choice of the EUTs..................................................................................................27

3.2 Testing of the EUTs .................................................................................................28



3.3.1 Determination of the circuit diagram............................................................29

3.3.2 Modelling of the emission for EUTs 5, 6 and 7............................................29

3.4.1 Analysis of the Harmonic Emission Testing ................................................29

3.4.2 Analysis of the Conducted Emission Testing ...............................................31

3.4.2.1 Conducted emission of item 5 (Printer SMPS) ......................................31

3.4.2.2 Conducted emission of item 6 (Plug- in SMPS) .....................................32

3.4.2.3 Conducted emission of item 7 (One way rotary light dimmer) .............34

3.4.3 Analysis of the Radiated Magnetic Field Emission (Van Veen Loop – Item 5) ........................................................................................................................36

3.4.4 Analysis of the Radiated Magnetic Field Emission (CISPR 0.6m Loop).....37

3.4.4.1 Radiated magnetic field emission of item 5 (Printer SMPS) .................37

3.4.4.2 Radiated magnetic field emission of item 6 (Plug- in SMPS) ................38

3.4.4.3 Radiated magnetic field emission of item 7 (One way rotary dimmer).40

3.4.5 Analysis of the Radiated Electric Field Emission.........................................41

4 Modelling .........................................................................................................................44

5 Code of Practice ...............................................................................................................45

6 Conclusions and Recommendations ................................................................................46

Appendices .............................................................................................................................47

Appendix A: Deliverable 1 – York EMC Services Ltd - Document 8202CR1 .......................48

Appendix B: EMC TEST PLAN - York EMC Services Ltd - Document 8214CR2...............49

Appendix C: Load Guidelines Booklet - York EMC Services Ltd - Document 8217CR2 .....50

Appendix D: Determination of Circuits Diagrams - York EMC Services Ltd - Document 8230CR1 .............................................................................................................51

Appendix E: Test Report - EMC Testing of EUT 5 – Printer Switched Mode Power Supply – York EMC Services Ltd - Document 5533/TR/1 ...............................................52

Appendix F: Test Report - EMC Testing of EUT 6 – Plug In Switched Mode Power Supply – York EMC Services Ltd - Document 5534/TR/1 ...............................................53

3.3 Investigation of the Circuit diagram and Modelling of the emission of 3 EUTs.....29

3.4 Analysis of the Testing ............................................................................................29

3.5 Conclusion of the Testing ........................................................................................42

6.1 Conclusions ..............................................................................................................46

6.2 Recommendations for further work .........................................................................46



Appendix G: Test Report - EMC Testing of EUT 7 – One Way Rotary Dimmer – York EMC Services Ltd - Document 5535/TR/1 ..................................................................54

Appendix H: –Modelling Report on SMPSs and SELCs- York EMC Services Ltd - Document 8273CR2............................................................................................55

Appendix I: –Code of Practice for the Design of SMPSs and SELCs - York EMC Services Ltd - Document 8268CR2...................................................................................56

References .............................................................................................................................57



LIST OF TABLES

Table 1: Summary of the basic topologies .................................................................................... 17

Table 2: Converter Topology with their associated maximum output power ................................... 19

Table 3: List of items based on SMPS Technology ....................................................................... 27

Table 4: List of items based on the SELC Technology .................................................................. 27



LIST OF FIGURES Figure 1: Basic Switched Mode Power Supply Block Diagram...................................................... 14

Figure 2: Buck regulator with the associated voltage and current waveform.................................... 15

Figure 3: Buck-Boost regulator with the associated voltage and current waveform.......................... 16

Figure 4: Boost regulator with the associated voltage and current waveform................................... 17

Figure 5: Diagram of a single -ended flyback converter circuit and typical waveforms (Ip=Primary

current, Is= Secondary current)............................................................................................ 19

Figure 6: Diagram of a forward converter circuit and typical waveforms ........................................ 21

Figure 7: Diagram of a Push-pull converter circuit ........................................................................ 22

Figure 8: Rectified output voltage of a Push-pull converter (before filter)....................................... 22

Figure 9: Diagram of Half-bridge converter circuit ....................................................................... 23

Figure 10: Diagram of Full-bridge converter circuit ...................................................................... 24

Figure 11: Description of the Triac as a symmetrical switch .......................................................... 25

Figure 12: A full-wave switching regulator using a Triac .............................................................. 26

Figure 13: Harmonic testing results of item 5 (Printer SMPS)........................................................ 29

Figure 14: Harmonic testing results of item 6 (Plug in power-supply) ............................................. 30

Figure 15: Harmonic testing results of item 7 (One way rotary dimmer) ......................................... 30

Figure 16: Conducted emissions results, (Live conductor, average measurement) ........................... 31

Figure 17: Conducted emissions results, (Neutral peak and quasi peak measurement) ..................... 32

Figure 18: Conducted emissions results, (Neutral average measurement)........................................ 33

Figure 19: Conducted emissions results, 1m extended cable (Neutral average measurement) ........... 34

Figure 20: Conducted emissions results, (Live average measurement)............................................ 35

Figure 21: Conducted emissions results, (Neutral peak and quasi peak measurement) ..................... 35

Figure 22: Radiated magnetic field emission for horizontal polarisation (Peak measurement - loop H1)

.......................................................................................................................................... 36

Figure 23: Radiated magnetic field emission for vertical polarisation (Peak measurement - loop V2) 37

Figure 24: Radiated magnetic field emissions for parallel polarisation (Peak measurement)............. 38

Figure 25: Radiated magnetic field emissions for perpendicular polarisation (Peak measurement).... 38







Figure 30: Maximum of the radiated emission for both horizontal and vertical polarisations for item 5

(Peak measurement)............................................................................................................ 41

Figure 31: Maximum of the radiated emission for both horizontal and vertical polarisations for item 6 (Peak

measurement)....................................................................................................................... 41

Figure 32: Maximum of the radiated emission for both horizontal and vertical polarisations for item 7 (Peak

measurement)....................................................................................................................... 42



LIST OF TERMS AND ABBREVIATIONS

AC Alternating Current

CENELEC Comité Européen de Normalisation Electrotechnique

CISPR Comité International Spécial des Perturbations Radioélectriques

COTS Commercial Off The Shelf

DC Direct Current

DSO Digital Signal Oscilloscope

EMC Electromagnetic Compatibility

EMI Electromagnetic Interference

EUT Equipment Under Test

FAR Fully Anechoic Room

FET Field Effect Transistor

FFT Fast Fourier Transform

ITE Information Technology Equipment

LISN Line Impedance Stabilisation Network

MOSFET Metal-Oxide Semiconductor Field-Effect Transistor

OATS Open Area Test Site

OSC Oscillator

PC Personal Computer

PCB Printed Circuit Board

PWM Pulse Width Modulation

RA Radiocommunications Agency

RF Radio Frequency

RMS Root mean square

RTCG Radio Technology and Compatibility Group

SELCs Switched Electronic Load Controllers

SMPSs Switched Mode Power Supplies

SPICE Simulation Program with Integrated Circuit Emphasis

UKAS United Kingdom Accreditation Service

YES York EMC Services Ltd



ACKNOWLEDGEMENTS We would like to thank the following people for their help during this study:

Mr. John Airs and Mr. Mahesh Dudhia from the Radiocommunications Agency (Radio Technology & Compatibility Group).

Dr Bryan Flynn for his contribution to the writing of the Code of Practice.



1 INTRODUCTION Over recent years, the demand within the electronics industry fo r power supplies that are physically small, perform efficiently and with stability over a “world-wide” range of input voltage and frequency has increased significantly, particularly with respect to manufacturers of Information Technology and consumer products.

The obvious choice for manufacturers of these products is the Switched Mode Power Supply (SMPS), which provides a low cost, high efficiency, compact solution. The use of SMPSs within these types of products is now practically universal. Although these SMPSs are typically single-phase low power (less than 500W), SMPSs are available for single and 3-phase uses at higher power levels.

The technique used by SMPSs relies on the rectification and high speed switching of the mains supply and it is this high speed switching (in the order of 50-500kHz for typical Information Technology Equipment –ITE- use) which can be the source of conducted emissions. Typically, the switching waveform has fast rise and fall times leading to the production of harmonics (odd and even), which can extend across the frequency range for conducted emissions (150kHz to 30MHz for EN55022) and occasionally above this into the radiated emissions frequency range.

In general, an improvement in the emission performance of SMPSs has been seen since the introduction in 1996 of the mandatory application of the EMC Directive 89/336/EEC for equipment intended to be sold into the European Economic Area and particularly in those products used in personal computers and other ITE. This is likely to be due to manufacturers appreciating the requirements of the EMC regulations more fully and incorporating better EMI filtering at both the input and the output of the supplies.

Although this improvement has been the case for personal computers and other ITE where it is known that the power supply will be fitted at the mains supply/equipment interface, there are still cases where an off the shelf SMPS has been used directly at the power input. Since these items can be classed as “components”, it can be argued that they have no legal requirement to meet the EMC regulations. They may carry a CE mark, which can (and does) mislead companies into thinking that they comply with the relevant EMC standards whereas the CE marking could have been applied in such a case with reference to another directive e.g. electrical safety. Filtering on some of these supplies can be either minimal or non-existent, since the final intended use for them is within larger pieces of equipment where filtering should be added separately.

SELCs are also becoming very common in the residential, commercial and light industrial environment and are used, for example, in light dimmer switches and variable speed control devices (e.g. drills). Their technology is based on a solid-state dimmer, which works by varying the "duty cycle" (on/off time) of the full AC voltage that is applied to the device, which is being controlled (e.g. lights, motors). For example, if the voltage is applied for only half of each AC cycle in the case of a light dimmer, the light bulb will appear to be dimmer than when voltage is applied for the full AC cycle, because the bulb receives less power to heat the filament.

Typical SELCs are built using thyristors and the exact time when the thyristor is triggered relative to the zero crossing of the AC power is used to determine the power level. When the thyristor is triggered it continues to conduct until the current passing though it reaches zero (exactly at the next zero crossing if the load is purely resistive, mostly like a light bulb). By changing the phase at which the thyristor is triggered the duty cycle is changed and therefore the brightness of the light can be controlled.



The thyristor (Triac) dimmer has one fairly severe drawback in its performance in that it dims by switching on the current to the load partway through each mains cycle. Cutting the leading smooth-part off a mains cycle produces a current with a very rapid turn-on time, which generates both mains distortion and EMI. Chokes are commonly included in dimmers to slow down the rapid switch-on (rise time) of the chopped current. The longer the rise time (i.e. the smaller the magnitude of the rate of change of current with respect to time, dI/dt) the less EMI and mains distortion is produced.

A typical thyristor/triac starts to fully conduct at around 1 microsecond after triggering, so the current change is very fast if not limited in some way. The fast voltage and current changes cause high frequency interference which can propagate through mains wiring, unless there are suitable radio frequency interference filters built into the circuit. The corners in the waveform effectively consist of 50/60Hz plus varying amounts of other frequencies that are multiples of 50/60Hz. In some cases, the interference can be observed in the 1 to 10MHz band and even higher. The wiring in a typical house acts as an antenna for this EMI and can broadcast the interference throughout the house. Cheap, poor quality light dimmers, which do not have adequate filtering, can easily cause a great deal of radio interference.

However, many pieces of equipment are intended to be upgradeable by the addition of sub units. Personal computers are a prime example of this where drives, printed circuit boards (e.g., graphics and sound cards) and other peripherals which are powered from the computer’s supply can be added during the lifetime of the main equipment. This variation in loading will affect emission characteristics of the SMPSs/SELCs and it is in the manufacturer’s interest that this does not adversely affect the EMC performance of the equipment. The method of ensuring that this is the case, however, is largely outside of the manufacturer control since it is dependent on the SMPS/SELC producer.

This project therefore considers the radiofrequency emission from Switched Mode Power Supplies (SMPSs) and similar Switched Electronic Load Controller (SELC) devices tested under different load conditions.

The objectives of this study were to:

1 Research the market in order to identify a range of products that are based on, or contain, SMPS or SELC technology, and purchase a number of these products (around 10 in total) such that they represent the range available on the electrical goods market;

2 Arrange with the Agency’s RTGG laboratory to test the purchased products for both conducted and radiated emissions over a range of agreed likely service conditions for the output load;

3 Investigate and identify the circuit configurations of the items under test;

4 Produce guidelines or a Code of Practice for the SMPSs and SELCs that would enable the industry to improve the EMC performance of their products when in service.



2 OVERVIEW OF SMPSS AND SELCS This section presents a brief overview of the working principle of Switched Mode Power Supplies and Switch Electronic Load Controllers. More detailed and specific information concerning SMPSs can be found in [1,2] and for SELCs in [3].

2.1 The Switched Mode Power Supplies (SMPSs)

For many years, the world of power supply design is gradually moving from the use of linear power supplies to the more practical Switched Mode Power Supplies. Linear power supplies are known to have a poor efficiency (typically 30%), to be very heavy and to be extremely large. Those characteristics, when compared to typical Switched Mode Power Supplies, make the SMPS a very strong candidate as power transformers. Advantages include efficiencies of between 70% and 80%, very compact size and light weight.

The most important section of the SMPS is the high frequency inverter, where the input supply is chopped at very high frequencies (up to 200kHz and now current research is being pursued in the MHz region), then filtered and finally smoothed to produce the required output.

There is a wide choice of topologies of SMPSs, and two main groups of configurations can be identified

1. First group where the transformer is not part of the chopper (Non-isolated converters):

a. Buck converter (series chopper);

b. Boost converter (parallel chopper);

c. Buck-Boost converter (chopper with inductive energy storage);

d. Cuk converter (chopper with capacitive energy storage).

2. Second group where the transformer is an integral part of the chopper (Isolated converters):

a. Flyback converter (isolated inductive storage circuit);

b. Forward converter (isolated buck circuit);

c. Combinations of the above (push-pull, half bridge and full bridge).

For the purpose of this report, a description of the basic SMPS circuit will be presented with a summary of the other topologies (Isolated and Non-Isolated converters).

2.1.1 Basic Switched Mode Power Supply A SMPS, as seen from Figure 1, can be quite a complicated circuit. It is composed of various sections (enclosed in the rectangles) and each of them have their specific functions in the power conversion.

When a SMPS is fed from a 50/60Hz main supply, the AC supply is first rectified and then filtered by the input reservoir which then produces a “DC” output (in the case of a AC to DC SMPS). The input capacitances are usually of high values to hold up the supply in the event of a large drop in the main supply. The unregulated “DC” is then fed to the high frequency switching section, where semiconductors devices such as MOSFETs or Bipolar transistors switch the input voltage across the primary of the transformer (switching frequency from 20 to 200kHz).



Hence, a voltage pulse train of suitable magnitude and duty ratio appears on the transformer secondaries which is then rectified and smoothed by the output filter (composed of either capacitor/inductor arrangement).

The regulation of the output stability is achieved by the control/feedback block. The output voltage is compared to an accurate reference supply, and the error voltage produced by the comparator is used by dedicated control logic to terminate the drive pulse to the main power switch/switches at the correct instance. With a correct design, this SMPS will provide a very stable DC output supply.

Figure 1: Basic Switched Mode Power Supply Block Diagram

2.1.2 Non Isolated Topology The majority of the topologies used today are derived from the three non- isolated topologies. Those are the buck, the boost and the buck-boost (also called non- isolated flyback) which is a combined topology of the previous two. The configurations presented below are simplified configurations and have the lowest component count to generate the required output.

2.1.2.1 The Buck Converter The operation of the Buck converter is straightforward. When the switch T1 (Figure 2) is turned on, the input voltage (VI) is directly applied to the inductor L and power is delivered to the load R. In the meantime, the inductor current builds up as V = L(di/dt). Then, when the switch (T1) is turned off, the voltage across the inductor (L) reverses and the freewheel diode becomes forward biased, which allows the energy stored in the inductor (L) to be delivered to the output. The capacitor C then smooths the output. The combination of the inductance L and capacitor C acts as a filter and produces a smooth DC output voltage and current. The average voltage at the input of the inductor is VID. In the steady state conditions, for the average voltage across the inductor to be zero, the basic DC equation of the buck is simply:

DVV

I

=0 Equation 1

where D is the transistor duty cycle, defined as the conduction time (usually called ton) divided by the switching period (T).



From above, it is clear that the Buck Converter is a step down type (output voltage lower than the input voltage) since D (Equation 1) never reaches one and the voltage regulation is obtained by varying the duty cycle of the switch.

Figure 2: Buck regulator with the associated voltage and current waveform

2.1.2.2 The Buck-Boost Converter The Buck-Boost Converter is also commonly called the Flyback Converter. The Flyback converter stores energy during the on time of the transistor T1 and delivers the stored energy from the inductor (½ Li2) during the switch off- time to the capacitor C and the load R. The flyback converter works as follows: when the switch T1 is on, the diode is reversed biased and the input is then connected to the inductor L, in which energy is being stored (VL=VI). When the switch turns off, the voltage inductor reverses and the energy stored in the inductor is passed to the capacitor C and load R through the forward biased rectifier diode (VL=-V0). Over a period, the average voltage across the inductor is equal to zero, therefore:

0.. 0 =− offonI tVtV Equation 2

Which can be expressed as

DD

TDTD

tt

VV

off

on

I −=

−==

1).1(.0 Equation 3

Equation 3 shows that the value of switch duty ratio, D, can be selected in such a way that the output voltage can either be higher or lower than the input voltage. The converter has the flexibility to be either a step up or step down converter, depending of the value of D.



Figure 3: Buck-Boost regulator with the associated voltage and current waveform

2.1.2.3 The Boost Converter The operation of the Boost Converter is more complex than the Buck Converter: when the switch T1 is on, the diode is reversed biased and VI is applied across the inductor L, where the current builds up to a peak value. When the switch T1 turns off, the voltage VL (voltage across the inductor L) reverses, which causes the voltage at the diode to rise above the input voltage VI. Then the diode conducts the energy stored in the inductor, L, as well as the energy from the input to the smoothing capacitor, C.

As before, the average voltage across the inductor over one complete cycle is zero, therefore

0).(. 0 =−− offIonI tVVtV Equation 4

which gives

TDVVTDV II ).1).((.. 0 −−= Equation 5

then

DVV

I −=

110 Equation 6

Hence, V0 is always greater than VI, which makes this converter a step-up converter with the output value dependent upon the duty cycle D.



Figure 4: Boost regulator with the associated voltage and current waveform

2.1.2.4 Summary of the basic converter in the non- isolated topology Topology Polarity V0 wrt VI Magnitude V0 wrt VI

Buck Same Step-down

Buck-Boost Inverse Step-down or step-up

Boost Same Step-up

Table 1: Summary of the basic topologies

The non- isolated topologies described above were only presented for the continuous mode of operation. There are two modes of operation for SMPSs, and their most important features are summarised below:

(A) Continuous Mode Operation:

• The current through the inductor L (IL) does not fall to zero at any time of the switching cycle;

• In Boost and Buck-Boost regulators, the current into the regulator output stage is discontinuous. For the Buck regulator, the current in the output stage is continuous with small ripple;

• In the Open Loop Regulation, the magnitude of V0 with respect VI (Table 1) is independent of the load resistance (Load R), but with V0 dependent on VI, making the open loop line regulation poor;



• In the Closed Loop Regulation, the requirement of a large inductor for the continuous mode (so as to maintain the current flowing) associated with the capacitor (C) results in poor closed loop response (delay).

(B) Discontinuous Mode Operation:

• In opposition to the Continuous Mode Operation, the current in the inductor, L, falls to zero at each cycle, which creates high inductor current inductor peaks;

• In the Open Loop Regulation, for the three types of converters, the output voltage depends on the loads resistance (R) as well the input voltage VI;

• In the Closed Loop Response, at the start of each cycle, no energy is being stored in the inductor, so that the inductor has no effect on the signal closed loop characteristic. Only the capacitor (C) is in this case used as the delay element in the loop making the regulators very stable and with good closed loop response.

Further information on continuous and discontinuous mode of operation can be found in [2].

2.1.3 Isolated Topology SMPS based on non- isolated topologies have limited use, since the output range is limited by the input and duty cycle (application such as single output DC to DC converter for example). The use of a trans former in the isolated topology helps to overcome these constraints and provide the following advantages:

• A direct isolation of the input/output is provided which does produce a degree of safety;

• The selection of the transformer turns ratio provides a choice of outputs, which can be very different from the input. The polarity of each output can also be selected, dependent upon the polarity of the secondary with respect to the primary;

• Multiple outputs can easily be obtained by adding more secondary windings to the transformer;

• But disadvantages such as size, weight, power loss will all the time be present when using transformers.

Isolated converters are usually split into two main sections (symmetrical and asymmetrical converter) depending on how the transformer is to be operated:

• In the symmetrical converter (case of the Push-Pull, the Half-Bridge and the Full Bridge types), the full B-H (Magnetic field – Flux) magnetisation loop is used, utilising the core more effectively, which allows more power to be produced (surface area of the B-H curve);

• In the asymmetrical converter, the flux (H) and magnetic field (B) never change sign, which means that only half of the power is being produced compared to the symmetrical converters. The flyback and forward converter are both asymmetrical converters.

Table 2 summarises the most commonly used topologies with their typical maximum output power.



Converter Topology Typical Maximum output Power (Watts)

Flyback 200W

Forward 300W

Push-pull 500W

Half-Bridge 1000W

Full-Bridge >1000W

Table 2: Converter Topology with their associated maximum output power

2.1.3.1 The Flyback Converter Figure 5 shows a typical single-ended flyback converter which is an asymmetrical converter since only one transistor switch is used. The converter works as follows: when the transistor is turned on, the current builds up in the primary coil of the transformer, stored in the core and then released to the output through the secondary coil of the transformer when the transistor switch is turned off. The polarity of the windings is such that the output of the diode blocks during the transistor on time.

Once the transistor is turned off, the secondary voltage is reversed, which forces the secondary current to flow through the diode (D1), directly to the output load.

Figure 5: Diagram of a single-ended flyback converter circuit and typical waveforms (Ip=Primary current, Is= Secondary current)



The output energy stored in the core is ½LI2 and is all the power that will be deliver to the output, which means that in the case of the flyback converter the transformer is extremely important.

During the continuous mode of operation,

DDn

VV

i −=

1.0 Equation 7

where 1

2

NN

n = , the ratio of turns in the transformer.

For the continuous mode of operation, the primary inductance is usually selected for the following criteria:

2

2

.2)1.(.

nDTR

L−

> Equation 8

The primary current ripple can be calculated from:

).(.L

VTDI i=∆ Equation 9

The flyback converter is more commonly used in the discontinuous mode, due to:

a. smaller inductor required (high peak current needed for high stored energy);

b. better closed loop response.

However it has the disadvantages in the discontinuous mode of

a. high peak transistor current;

b. large filter capacitor.

2.1.3.2 The Forward Converter As seen from Figure 6, the Forward Converter is based on the Buck converter with an added transformer at the input and an added diode on the output of the circuit. In the case of the Forward Converter, the energy stored in the transformer is directly transferred to the output through the inductor during the on-time transition (diode D1 forward biased). During the off-time transition, the diode D1 is reversed biased and the diode, D2 is forward biased to maintain a continuous current in the output circuit. The Forward Converter is usually operated in continuous mode since this produces very low peak input current, output current and ripple components. The discontinuous mode would increase these values as well as the generated noise.

In the case of the Forward Converter, there is an issue with the removal of the core magnetisation energy by the end of each switching cycle. A common technique is to include a tertiary winding on the transformer core, which conducts during the off period, therefore resetting the core.

The DC gain of the forward converter is

DnVV

i

.0 = Equation 10



The Forward Converter is usually used in the continuous mode due to low current ripples which do place a heavy duty on the filter capacitor but has in this mode the disadvantages of:

a. poor closed loop response (leading to instability);

b. bulky design at high power (>500 watts) due to the poor transformer use.

Figure 6: Diagram of a forward converter circuit and typical waveforms

2.1.3.3 The Push-Pull Converter The Push-Pull Converter, also derived from the Buck Converter, uses both transistors TR1 and TR2 alternatively, which drives the transformer in both directions. The corresponding voltage waveform of the secondary is shown in Figure 8, which is then rectified and filtered with L1 and C0.

The input voltage Vi is applied across the half of the primary (through TR1 or TR2), which means that the voltage across the full primary is 2Vi. The DC gain of the Push-Pull Converter is

DnVV

i

..20 = Equation 11



The duty ratio of each switch is usually less than 0.45, which provides enough dead time to avoid transistor cross conduction. The use of both transistors TR1 and TR2 provides an output which operates at twice the switching frequency, and therefo re offering a very compact design of the transformer and output filter, while producing a very low output ripple.

Figure 7: Diagram of a Push-pull converter circuit

Figure 8: Rectified output voltage of a Push-pull converter (before filter)

The Push-Pull converter has the following advantages:

a. compact design (importance of space is an issue);

b. efficient (energy is transferred from the primary to the secondary during both halves of the cycle – using the transformer efficiently);

c. Isolation is provided between the input and the output.

Its mains disadvantages are:

a. each of the transistors TR1/TR2 must block twice the input voltage due to the doubling effect of the primary winding of the transformer, therefore requiring two transformers;

b. the flux swing needs to be symmetrical for each cycle. In the case of a symmetry imbalance, the transformer will saturate. Imbalance can be caused by different characteristics in both transistors such as storage time and on state losses.



2.1.3.4 The Half-bridge Converter The Half-bridge is also derived from the Buck Converter and is usually the first choice for the high power applications (500 to 1000W range). From Figure 9, the capacitors C1 and C2 are connected in series and form a voltage divider of the input voltage. The two transistors are driven alternatively, and this connects each capacitor across the single primary winding each half cycle, switching the primary voltage from +½Vi to -½Vi. Similarly to the Push-Pull converter, since the transistors are driven alternatively, the duty cycle, D, of each transistor is limited to 0.5 to prevent both transistors conducting simultaneously. The DC gain of the Half-bridge is:

DnVV

i

.0 = Equation 12

Some of the advantages of the Half-bridge are:

a. using the series capacitors C1 and C2, any DC build up of flux in the transformer is blocked which minimises risk of saturation of the core of the transformer;

b. both the series reservoir capacitor already exist and make the Half-bridge converter ideal for use in either 110V/220V mains.


a. the use of the input capacitors since they are large in size;

b. TR1 (top transistor) must have isolated drive since the gate is at floating potential.

Figure 9: Diagram of Half-bridge converter circuit

2.1.3.5 The Full-bridge Converter The Full-bridge converter is a higher power conversion of the Half-bridge. It contains four transistors which are alternatively driven in pairs, TR1 and TR3, then TR2 and TR4. This produces a square AC voltage waveform equal to ±Vi on the transformer primary. On the secondary side, as for the Half-bridge, the Full-bridge produces very low ripple outputs at very high current levels. The DC gain of the Full-bridge is:

DnVV

i

..20 = Equation 13



The capacitor C2 is used to block any DC component which would saturate the transformer core as for the Half-bridge converter.

Some of the advantages of the Full-bridge converter are:

a. ideal for the high power output (1kW and above);

b. only one main smoothing capacitor is required in the Full-bridge compared to two in the case of the Half-bridge.


a. most complex and costly design compared to the other types of converter;

b. requirement of four transistors and clamp diodes;

c. for two of the transistors, isolated drives are required.

Figure 10: Diagram of Full-bridge converter circuit

2.2 The Switched Electronic Load Controllers (SELCs)

Switched Electronic Load Controllers are also becoming very common in the residential, commercial and light industrial environment and are used, for example, in light dimmer switches and variable speed controlled devices (e.g. drills). Their technology is based on a solid state dimmer, which works by varying the duty cycle (or On/Off time) of the full AC voltage applied to the device which is being controlled. SELCs are built using thyristors (Silicon Controlled Rectifier and Triac) and their features are presented below.

2.2.1 Silicon Controlled Rectifier (SCR) Silicon Controlled Rectifiers, like switching transistors, are available in a wide variety of types. They work on the following principle: the thyristors enable the control of the load power by controlling the amount of the load current. Once triggered, the thyristor supplies its own gate drive and very rapidly goes into saturation, thereafter, the gate loses its control entirely. Once the thyristor is in its on state, it is latched as a closed circuit.

The thyristor remains on until the current going through it, is either stopped or reduced below a set value which is usually called the “holding current”. It then reverts to a non-conductive state or off state. Now since being reset, it is again receptive to a gate trigger pulse. Since simple thyristor circuits are fed from an AC source, the load current is determined by timing the occurrence of gate triggers, which means that more or less half-wave cycle is allowed to



pass to the load. This method is also known as phase control. More detailed explanations can be found in [3].

2.2.2 Triac Triacs are commonly known as full-wave rectifier thyristors, which permits the passage of both half cycles of the AC wave to the load (SCR only operates as a half wave-rectifier).

The triac structure is based on a pair of oppositely poled SCRs connected in parallel and is used when AC power is to be controlled. Triac based circuits (SELCs in our case) work on the following principle: when the thyristors is triggered, it continues to conduct until the current passing through it reaches zero. Then, by changing the point (phase) at which the thyristor is triggered, the duty cycle is changed and therefore the output power is altered.

Figure 11 illustrates a triac and its switching characteristics. The triac has the advantages of exhibiting both forward and reverse breakdown voltage, BVF and BVR, which means that the conduction characteristics are symmetrical (see above). A gate pulse can turn on the triac through the electrodes M1 and M2. Again, the triac is very much like two SCRs connected in parallel, with opposite polarisation, but with only one single gate terminal for the triac [3].

Figure 11: Description of the Triac as a symmetrical switch



Figure 12 shows a switching-type regulator using a triac with a symmetric phase controlled wave.

Figure 12: A full-wave switching regulator using a Triac



3 TESTING OF THE SMPSS AND SELCS This study is based on the investigation into the EMC emissions from SMPSs and SELCs under different load conditions.

It was agreed with both the Radiocommunications Agency and York EMC Services Ltd that 6 items based on the SMPS technology and 4 items based on the SELC technology were to be purchased for this project and then be tested at the Agency’s RTCG laboratory for mains harmonics emissions, conducted emissions and radiated emissions under 25, 50 and 100% load conditions. Three items (items 5, 6 and 7) were also tested at York EMC Services Ltd test facility at Castleford, and this report only contains the results and data analysis of these three items as agreed with the Radiocommunications Agency. The work related to the purchase of the EUTs and EMC testing is presented in the next sections.

3.1 Choice of the EUTs

The products for this study were chosen to cover the widest range of applications while still remaining easily available for the general UK consumers. Those products were either purchased in local stores (DIY stores), ordered from the internet, purchased through general catalogues or already available at YES.

3.1.1 Products based on the Switched Mode Power Supply Technology

As agreed with the Radiocommunications Agency, a total number of 6 items based on the SMPS Technology were procured. Those items are listed in Table 3.

Item Type of Product Characteristic Origin

1 Mass produced SMPS for PC PSU-ATX 250 watts Catalogue

2 Replacement PC SMPS 250 watts, CE approved Catalogue

3 DVD player Internet

4 Sky Digital Box Already available at YES

5 Printer Stores

6 Plug- in power supply Catalogue

Table 3: List of items based on SMPS Technology

Details of each of the SMPS based items (power, output characteristics) can be found in Appendix A [4].

3.1.2 Products based on the Switched Electronic Load Controller Technology As agreed with the Radiocommunications Agency, 4 items based on the SELC Technology have been purchased. Those items are listed in Table 4.

Item Type of Product Characteristic Origin

7 One way rotary dimmer 250 watts Stores

8 Power corded Hammer drill 760 watts Stores

9 Separate speed transformer 24VA output Catalogue

10 Courtesy Wall Light 60 watts Stores

Table 4: List of items based on the SELC Technology



Details of each of the SELC based items (power, output characteristics) can be found in Appendix A [4].

3.2 Testing of the EUTs

EMC measurements were performed at the Agency’s RTCG laboratory for all the purchased EUTs and at York EMC Services Ltd test facility at Castleford for three items only (EUTs 5, 6 and 7). The EUTs were tested with their outputs terminated into loads representing maximum (100%), half (50%) and a quarter (25%) of their specified maximum operating loads. Bespoke loads were designed and built by YES for tests purposes.

Prior to the testing, an EMC Test Plan was issued which specified the EMC testing to be performed on the 10 items [5], which is enclosed in Appendix B. A Load Guidelines Booklet was also drafted to provide guidance how to perform the connections to the output of the SMPSs/SELCs to ensure that the EUTs were tested under the correct load conditions. A brief summary of both the Test Plan and Load Guidelines Booklet [10] is presented below.

3.2.1 EMC Test Plan The issued EMC Test Plan [5] (enclosed in Appendix B) identified the EMC standards which were used in order that comparisons can be made between the EUTs and defined the test methodology and how the equipment is to be exercised and monitored while the EMC testing is being performed.

SMPSs and electrical devices that contain SELCs are required to be in conformance with the EMC directive 89/336/EEC as well as the implementing regulation SI 2372 1992. The EUTs were subject to the following tests:

a. Harmonic Emission Testing (EN61000-3-2:2000 [6]);

b. Conducted Emission Testing (EN55015:2001 [9] and EN55022: 1998 [7]);

c. Radiated Magnetic Field Emission Testing (EN55011:1998 [8] and EN55015:2001 [9];

d. Radiated Electric Field Emission Testing (EN55022: 1998 [7]).

Conducted emission measurements are usually performed between 150kHz to 30MHz as specified in EN55022, but for this project, the conducted emission measurements were extended down to 9kHz for investigation (as specified by EN55015 [9] for quasi peak measurements only).

All the details of the standards are specified in the Test Plan [5] with their frequency range and limits.

3.2.2 Load Guidelines Booklet A Load Guidelines Booklet [10] was issued to provide instructions for the use of the variable and static loads during the EMC testing. The Load Guidelines Booklet is enclosed in Appendix C.

3.2.3 EMC Test Reports Three EMC Test Reports were issued to describe the results obtained for the EMC testing. A report was issued for each of the EUTs and are included in Appendix E (EUT5), Appendix F (EUT6) and Appendix G (EUT7).



3.3 Investigation of the Circuit diagram and Modelling of the emission of 3 EUTs

3.3.1 Determination of the circuit diagram For each of the items, the circuit diagrams have been extracted and component values recorded to enable some modelling of emission to be performed on some of the items (EUTs 5, 6 and 7). Information, such as circuit configuration, type of filtering, type of rectification are detailed in the report. Details of each of the circuit diagrams of the SMPSs/SELCs based items can be found in Appendix D [11].

3.3.2 Modelling of the emission for EUTs 5, 6 and 7. Harmonic emissions and conducted emissions (9kHz to 30MHz) were modelled for three of the items: EUT 5 (Printer SMPS), EUT6 (Plug- in power supply) and EUT7 (One way rotary dimmer). Results of the testing are presented in Appendix H [12]. A summary of the modelling findings is enclosed in section 4 of this report.

3.4 Analysis of the Testing

The results of the testing of the three EUTs are enclosed in Appendix E for EUT5 [13], in Appendix F for EUT6 [14] and finally in Appendix G for EUT7 [15]. The most important results of the testing are enclosed in the next sections with the data analysis.

3.4.1 Analysis of the Harmonic Emission Testing The power frequency harmonic emission testing was performed according to EN61000-3-2:2000 [6]. The results of the harmonic testing for the three EUTs are presented in Figure 13 to Figure 15. For each of the items, the harmonic results for the 100% load (red bar), 50% load (yellow bar) and 25% load (blue bar) are presented. The limit line for the 100% load condition is also present for indication. As specified by EN61000-3-2:2000, there are no requirements to perform harmonic testing for items of power less than 75 watts (except for Class C equipment – lighting equipment). The limit lines were derived from the power values (Class D limit).

0

10

20

30

40

50

60

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Harmonic Number

Cur

rent

(m

A)

100% limit 100% load 50% load 25% load

Figure 13: Harmonic testing results of item 5 (Printer SMPS)

The harmonic emission measurements were still performed on item 5 (printer SMPS – maximum rated power of 12 watts) and on item 6 (plug- in SMPS – maximum rated power of 6 watts) even though their rated power was less than 75 watts.



In the case of Item 5 (Figure 13), the harmonic emission results show that the EUT would fail the harmonic emissions testing if it was required. The overall levels of harmonic emissions are still low (less than 60mA). The amount of generated harmonic current is approximately proportional to the value of the load (the larger the load, the more current is being generated).

0

5

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15

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25

30

35

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40Harmonic Number

Cu

rren

t (m

A)


Figure 14: Harmonic testing results of item 6 (Plug in power-supply)

For item 6 (Figure 14), once again the EUT would also fail the harmonic emission test if it was required to be tested (as per item 5). Again, the amount of harmonic current produced is approximately proportional to the value of the load and the overall emission levels remain low (less than 35mA).

0

100

200

300

400

500

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700

800

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40Harmonic Number

Cu

rren

t (m

A)


Figure 15: Harmonic testing results of item 7 (One way rotary dimmer)

The results of the testing of item 7 (Figure 15 - one way rotary light dimmer – maximum power rating of 250 watts) shows that the EUT passes the harmonic emission testing for the 100% load condition but fails the testing when tested under 50% and 25% load (Appendix



G). For this item, the amount of harmonic current generated is inversely proportional to the load value (opposite to items 5 and 6).

In the case of the SMPS items, the input current flows as a short pulse when the mains voltage reaches its peak value. Therefore the current required increases with the load current and hence a higher level of harmonic emissions tends to be generated.

For SELC (item 7), the harmonic emissions are due to the dimmer switching part-way through each mains cycle, distorting the waveshape of the mains supply. The lower the load (in our case 25%), the more the waveshape is distorted (deviates from the 50Hz sinewave), and therefore creating more harmonic emissions (25% load condition distorts the waveshape more than the 50% load condition). In the case of the 100% load condition, the mains cycle remains largely unchanged and little harmonic emissions are measured.

3.4.2 Analysis of the Conducted Emission Testing The conducted emission tests were performed on the three items in accordance with EN55015 [9] (9kHz to 150kHz – Peak and Quasi Peak) and EN55022 [7] (150kHz to 30MHz). Measurements were performed on both live and neutral lines, for peak, quasi-peak and averaged measurements. The most relevant results are presented in the following sections. Further details on measurements results can be found in Appendix E (Item 5), Appendix F (Item 6) and finally Appendix G (Item 7).

3.4.2.1 Conducted emission of item 5 (Printer SMPS)

Conducted Emission (Live) Average Measurement

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100

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0.001 0.01 0.1 1 10 100

Frequency (MHz)

Con

duct

ed E

mis

sion

(dB

uV)

100% Load 50% Load 25% Load Noise Floor AV - 100% Load AV- 50% Load AV - 25% Load Limit EN55022 - AV

Figure 16: Conducted emissions results, (Live conductor, average measurement)



Figure 17: Conducted emissions results, (Neutral peak and quasi peak measurement)

Figure 16 and Figure 15 show the results of the conducted emissions in the cases of the live conductor average measurements (Figure 16) and neutral conductor peak/quasi peak measurements (Figure 15). The graphs show that the emissions from the EUT are below the limits (20dB over most of the frequency range) and that there is relatively little variation when the load is varied, with the 100% load condition being the worst-case. A strong emission is measured at approximately 59kHz, (main switching frequency of the power supply), for each of the load conditions, and with its second harmonic present at 118kHz, third harmonic at 180kHz and fourth harmonic at 235kHz. Harmonics of the main switching frequency of the power supply can be clearly seen for frequencies up to 10MHz.

3.4.2.2 Conducted emission of item 6 (Plug- in SMPS) Figure 18 shows the results of the conducted emissions in the case of the neutral average measurement, with the EUT directly plugged into the LISN and Figure 19 shows the same measurement, but with a 1 meter extension cable between the LISN and the EUT (as specified by EN55011). The test results show that the emissions from the EUT are below the limits even though this EUT produces emissions over most of the frequency range. The measurements below 60kHz show substantial differences between the emission levels with the 100% load and the 50% and 20% loads of approximately 20 dB. Emissions can be clearly seen for frequencies above 100kHz, with variations up to 25dB between some load conditions. Conducted emissions due to the harmonics of the main switching frequency (positioned at around 100kHz) can be clearly seen up to approximately 5MHz, especially in the 100% load condition. The levels of conducted emissions being measured seemed to be approximately proportional to the load condition (100% being the worst emission case).

Conducted Emission (Neutral) Peak and Quasi Peak Measurement

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0.001 0.01 0.1 1 10 100

frequency (MHz)

Co

nd

uct

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mis

sio

n (

dB

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)

100% Load 50% Load 25% Load Noise FloorQuasi-Peak - 100% Load Quasi-Peak - 50% Load Quasi-Peak - 25% Load Limit EN55015 and EN55022 - QP



Conducted Emission (Neutral) Average Measurement

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frequency (MHz)

Con

duct

ed E

mis

sion

(dB

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100% Load 50% Load 25% Load Noise Floor AV - 100% Load" AV - 50% Load" AV - 25% Load" Limit EN55022 - AV

Figure 18: Conducted emissions results, (Neutral average measurement)

The measurements also show that the load condition affects the positioning of the switching frequency as maybe expected in SMPS (changing of the duty cycle by keeping pulse width constant and altering the switching frequency which will affect the frequency of emission depending on the load). This is not the case for the printer SMPS and hence shows a different control mechanism.

Differences between Figure 18 and Figure 19 lie in the overall amplitude of the emission (emission with the 1m cable approximately 5dB less than in the case with the EUT directly plugged in the LISN). The emission pattern is approximately the same in both cases.



Conducted Emission (Neutral) Average Measurement, with 1m extended cable

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frequency (MHz)

Con

duct

ed E

mis

sion

(dB

uV)

100% Load 50% Load 25% Load Noise Floor AV - 100% Load" AV - 50% Load" AV - 25% Load" Limit EN55022 - AV

Figure 19: Conducted emissions results, 1m extended cable (Neutral average measurement)

3.4.2.3 Conducted emission of item 7 (One way rotary light dimmer) Results of the conducted emissions in the case of the one-way rotary dimmer are presented in Figure 20 (Live average measurements) and Figure 21 (Neutral Peak and Quasi peak measurement). The results show that the emissions are close to the limit line in the case of the average measurements and above the limit lines for the 25% and 50% loads in the case of the quasi peak measurements. The conducted emission results in the case of the 100% load are largely different to the results obtained for the 25% and 50% loads. At frequencies around 150kHz, the measured levels are approximately 20dB above the EN55022 limit line. Such large differences were also seen in the average measurements results for both live and neutral cases. The shape of the response for the 25% and 50% load condition cases both show a typical (sinx)/x decay, characteristic of the attenuation of the emissions expected when a square wave component is present (the phase control may be through multiplying the sinusoidal voltage by a square wave).



Conducted Emission (Live) Average Measurement

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0.001 0.01 0.1 1 10 100

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Con

duct

ed E

mis

sion

(dB

uV)

100% Load 50% Load 25% Load Noise Floor AV - 50% Load" AV - 25% Load" Limit EN55022 - AV

Figure 20: Conducted emissions results, (Live average measurement)

Figure 21: Conducted emissions results, (Neutral peak and quasi peak measurement)

Conducted Emission (Neutral) Peak and Quasi Peak Measurement

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Frequency (MHz)

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duct

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mis

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(dB

uV)

100% Load 50% Load 25% Load Noise FloorQuasi-Peak - 100% Load Quasi-Peak - 50% Load Quasi-Peak - 25% Load Limit EN55015 and EN55022 - QP



3.4.3 Analysis of the Radiated Magnetic Field Emission (Van Veen Loop – Item 5) Radiated magnetic field emission was performed on Item 5 using a 2m diameter magnetic loop [9]. The Van Veen loop consists of three orthogonal loops: one horizontal and two vertical (orthogonal to each other). The measurements were performed in accordance to EN55015 [9]. Results of the horizontal and one vertical loop are presented in Figure 22 and Figure 23 (the results of both vertical loop measurements were similar and therefore are not duplicated here. As can be seen from the results, the measured levels are extremely low (approximately 15dB below the limit line for the worst case emission) over most of the frequency range (9kHz to 30MHz), there are few differences between the noise floor and the measured radiated emission. In the case of the vertical polarisation measurements (Figure 23), it is possible to observe the same peak emissions (harmonic emission) at 59, 118, 180kHz and following harmonic emissions. Such emission peaks were also measured in the conducted emission measurements (Figure 16). There are no obvious differences in the radiated magnetic field emissions between each of the load condition.

Van Veen Peak Measurement (Loop H1)

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0.001 0.01 0.1 1 10 100

frequency (MHz)

Rad

iate

d E

mis

sio

n (d

Bu

A/m

)

100% Load 50% Load 25% Load Noise FloorQuasi-Peak - 100% Load Quasi-Peak - 50% Load Quasi-Peak - 25% Load Limit EN55015 - QP

Figure 22: Radiated magnetic field emission for horizontal polarisation (Peak measurement - loop H1)



Van Veen Peak Measurement (Loop V2)

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0.001 0.01 0.1 1 10 100

frequency (MHz)

Rad

iate

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mis

sion

(dB

uA/m

)

100% Load 50% Load 25% Load Noise FloorQuasi-Peak - 100% Load Quasi-Peak - 50% Load Quasi-Peak - 25% Load Limit EN55015 - QP

Figure 23: Radiated magnetic field emission for vertical polarisation (Peak measurement - loop V2)

3.4.4 Analysis of the Radiated Magnetic Field Emission (CISPR 0.6m Loop) The CISPR loop measurements were performed with a 0.6m diameter active magnetic loop for both polarisations (vertical and horizontal), at a 3m distance in a Fully Anechoic Room (FAR). The measurements were performed in accordance with EN55011 [8], for frequencies ranging from 9kHz to 30MHz.

3.4.4.1 Radiated magnetic field emission of item 5 (Printer SMPS) Figure 24 and Figure 25 show the response of the radiated magnetic field emission in the case of the parallel (horizontal) polarisation (Figure 24) and perpendicular (vertical) (Figure 25). The results show that the emission from the EUT is clearly well below the limit line (emission at noise floor level for frequencies above 1MHz), and that very little emission is being radiated from the EUT. Radiated measurements in the case of the parallel polarisation seem to give slightly higher level of radiated interference (3 to 4dB). There are no obvious differences in the radiated magnetic field emission between each of the load conditions.



Parallel Loop Peak Measurement

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0.001 0.01 0.1 1 10 100

Frequency (MHz)

Mag

net

ic F

ield

(d

Bu

A/m

)

Noise Floor 25% Load 50% Load 100% Load Limit EN55011 - QP

Figure 24: Radiated magnetic field emissions for parallel polarisation (Peak measurement)

Perpendicular Loop Peak Measurement

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0.001 0.01 0.1 1 10 100

Frequency (MHz)

Mag

netic

fiel

d (d

BuA

/m)


Figure 25: Radiated magnetic field emissions for perpendicular polarisation (Peak measurement)

3.4.4.2 Radiated magnetic field emission of item 6 (Plug- in SMPS) Figure 26 and Figure 27 show the response of the radiated magnetic field emission in the case of the parallel polarisation (Figure 26) and perpendicular (Figure 27). As for item 5, the results show that the emission from the EUT is clearly well below the limit line (emission at



noise floor level for frequencies above 150kHz), and that little to no emission is being radiated from the EUT. Radiated measurements in the case of the perpendicular polarisation seem to give slightly higher level of radiated interference (7 to 9dB).

Parallel Loop Peak Measurement - 3metres

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ld (d

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/m)



Perpendicular Loop Peak measurement - 3metres

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/m)





3.4.4.3 Radiated magnetic field emission of item 7 (One way rotary dimmer) Figure 28 and Figure 29 show the response of the radiated magnetic field emission in the case of parallel polarisation (Figure 28) and perpendicular (Figure 29). As for the two previous items (item5 and item 6), the results show that the emission from the EUT is clearly well below the limit line (emission at noise floor level for frequencies above 150kHz), and that little to no emission is being radiated from the EUT.


Perpendicular Loop Peak Measurement

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ield

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Parallel Loop Peak Measurement

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3.4.5 Analysis of the Radiated Electric Field Emission Radiated electric field measurements were performed on all three items as specified in EN55022 [7] in a FAR.

Combined Horizontal & Vertical Polarisations Peak Measurement

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Frequency (MHz)

Rad

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s (d

BuV

/m)

Noise Floor 100% Load 50% Load 25% Load Limit EN55022 Class B

Figure 30: Maximum of the radiated emission for both horizontal and vertical polarisations for item 5 (Peak measurement)


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Noise Floor 100% 50% Load 25% Load Limit EN55022 Class B




Only peak measurements were performed since the measured levels of emission were low. The results of the testing for the three items are presented in Figure 30 (Item 5), Figure 31 (Item 6) and Figure 32 (Item 7) for the maximum of horizontal and vertical polarisations. The measured levels are in most cases comparable to the noise floor of the measurement system, except for item 5 (Figure 30), where between 100MHz and 200MHz some emission is present. However, the emission remains 20dB below the limit line. No specific radiated electric field characteristics are noted for these items. The limits line shown is that for EN55022 Class B measurements at 3m. The measurements were made in a FAR rather than on an OATS and therefore the limits should be used for guidance only.


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Noise Floor 100% Load 50% Load 25% Load Limit EN55022 Class B


3.5 Conclusion of the Testing

EMC testing has been performed for three items, two based on the SMPS technology and one item based on the SELC technology. Each of the items has been tested for harmonic emission (EN61000-3-2 [6]), conducted emission (EN55022 [7] and EN55015 [9]), radiated magnetic field emission (EN55011 [8] and EN55015 [9]) and finally radiated electric field emission (EN55022 [7]).

The harmonic emission testing showed that in the case of the SMPS based devices (Items 5 and 6), the amount of harmonic current produced is approximately proportional to the value of the load (the larger the load, the more harmonic current is being generated). The results of item 7 (One way rotary light dimmer) showed that the EUT passes the 100% load condition but did fail the 50% and 25% load condition testing (Appendix G).

The conducted emission results showed that items 5 and 6 passed the conducted emission testing (despite high levels of emission being measured for item 6 - Figure 18). High order harmonics of the switching frequency were measured in both cases up to the MHz frequency range. The results showed that the amount of conducted emission being measured in both cases seemed to be dependent on the load condition (i.e. 100% load condition being the worst case). The conducted emissions results of item 7 show that the EUT failed the conducted



emission testing in the case of the Quasi-Peak measurement (by 20dB), and that overall, the worst case emission is being measured for the 25% and 50% load conditions.

Radiated magnetic field emission measurements were performed with both the 2m diameter Van Veen loop and the 0.6m CISPR loop. For all items, the levels of radiated magnetic emission were low and for frequencies above 1MHz, lie within the noise floor. No obvious differences in the radiated magnetic field emission could be measured between each of the load conditions.

Radiated electric field emission measurements were performed in a FAR at a 3 metres distance for peak measurements. The measured levels of radiated emission were in most cases comparable to the noise floor of the measurement system.



4 MODELLING Modelling was performed to determine the feasibility of using SPICE models to predict harmonic current levels, and conducted interference (9kHz to 30MHz) produced by a phase controlled lamp dimmer and flyback converter switched-mode power supplies (SMPSs).

The SPICE models were constructed using realistic models for the passive components and approximate models for the switching and control circuitry so that switching rates and duty cycles can be precisely controlled. The SPICE time-domain data was processed to predict the harmonic current, average and peak conducted interference values.

The models were compared with measured results. Overall, the modelling of power frequency harmonic currents in SMPSs/SELC is feasible and can produce accurate results for phase controlled circuits if the switching phase angle is known. In circuits with rectifiers the currents in the rectifier diodes determine the mains harmonic performance. This can be modelled with good accuracy if the model includes the parasitic elements in the rectifier and reservoir capacitor.

The modelling of conducted emissions in SMPSs/SELC depends on the details of the switching waveform (rise-time) and knowledge of the behaviour of the filter components in the circuit. It has been shown that the correct general trends can be seen in the model, but the overall accuracy is poor. More work is required if the reasons for this and sources of inaccuracy are to be discovered. In order to achieve the waveform resolution required to resolve frequency components up to 30Hz a print time-step of 15ns was used resulting in SPICE output files of over 100MB (30ms simulation) and run times of tens of minutes on a 1.4GHz Pentium processor – a similar time was required for post processing the data.

A copy of the modelling report can be found in Appendix H.



5 CODE OF PRACTICE Switched mode power supplies are becoming the norm in most electronic equipment requiring power leve ls greater than around a few tens of watts. Specialist designers largely undertake the design of such supplies with expertise in the area. In many cases, power supplies are Original Equipment Manufacturer (OEM) items, which are bought in as modules to be incorporated into products.

One of the major problem areas with SMPS is electromagnetic interference, EMI, both within the supply itself and external to the supply. These problems are partially taken care of in the design of the power supply itself but there are generally still EMI problems when incorporating SMPS into a design. As a result most electronic designers and engineers have to become familiar with the problems and pitfalls of using and designing switching supplies.

The Code of Practice is aimed at engineers who have experience of general electronic design but need to gain knowledge of the EMI problems and solutions peculiar to SMPS. It begins by considering how EMI arises in switching circuits and identifies the most common sources of interference and noise in SMPS It goes on to identify the different noise reduction techniques that are used to make SMPS meet the internationally agreed specifications for conducted and radiated noise. It gives a series of guidelines to take into account to minimise EMI and then goes on to discuss the specia lised components that are used to solve EMI problems including, ferrite beads, feed through filters, feed through capacitors, bi- filar or common mode chokes.

Additionally detailed guidelines are given on the choice of inductor and capacitor types for EMI suppression. The specialised grounding and screening techniques used in SMPS design are discussed at length. The incorporation of screens in transformers and heat sinks are described along with their use to bypass noise. The topic of grounding is considered, as this is important in interference control, particularly within the supply itself. It is also important in interference control terms when interfacing the supply to the electronic system being powered.

The topic of screening is dealt with by considering the nature of the interfering field i.e., whether it is magnetic, electric or an electromagnetic plane wave. Guidelines are given for dealing with each different case.

The design of filters for EMI control is outlined and the design problems are highlighted.

Overall, the Code of Practice aims to point an engineer to the particular areas in the design of a SMPS that need attention if it is to meet its specification for noise emissions. By being forewarned of the potential problems in advance it should be possible to obtain a working design which is within specification with less time, effort and cost.

A copy of the code of practice can be found Appendix I.



6 CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

The characteristics of emissions from SMPS/SELC based devices have been examined by a combination of EMC measurements (Harmonic power frequency emission, conducted emission, radiated magnetic and electric field emission) and numerical modelling, when subject to different load conditions (25%, 50% and 100%).

The EMC measurement results show that for the SMPS based devices (items 5 and 6), the 100% load condition was found to be the most appropriate load to measure the maximum emission. In the case of SELC based device (item 7), the results show that the worst emission is being measured when lower value loads (25% and 50%) are applied to the EUT. Substantial emission levels were measured in the harmonic and conducted emission tests (with some items failing the test under some specific load conditions) and little emission was measured for the radiated type emission (magnetic and electric).

The results also show that there are no present requirements to extend the frequency range of the conducted emission from 9kHz to 30MHz (instead of 150kHz to 30MHz) since in the case of these items, the levels of conducted emissions between 9kHz to 150kHz were well below the limit lines (with comparison to the chosen limit line of EN55015).

Numerical modelling was performed on the three items to determine the feasibility of using SPICE models to predict harmonic power frequency emission and conducted interference between 9kHz and 30MHz. The modelling of the harmonic results showed that it is possible to accurately model the amount of harmonic emission in the case of both SMPSs and SELC based devices.

The modelling of the conducted emission was found to be more difficult but, overall, the general emission trend was predicted but only with poor accuracy. Further work would be required to achieve better modelling results.

A Code of Practice was written as part of this investigation for the design of SMPSs and SELCs giving guidance on how to design such devices to ensure good EMC characteristics. The Code of Practice aims to point an engineer to particular areas in the design of a SMPS or SELC that need attention if it is to meet its specification for noise emissions.

6.2 Recommendations for further work

This investigation has brought to light several points which are recommended for further study:

a. This study has only concentrated on a few items (3 in total). Further work should concentrate on the testing of further products to validate and support the conclusions of this report.

b. There has been very little attempt to perform numerical modelling on SMPS and SELC based products. Further work should concentrate on understand ing the differences between the measured and modelled emission in the case of the conducted emission. The work should also include an attempt of numerical modelling for radiated emissions (magnetic and electric)

c. Some work related with this study has shown that the emission levels measured on specific EUTs by different laboratories can lead to different emission results. A Round Robin type experiment would ascertain the differences amongst a variety of EMC laboratories and to understand the reasons for such differences.



APPENDICES



APPENDIX A: DELIVERABLE 1 – YORK EMC SERVICES LTD - DOCUMENT 8202CR1



APPENDIX B: EMC TEST PLAN - YORK EMC SERVICES LTD - DOCUMENT 8214CR2



APPENDIX C: LOAD GUIDELINES BOOKLET - YORK EMC SERVICES LTD - DOCUMENT 8217CR2



APPENDIX D: DETERMINATION OF CIRCUITS DIAGRAMS - YORK EMC SERVICES LTD - DOCUMENT 8230CR1



APPENDIX E: TEST REPORT - EMC TESTING OF EUT 5 – PRINTER SWITCHED MODE POWER SUPPLY – YORK EMC SERVICES LTD - DOCUMENT 5533/TR/1



APPENDIX F: TEST REPORT - EMC TESTING OF EUT 6 – PLUG IN SWITCHED MODE POWER SUPPLY – YORK EMC SERVICES LTD - DOCUMENT 5534/TR/1



APPENDIX G: TEST REPORT - EMC TESTING OF EUT 7 – ONE WAY ROTARY DIMMER – YORK EMC SERVICES LTD - DOCUMENT 5535/TR/1



APPENDIX H: –MODELLING REPORT ON SMPSS AND SELCS- YORK EMC SERVICES LTD - DOCUMENT 8273CR2



APPENDIX I: –CODE OF PRACTICE FOR THE DESIGN OF SMPSS AND SELCS - YORK EMC SERVICES LTD - DOCUMENT 8268CR2



REFERENCES

[1] SIMPLIFIED DESIGN OF SWITCHING MODE POWER SUPPLIES – John D. Lenk – Butterworth-Heinemann – 1994 [2] SWITCHED MODE POWER SUPPLIES – design and construction – Second edition – H.W. Whittington, B.W. Flynn and D.E. Macpherson – Research Studies Press Ltd – 1997. [3] POWER SUPPLIES SWITCHING, REGULATORS, INVERTERS AND CONVERTERS – Second Edition – Irving M. Gottlieb – TAB Books – 1994 [4] Investigation into the EMC emissions from off- load Switched Mode Power Supplies (SMPSs) and similar Switched Electronic Load Controller (SELCs) – Deliverable 1 – Interim Report – York EMC Services Ltd - Document 8202CR1

[5] Investigation into the EMC emissions from off- load Switched Mode Power Supplies (SMPSs) and similar Switched Electronic Load Controller (SELCs) – EMC TEST PLAN - York EMC Services Ltd - Document 8214CR2

[6] EN 61000-3-2:2000 Electromagnetic Compatibility (EMC) Part 3-2: Limits – Limit for harmonic current emissions (equipment input current up to 16A per phase)

[7] BS EN55022:1998 Information technology equipment – Radio disturbance characteristics – Limits and methods of measurement

[8] EN55011:1998 (+ Amendement A1:1999 to EN55011:1998) Industrial, scientific and medical (ISM) radio-frequency equipment – Radio disturbances characteristics – Limits and methods of measurement

[9] BS EN55015:2001 Limits and methods of measurement of radio disturbances characteristics of electrical lighting and similar equipment

[10] Investigation into the EMC emissions from off- load Switched Mode Power Supplies (SMPSs) and similar Switched Electronic Load Controller (SELCs) – Load Guidelines Booklet - York EMC Services Ltd - Document 8217CR2

[11] Investigation into the EMC emissions from off- load Switched Mode Power Supplies (SMPSs) and similar Switched Electronic Load Controllers (SELCs) - Determination of Circuit diagrams - York EMC Services Ltd - Document 8230CR1

[12] Investigation into the EMC emissions Switched Mode Power Supplies (SMPSs) and similar Switched Electronic Load Controllers (SELCs) under different load conditions - Modelling Report on SMPSs and SELCs - York EMC Services Ltd - Document 8273CR1

[13] Test Report - EMC Testing of EUT 5 – Printer Switched Mode Power Supply – York EMC Services Ltd - Document 5533/TR/1

[14] Test Report - EMC Testing of EUT 6 – Plug In Switched Mode Power Supply – York EMC Services Ltd - Document 5534/TR/1

[15] Test Report - EMC Testing of EUT 7 – One Way Rotary Dimmer – York EMC Services Ltd - Document 5535/TR/1.

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