An Evaluation Method of Transformer Behaviors on Common-mode Conduction Noise in SMPS

Qingbin Chen1, Wei Chen2, Qingliang Song3, Yongfa, Zhu4

1, 2Fuzhou University, Fuzhou, Fujian, 350108, China 3, 4Huawei Technology Co., Ltd., China

E-mail: CQB1012@163.com Abstract-The electric coupling between primary and secondary

windings of the transformer is usually evaluated by the

capacitance between the primary and secondary windings

which is tested by one-port test devices, such as LCR meter.

This method can’t completely characterizes the EMI behaviors

of the transformer on common-mode noise, because this method

does not account electrical potential distribution along the

winding turns and the shielding copper foil effect in the

transformer. A new evaluation method is proposed in this paper

which can overcome the traditional shortages on CM noise by

spectrum analyzer, EMI receiver or network analyzer. The

method provides an effective test tool to optimize the structures

of the windings and shielding of a transformer in design and to

control the quality of the transformer in mass production with

its CM EMI behaviors. Finally, the experimental results verify

the proposed method to be correct and feasible.

I. INTRODUCTION

Electromagnetic conduction emission is an important part of the EMC standard in the power converters, which includes common-mode (CM) emission and different-mode (DM) emission. CM noise is the result of the induced charge by the electric coupling between voltage change conductors and the ground. The path of the CM noise is complex, which is the most difficult issue of EMC [1-2].

The isolated converter is widely used because of its turn ratio adjustment and isolation between primary and secondary.

The operation of switch is usually considered as EMI noise sources. There are two main CM noise paths in isolated converters. The first path goes through the heat sink of the switch to the ground. The second path goes through the electric coupling between primary and secondary windings of the transformer to the secondary ground. Usually, the first path of CM noise can be eliminated by connecting the heat sinks to primary or secondary minus. And the second path through transformer becomes the domination. In this case, the electric coupling between primary and secondary windings of

the transformer becomes the key factor to the CM noise in isolated converters [3-6].

Traditionally, the electrical coupling between primary and secondary windings of the transformer is evaluated by the coupling capacitance between the primary and secondary windings with one-port test method, such as LCR meter. However this doesn’t take the shielding copper foil effect and electrical potential distribution along the winding turns in the transformer into consideration. In this paper the new evaluation method is proposed which can consider the effects of shielding foil and potential distribution along winding turns on CM noise by two-port test method, such as spectrum analyzer, network analyzer or EMI receiver. Finally, A 2.4kW (Input: 220V@50Hz, Output: 48V&50A) front-end power supply module is taken as sample to verify the proposed method to be correct and feasible.

II. TRADITIONAL EVALUATION METHOD BY LCR

METER

Usually, there are four possible connections of coupling capacitance evaluation by LCR meter as in Fig. 1 and 2. Fig. 1 shows the connections for the transformer without shielding copper foil; Fig. 2 shows the connections for the transformer with shielding copper foil.

In Fig. 1 and 2, Cab is the LCR meter tested terminal capacitance between “a” and “b” terminals; Cps is the structure capacitance between the primary and secondary windings of the transformer; Cshp is the structure capacitance between the shielding copper foil and the primary winding; Cshs is the structure capacitance between the shielding copper foil and the secondary winding.

a. b.

Fig. 1. Traditional evaluation method by LCR meter

(The transformer without shielding copper foil)

The work was supported by National Science Foundation of China (#50877010)and National Science Foundation of Fujian Province, China(#2009J01242).

IEEE PEDS 2011, Singapore, 5 - 8 December 2011

978-1-4577-0001-9/11/$26.00 ©2011 IEEE 782

(1). With the test as in Fig. 1 by LCR meter, the electric

potential in all winding turns of primary or secondary winding is the same. And the electrical potential distribution along winding turns cannot be considered. So Cab isn’t the dynamic capacitance to characterize the CM noise but the static capacitance between primary and secondary windings of transformer, which is only determined by the winding structure, called structure capacitance (C0) and nothing to do with the electric potential distributions. On the other hand, when the capacitance taking electrical potential distribution of primary and secondary windings and shielding copper foil effect into consideration, which can characterize the CM noise of the actual circuit, called CM port effective capacitance (CQ).

(2). In the Fig. 2, when the transformer is with shielding copper foil, the evaluation method not only doesn’t consider the potential distribution along the winding turns, but also doesn’t consider the shielding effect of copper foil. The capacitance between “a” and “b” terminals is Cab=Cps+Cshp//Cshs, Cab isn’t equal to Cps, as in Fig. 2a, while the capacitance between “a” and “b” terminals is Cab=Cps+Cshs, Cab isn’t equal to Cps too, as in Fig. 2b. So, whether as in Fig. 2a or 2b, the test results Cab can’t stands for the structure capacitance between the primary and secondary windings.

A. Structure capacitance calculation method

In order to understand the effective capacitance, or dynamic capacitance of the transformer windings considering the voltage potential distribution in the winding turns, the structure capacitance, or static capacitance is reviewed. The transformer interlayer structure capacitance generally can use approximate parallel plate capacitance formula to calculate when the transformer windings are tight and the distance between winding layers is much less than the width of the transformer windings. And the transformer layer insulation tape, wire insulation layer and air space can be equivalent to be a parallel plate capacitor as in Fig. 3. So the transformer interlayer structure capacitance can be got by (1).

dAC r ⋅⋅= εε 00

(1)

a b.

Fig. 2. Traditional evaluation method by LCR meter

(The transformer with shielding copper foil)

Fig. 3. Insulation tape, wire insulation and air gaps equivalent figure

where, A is the relative area between the primary and secondary windings; do is the diameter of the solid wire including wire insulation; di is the diameter of the solid wire; d is equivalent distance. (d=d1+dequ, d1: the thickness of insulation tape; dequ: interlayer equivalent distance, including wire insulation and air gap. When one of transformer windings is solid wire and the other is copper foil, dequ=1.26do-1.15di [7])

B. CM port effective capacitance calculation method

Transformer CM port effective capacitance should be calculated by the induced charge when electric potential distribution along winding turns have been taken into consideration. Generally, in transformer with magnetic core, each turn of the winding links almost the same excitation magnetic flux and so has the same induced electromotive force. So the electrical potential distribution along the winding turns can be considered as linear [5-6]. For easy analysis, suppose both primary and secondary windings have single layer without shielding copper foil as in Fig. 4. The width of windings is w; the voltage across primary winding is Up and the induced voltage across secondary winding is Us. With the consumption of linear electrical potential distribution along winding turns, the voltage difference between the layer of primary and secondary winding can be shown in (2):

( )( ) p sU U x

u xw

− ⋅Δ = (2)

If the transformer interlayer structure capacitance is C0, the induced charge in the secondary winding can be calculated in (3):

0

0( )

w CQ u x dx

w= ⋅Δ∫ (3)

a. The electrical potential distribution b. CM port effective capacitance CQ

Fig. 4. Equivalent figure

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Taking (2) into (3), and the CM port effective capacitance

reflected to primary can be got as in (4).

01 ( )2

p sQ

p p

U UQC CU U

−= = ⋅ ⋅ (4)

When the secondary winding turns are far less than primary winding turns as most used in power supplies, that is Up>>Us, the CM port effective capacitance CQ can be simplified as (5):

0 0

12QC C C≈ ⋅ ≠ (5)

Equation (5) shows that CM port effective capacitance is not the same as structure capacitance, but is proportional to the structure capacitance with the simple winding structure. However, in a real transformer, the winding structure is much more complex and even with magnetic core and shielding foil, the CM port effective capacitance will be much more complex. So the proportional relationship of structure capacitance and CM port effective capacitance will be invalid in the real case.

III. THE PROPOSED EVALUATION METHOD

From the analysis of part II, the traditional evaluation method can’t completely characterize the transformer behaviors on CM EMI conduction noise just because the LCR meter method can not impose the voltage distribution in winding turns. So the new evaluation method by two-port test method is proposed in Fig. 5.

Two-port high-frequency test device, such as spectrum analyzer with TG (tracking generator), network analyzer or EMI receiver with TG should be used in the new method. The excitation, such as TG output, positive terminal is connected to the primary winding dynamic terminal with voltage changing. And the TG reference is connected to the primary winding terminal without voltage changing, or primary minus. The RF input positive terminal is connected to the static terminal of secondary winding and RF reference to the TG reference.

When the TG output is applied to the primary winding, the electrical potential distribution is set up in both the primary and secondary windings and the charges induced in the secondary winding by TG output excitation will flow into the RF input port to become CM noise current.

Fig. 5. The proposed evaluation method

The proposed method is actually the insertion loss test principle for a transformer, which shows that the transformer can be considered as a part of the EMI filter in considering its behaviors on CM EMI.

For easy understanding, a simple flyback transformer is taken as sample to compare the test result with the proposed evaluation method and the simulation with the structure capacitance and the simulation with CM port effective capacitance respectively.

The Flyback transformer structure is as followed: Core: EI33 Primary winding: 22 turns (φ=0.8mm, solid wire) Secondary winding: 1 turn (16mm*0.2mm, copper foil) Both windings have one layer with the primary winding in

the inner layer and the secondary winding in the outer layer. The transformer structure capacitance is tested to be

C0=48.7pF@100kHz by WK6502A impedance analyzer. From (5), the transformer CM port effective capacitance can be calculated as CQ=0.5C0=23.75pF@100kHz.

The simulation circuit with the structure capacitance (C0) and with CM port effective capacitance (CQ) is shown in Fig. 6a and the results are in Fig. 6b. Then the flyback transformer is tested by spectrum analyzer GSP-810 with the proposed evaluation method. The result is shown in Fig. 6b. (The X-axis is in linear, while the Y-axis is in logarithmic.)

Fig. 6b shows that the simulation result of the CM port effective capacitance is nearly identical to the test result in the frequency range (0-20MHz). The comparison proves the analysis in part II to be correct.

Ri=50

TGC0 or CQ

vac

+

-vo

RF

Ro=50

a. Simulation circuit

b. The comparison of the test and simulation results

of the flyback transformer

Fig. 6. The comparison of the test and simulation results

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IV. EXPERIMENTAL RESULTS

A 2.4kW front-end power supply module was taken as sample for experiment.

The input vi(AC)： 220V@50Hz. The output Vo(DC)： 48V&50A. The module has two stages. The first stage is a bridgeless

PFC topology with boundary current mode control. The switching frequencies vary from 30kHz to 120kHz under the full load; the secondary stage is LLC topology with switching frequency from 110kHz to 135kHz under the full load.

Two same transformers in the LLC stage with their primary windings in series and secondary windings in parallel are used. The transformer is with PQ32/30 core and sandwich winding structure. The primary and secondary turns are 7 and 4, respectively.

Three kinds of transformer are taken into comparison: 1. the original transformer without shielding copper foil; 2. the transformer with the traditional shielding copper foil connected to primary minus; 3. the transformer with the adjusted shielding copper foil connected to primary minus.

The tested results for three different transformers by spectrum analyzer FSH3 are shown in Fig. 7.

The test results show that the transformer with the traditional shielding copper foil has great improvement comparing to the original transformer in the whole frequency range (100kHz-30MHz). There is around 14dB IL (insertion loss) improvement in 100kHz-2.4MHz; The transformer with the adjusted shielding copper foil has greater improvement comparing to the original transformer than the transformer with the traditional shielding copper foil in 100kHz-2.4MHz. There is around 27dB IL improvement from 100kHz to 2.4MHz.

In order to verify the relationships of transformer IL to the CM EMI conduction noise level, the CM EMI noise levels by CM and DM noise separator with the three kinds of transformers in the front-end power supply module without EMI filter were tested as in Fig. 8 to compare with the IL of the three transformers as in Fig. 7, respectively.

Fig. 8a shows the CM EMI noise of the front-end power supply module with the traditional shielding transformer has great improvement in the whole frequency rang (150kHz-10MHz) both in peak value and in average value comparing to the one of the module with the original transformer. The CM EMI noise of LLC converter reduced

Fig. 7. The test results with the proposed evaluation method

nearly 10dB from 100kHz to 2MHz. From Fig. 8b, the CM EMI noise of the front-end power supply module with the adjusted shielding transformer has greater improvement in the whole frequency rang (150kHz-10MHz) both in peak value and in average value than the front-end power supply module with the traditional shielding transformer comparing to the one of the module with original transformer--nearly 17dB improvement has been taken from 150kHz to 2MHz. The tendency of the CM EMI noise test is the same as the tendency of the proposed evaluation method, what verifies the proposed evaluation method to be correct and feasible.

There is a big difference by comparing the Fig. 6 with the Fig. 7. It is because with magnetic core and the shielding copper foil as well as the complex windings structures of the LLC transformer, the CM port effective capacitance (CQ) of the transformer is very complex. It can’t use a single capacitance to characterize the CM EMI behavior of the transformer. At the same time, there isn’t a proportional relationship between the insertion loss test result (as shown in the Fig. 7) and CM noise test result (as shown in the Fig. 8). It is because the impedance in the insertion loss test is standard 50 ohms which is different from the actual condition in the front-end power supply module. Also, the CM noise test result contains other CM noise besides transformer CM noise.

a. The CM EMI noise comparison between the original transformer and the

transformer with the traditional shielding foil

b. The CM EMI noise comparison between the original transformer and the

transformer with the adjusted shielding foil

Fig. 8. CM EMI noise test results

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V. CONCLUSION

The methods to evaluate the behaviors of transformer IL effect on CM EMI noise were analyzed. The conclusions are as below:

1. The traditional evaluation method by one-port test device, such as LCR meter or impedance analyzer can’t fully characterize the CM EMI behaviors of the transformer, because the effects of electric potential distribution along winding turns and shielding foil are not accounted with the method;

2. For a real transformer, the windings structures, electric potential distributions along winding turns, magnetic core existing and shielding foils all have effects on its CM behaviors. Using a simple coupling capacitance to characterize the transformer on its CM behaviors is no good enough;

3. The proposed method by using two-port test device can effectively characterize the EMI behaviors of the transformer on the CM noise, which provide a simple method to compare and to control the transformer qualities on its CM EMI specification;

4. The consistency of the tendency between the evaluation test results with the proposed method and the EMI CM noise test results verify the proposed method to be correct and feasible.

REFERENCE

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[2] Shuo Wang, Lee, F.C., Odendaal, W.G. “Characterization and parasitic extraction of EMI filters using scattering parameters,”． IEEE Transactions on Power Electronics, vol. 20, no. 2, pp. 502-510, 2005．

[3] Yazdani, M., Farzanehfard, H., Faiz, J. “EMI Analysis and Evaluation of an Improved ZCT Flyback Converter,” IEEE Transactions on Power Electronics, vol. 26, no. 8, pp. 2326-2334, 2011.

[4] Yazdani, M.R., Farzanehfard, H., Faiz, J. Conducted EMI modeling and reduction in a flyback switched mode power supply, PEDSTC 2011, pp. 620-624．

[5] Pengju Kong; Lee, F.C. Transformer structure and its effects on common mode EMI noise in isolated power converters, APEC 2010, pp. 1424 - 1429.

[6] Dianbo Fu, Pengju Kong, Lee, F.C., Shuo Wang, Novel techniques to suppress the common mode EMI noise caused by transformer parasitic capacitances in DC-DC converters, ECCE 2010, pp. 1252 -1259.

[7] E.C Snelling, B.Sc (Eng), C Eng FIEE, “Soft Ferrites Properties and Applications”, Second Edition, Butterworths,1988. pp331-335.

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