Pontificia Universidad JaverianaBogotá, Colombia

UNDERGRADUATE THESIS

BA L L A S T F O R F L U O R E S C E N T L A M P P OW E R E D B Y A D C VO LTAG E S U P P LY

Authors:Nini Vanesa Rueda AlgarraAndrea Pérez Barbosa

Advisors:Rafael Díez, M.Sc., Ph.D.

Gabriel Perilla, M.Sc.

May 20, 2013

Artículo 23 de la resolución No. 13 de junio de 1946

“La universidad no se hace responsable por los conceptos emitidospor sus alumnos en sus trabajos de tesis. Solo velará porque no sepublique nada contrario al dogma y la moral católica y porquelas tesis no contengan ataques personales contra persona alguna,antes bien se vea en ellas el anhelo por buscar verdad y justicia”

C O N T E N T S

1 theoretical framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.1 Light Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.1.2 Types of Lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.1.2.1 Incandescent Lamps . . . . . . . . . . . . . . . . . . . . . . . . . . 71.1.2.2 Solid-state Lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.1.2.3 Discharge Lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.1.3 Fluorescent Lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.1.3.1 Working Principle and Parts . . . . . . . . . . . . . . . . . . . . . . 81.1.3.2 Lamp Operating Frequency . . . . . . . . . . . . . . . . . . . . . . 81.1.3.3 Modeling Fluorescent Lamps . . . . . . . . . . . . . . . . . . . . . 9

1.2 Ballasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.1 Types of Ballasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.2 Lamp-Ballast Starting Methods . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.3 Resonant Inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.3.1 Half Bridge Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.3.2 Push-Pull Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.3.3 Class E Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.4 Comparison between Inverters . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 analysis and design of a class e inverter . . . . . . . . . . . . . . . . . . . . . . . . 172.1 Steady-State Analisys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.1 Basic Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.1.2 Power and Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.1.3 Final Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2 Resonant Tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.3 Final Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3 control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.1 Control Circuit Design and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1.1 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.1.1.1 Sensing Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.1.1.2 Switched Control for Operation Phases . . . . . . . . . . . . . . . . 263.1.1.3 Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.1.1.4 Voltage Controlled Oscilator (VCO) and MOSFET Driver . . . . . . 26

4 simulations and experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . 274.1 Simulations Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.2.1 Prototype Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284.2.2 PCB Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.2.3 Comparison between Sensing Circuits . . . . . . . . . . . . . . . . . . . . . . 324.2.4 Comparison with others implemented Ballasts . . . . . . . . . . . . . . . . . 33

3

Contents

5 conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

a pspice models code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

b full ballast schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

c experimental waveforms of the inverter in the prototype . . . . . . . . . . . . . 42

d printed circuit board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4

I N T RO D U C T I O N

Due to the changes in illumination technologies, the influence of lighting on the global energy consump-tion (15% of the total electricity consumption[1]), and its indirectly effects on the environment; one of theapplications of Power Electronics is the development of efficient lighting systems[2].

Fluorescent lamps have been preferred over incandescent lamps and have become a major method oflighting, because of its high efficiency, long lifetime and low heat dissipation[3]. For a correct performance,each fluorescent lamp requires a ballast to ignite the discharge and to prevent an uncontrolled current flowinginto the lamp [4].

Electronic ballasts compared with electromagnetic ballasts: get rid of flicker, have lower size and weight,can be fed by a DC or AC source, and improve the luminous efficacy, as they work in higher frequencieswhere fluorescent lamps produce more lumens per watt [4].

The main objective of this work is to present the design and implementation of an electronic ballast for afluorescent lamp, powered by a 12 V battery with frequency control. Applications of this technology mightinclude emergency lighting, outdoor equipment, or any system in which the battery portability provides alighting system where a connection to an electric grid is not available.

In order to reach the main objective, the following specific objectives were set:

1. Design a resonant converter DC/AC (inverter).

2. Design a control circuit to compensate the battery voltage variations and regulate the light intensity orthe current in the lamp.

3. Compare the performance of the control variables in the circuit (Light intensity and Current), measur-ing the light intensity and the lamp power.

4. Compare the final results with other systems as the electromagnetic ballast, through the efficiencyLPW (Lumen per Watt).

This document is divided into five chapters:In the first chapter the theoretical framework is presented. It contains the initial study of the most relevant

subsystems of the circuit: fluorescent lamps, ballasts and resonant inverter.The second chapter presents the specifications, restrictions and design of the selected inverter. First, the

analysis of the circuit will be explained in detail, then the resonant tank will be included in the inverter, andfinally the complete design will be presented.

In the third chapter the control circuit is described. The proposed solution will be explained with its twocontrol variables (Light Intensity and Lamp Current), so that all the operation phases and the power controlcan be performed.

In order to achieve all the operation phases and carry out the power regulation, the proposed circuit willbe explained in detail with its two variables of control (Light intensity or Lamp Current).

The fourth chapter is divided in two sections. In the first section, the simulation of the resonant inverteris displayed. In the second section the behavior of the inverter is presented, followed by the results of thecontrol circuit and a comparison between the two variables of control. In order to complete the analysis ofresults, is carried out a performance comparison of the designed ballast with others ballasts.

Based on the objectives of this work and its results, in the final chapter the conclusions are presented.

5

1T H E O R E T I C A L F R A M E W O R K

1.1 light sources

This section will introduce the light sources, it presents the description and working principles of the varietyof lamps available in the marketplace, deepening in fluorescent lamps.

1.1.1 Background

Light sources can be classified according to the type of light generated (natural - artificial) or the type ofenergy conversion [5]:

• Thermal radiation: Caloric radiation that depends on temperature of the transmitter body. The emis-sion in the visible spectrum is called incandescence, some examples are the sunlight, incandescentlamps and halogen lamps.

• Luminescence: Emission of light from a body caused by the excitation of atoms by a external agent.According to the agent it can be classified as electrical or no electrical.

• Electroluminescence: It is produced when the body emits light in response to the passage of an electriccurrent or to a strong electric field (e.g. LED).

• Photoluminescence: It is produced when the body absorbs the radiation and then re-radiates it ina different wavelength. The most outstanding effect is fluorescence, which converts the ultravioletradiation into light in the visible region of the spectrum.

To be able to compare the different light sources, and types of lamps, is important to define some basicterms and properties [6]:

Correlated color temperature (CCT): Measured in kelvins (K), describes the appearance of light generatedby a hot object. At lower temperatures reddish "warm" light is generated. As the temperature increases thelight appears bluish "cool".

Color rendering index (CRI): Measure the capability of the light source for maintaining the natural colorsof the surfaces. High values represent a better CRI and a more natural light, the maximum value is 100.

Luminous Efficacy: It is the rating of the total luminous flux emitted in relation to the total lamp powerinput. It is expressed in lumen per watt LPW (lm/W).

1.1.2 Types of Lamps

The electric light sources (lamps) fall into three general classes [6]: incandescent, solid state and dischargelamps. In the first class, when a current flows through the filament, it is heated until it glows and produceslight. The solid state lamps work due to a semiconductor devices that generate light by the movement of

6

1.1 light sources

electrons in the material. And the discharge lamps produce light by ionizing a gas through electric dischargeinside the lamp.

1.1.2.1 Incandescent Lamps

According to the Thermal Radiation, the exposure of a body to high temperatures produces a certain amountof light. Therefore, this principle is used to analyse the simple operation of an incandescent lamp.

To generate visible light, the coiled tungsten filament of the incandescent lamp is heated to the incan-descence, by passing a current through it. The glass bulb is filled with a mixture of nitrogen and argon[6].

Although, the incandescent lamp is simple, economic and does not need an auxiliary circuit to ignite it,the principal disadvantage is its low luminous efficacy; 95 % of the energy is dissipated as heat and just theremaining 5% is converted into light [7].

Unlike incandescent lamps, halogen lamps are filled with a halogen gas that extends the lifetime from1000-2000hrs to 2000-5000hrs, also they increase the luminous efficacy and produce a "cooler" light with ahigher correlated color temperature (CCT) [6].

1.1.2.2 Solid-state Lamps

Light Emitting Diodes (LEDs), convert electrical energy directly into light. When a current flows throughthe p-n junction in semiconductors the light is generated. The materials forming the junction determine thewavelength and light color.

As they do not have filaments that burn out, they last longer than traditional lamps (lifetime up to 100000hours) [6].

1.1.2.3 Discharge Lamps

The working principle of this type of lamps is the luminescence effect, in which by applying a high volt-age between the electrodes of the tube (filled with ionized gas), a flux of ions called electric discharge isgenerated.

The main two categories of discharge lamps are: high-intensity discharge and fluorescent lamps. In thefirst category the three lamps most widely available in the marketplace are [6]:

• High-Pressure Mercury Vapor Lamps: Light is produced by an electric discharge through gaseous mer-cury. The Spectral Power Distribution (SPD) of this lamp has discrete spikes at specific wavelengths,reason why, it has low values in the chromatic properties CRI and CCT.

• Metal-Halide Lamps: To improve the chromatic performance and luminous efficacy of the MercuryVapor Lamps, it was added metal components (known as halides) to the arc tube.

• High-Pressure Sodium Lamps: Sodium dominate the spectral radiation, in which the electric dischargeproduces light. The sodium is combined with a low amount of mercury to improve the spectrum.

Finally, fluorescent lamps will be thoroughly studied in the next subsection. An explication of its operation,parts, construction, and electrical model will be presented.

1.1.3 Fluorescent Lamps

Due to the high luminous efficacy of fluorescent lamps, two-thirds of all electric light systems use theminstead of incandescent lamps [6].

7

1.1 light sources

To define the specifications of the ballast and have a greater understanding of fluorescent lamps, theworking principle is presented below.

1.1.3.1 Working Principle and Parts

The mixture of gases contained in the glass bulb (mercury vapor at low pressure, with a small amount of inertgas) has an initial high resistance to current flow. To ignite the electric discharge and allow the emission oflight, is necessary to apply a high enough voltage between the filaments of the lamp to preheat them. Oncethe filaments have been preheated, the voltage is increased further and the discharge is established, the gas isionized and electrons can flow through it [6].

While the current is flowing through the lamp, the mercury vapor emits ultraviolet radiation. To convertthis radiation into visible light, a phosphor coating in the inner walls of the bulb is used, which respondsemitting wavelengths in the visible spectrum [6].

Figure 1.: Parts of Fluorescent Lamps

Figure 1 shows the parts of an instant-start fluorescent lamp [6]:

• Electrodes has a coiled tungsten filament wrapped by a substance that easily allows the emission ofelectrons. The lifetime of the lamp has a directly relation with the quality of that substance, when itloses its properties is harder to ignite the electric discharge.

• The gas inside the lamp facilitates the discharge start, when the electric discharge is established allowsa faster transport of electrons, and protects the substance of the electrodes.

• Finally, the phosphor-coating converts the UV radiation into visible light and allows obtaining a whitelight.

1.1.3.2 Lamp Operating Frequency

The operating frequency is an important factor to consider in fluorescent lamps due to its effects in theluminous efficacy, lifetime and flicker[8]. When the lamp is powered by a low frequency AC signal, thetransition from positive to negative currents happens each half-cycle; as it is too slow, the ionized gas iscooled and the electric arc disappears. Each time that the current decreases to zero, the electric discharge isextinguished, so to restore it, the voltage should sufficiently increase to ignite the discharge again [5].

Some effects of continuously restoring of the arc are: stroboscopic effect caused by the flicker of light,wear of the electrodes and consequently the reduction in lifetime[8].

As the frequency is increased, the ionized gas and the electrodes can not be cooled, so the electric dischargeis continuous, the aforementioned problems are solved and the luminous efficacy is improved. As can beshown in the Figure 2 at frequencies over 20kHz, the luminous efficacy increases by 10 %.

8

1.1 light sources

Figure 2.: Lamp Efficacy vs. Operation Frequency

1.1.3.3 Modeling Fluorescent Lamps

Fluorescent Lamps can be statically represented as a positive resistance with a negative dynamic behavior, inother words, when the RMS current in the lamp decreases the voltage increases [9]. Figure 3 shows the staticcharacteristic of the 32W T8 Sylvania lamp 1, it can be seen that the RMS lamp V-I (Voltage vs Current)curve has a negative slope.

Figure 3.: 32W T8 lamp RMS V-I characteristics

In order to verify the ballast operation and execute computer simulations, lamps can be replaced by theirmodels. The most complete models of fluorescent lamps are focused on physical performance, they canpredict the electrical behavior, as well as the spectral distribution of light and luminous efficacy [10]. Thesemodels will not be considered, since they are very complex and are used for lamps design.

The electrical models of the lamps are complete enough to design the ballast. They just describe theelectrical behavior without thermal or chemical effects. All the electrical models found in the literature donot consider the preheating stage, in which the electric discharge is still not generated, so the lamp can beconsidered as an open circuit.

Electrical models consider the equivalent resistance of the lamp as a function of the power or the RMScurrent. The V-I characteristics of the models that will be evaluated are summarized in Figure 4. Figure 4.cshows the simplest model, as it use a fixed resistance, does not consider the dynamic characteristics of thelamp [11].

In addition to have the dynamic behavior of the lamp represented by the negative slope, the model pre-sented in Figure 4.a considers a non linear relation function, where the instantaneous lamp voltage is givenby a cubic polynomial function of the instantaneous lamp current, and depends on the average power [12] infunction of the average power. The equation describing this model is shown in (1):

1 Figure 3 was experimentally obtained with the 32WT8 Sylvania lamp used to in the experimental results.

9

1.1 light sources

Figure 4.: Instantaneous V-I characteristics at different power

vLamp(t) = APLampiLamp(t) + [BPLampiL(t)]3 (1)

where, APLamp = a1P−1Lamp + a2 + a3PLamp + a4P2

Lamp

BPLamp = b1P−1Lamp + b2 + b3PLamp + b4P2

Lamp

Values for A and B depend on the average lamp power PLamp, and the parameters a1, a2, a3, a4, b1, b2, b3, b4

should be experimentally determined.On the other hand, the model presented in [13] does not take into account the variations of instantaneous

lamp voltage, and it describes the lamp as a rms current-controlled resistor (Figure 4.b).As can be seen in Figure 5, the dynamic behavior of the lamp can be approximated to a line equation.

Thus, to be able to describe the line, this model proposes the measuring of the rms voltage and current of thelamp at 20% and 100% of the full power rating [13].

RS =V100% − V20%

I100% − I20%(2)

VH = V20% − I20% ∗ RS (3)

Figure 5.: 32W T8 lamp RMS V-I characteristics [13]

The voltage VH denotes the value at the intersection of line and the V axis and RS is the slope of the linewhich links the points (I100%,V100%) and (I20%,V20%). Therefore, the steady-state lamp equivalent resistor(RLamp) is determined by the slope of the line connected between the operating point on line with the originand the instantaneous voltage is given by: [13]

10

1.1 light sources

RLamp =(RS +

VH

IO

)(4)

vLamp(t) =(RS +

VH

IO

)∗ iLamp(t) (5)

Where IO is the rms current of the lamp at the operation point.The simulations of both models were carried out in Pspice and their code can be found in Appendix A.

Figures 6 and 7 show the simulation results of the nonlinear model, and Figures 8 and 9 shows the result ofthe linear model.

Figure 6.: Current and Voltage waveforms of theNonlinear Model for different powers

Figure 7.: Dynamic Characteristics V-I of theNonlinear Model for different powers

Figure 8.: Current and Voltage waveforms of theLinear Model for different powers

Figure 9.: Dynamic Characteristics V-I of theLinear Model for different powers

Both simulations showed the negative dynamic behavior of the lamp, in which by applying a high voltage(RED Curves) the current is lower than by applying a lower voltage (BLUE Curves).

Also, the non-linearity of the first model is clearly seen in its current waveforms (Figure 6).To compare the aforementioned models, Table 1 summarize them.

11

1.2 ballasts

Table 1.: Comparison between ModelsMODEL EXPERIMENTAL

PARAMETERSCHARACTERISTICS

Fixed Resistance None It does not modelate the dy-namic characteristics, and doesnot require experimental param-eters since the resistance canbe calculated with the nominalpower and nominal rms currentin the lamp.

Linear Model 2 Parameters (RS

and VH)Describes the dynamic behaviorof the lamp reducing it to a linearequation.

Nonlinear Model 8 Parameters(a1...a4 and b1...b4)

Describes the dynamic behaviorof the lamp by a nonlinear func-tion allowing a better approxima-tion.

1.2 ballasts

In order to correctly operate fluorescent lamps a ballast is required to stabilize the current flow. Otherwise thenegative dynamic behavior of the lamps make them inoperable [14]. Besides to limit the current delivered tothe lamp, the ballast also [4], [14]:

1. Provide a symmetrical bipolar signal, to avoid damages an preserve lamp lifetime.

2. Provide a high enough voltage to ignite the lamp. Unlike incandescent lamps, fluorescent lamps cannot be connected directly to the electric line because the ignition voltage in a fluorescent light tube ishigher than the peak voltage of the line (170VP) [15].

3. Maintain a preheating voltage for a defined time to increase the lamp lifetime and reduce ignitionvoltage.

1.2.1 Types of Ballasts

There are three types of ballasts commonly used for commercial applications [8]: magnetic ballast, hybridballast, and electronic ballast.

• The magnetic ballast contains a magnetic core of several laminated steel plates wrapped with copperwindings [8]. Although, the power losses in the magnetic ballast are usually greater than the lossesin electronic ballast, the magnetic ballast has a simple design which makes it cheaper. It operates thelamp at the line frequency [8],[15], which produces a flashing light emission and also produces anslight audible sound.

• The Hybrid ballast uses a magnetic core-and-coil transformer and an electronic switch to the electrode-heating circuit. As with the magnetic ballast, the hybrid ballast operates the lamp at the line frequency,but it has lower power losses [8].

12

1.3 resonant inverters

• The electronic ballast revolutionized the design and specifications of fluorescent lighting systems [14].The advances in solid state technology allow the replacement of the core-and-coil transformer withelectronic components [8]. As a consequence, electronic ballasts reduce power losses and operate atmuch higher frequency than hybrid and magnetic ballasts [4],[8]. This ballasts can be powered by ACor DC voltage, an important characteristic for the development of this work.

In Figure 10.a and Figure 10.b the general stages of the electronic ballast are shown, according to itssupply voltage. The most important part is the high frequency resonant inverter.

Figure 10.: Stages of Electronic Ballast Powered by: a) AC Volage b) DC Voltage

If the power supply is AC, the ballast is connected directly to the electric line and a rectifier, a power factorcorrection circuit and the control circuit along with the inverter are necessary. If the power supply is DC,only the control system and the frequency resonant inverter is needed.

1.2.2 Lamp-Ballast Starting Methods

In order to ignite fluorescent lamps, ballasts use one of the following methods [8]:

1. Preheat: The ballasts that use this method are all magnetics ballasts. Initially, ballast heats the lampelectrodes for several seconds at high temperature, then, a switch opens in order to apply a voltageacross the lamp and ignite it.

2. Instant-Start: This method starts the lamps with a high initial voltage and without delay. Instant-Startballast do not provide heating voltage to the electrodes either before or during operation, so that thisballast has lower losses t han the rapid start ballast. This method reduces lamp lifetime because startinga lamp without heating the electrodes accelerates degradation of the electrodes emissive coating.

3. Rapid-Start: This ballast extends lamp lifetime while preventing lamps from flashing. They have aseparate set of windings in order to heat the electrodes. This reduces the necessary voltage to ignitethe lamp. While the lamp is heating, rapid-start ballasts apply the ignition voltage to start it.

4. Programmed-Start: Unlike rapid-start, programmed-start ballast preheat the lamp for a few secondsfirst, and then apply the ignition voltage. This was the starting method developed in this document.

1.3 resonant inverters

Inverters use a continuous electrical power supply to generate an alternating output waveform of a givenmagnitude and frequency (current or voltage). By operating a fluorescent lamp at high frequencies is possibleto obtain a higher luminous efficacy than at line frequency, as said before. To achieve the desired frequency,a resonant inverter must be used. This generates a high frequency sinusoidal output, produced with a squarewaveform filtered by the resonant tank.

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1.3 resonant inverters

To be able to select an appropriate resonant inverter for this application, some current-fed resonant invert-ers will be outlined, and a comparison between them will be presented.

1.3.1 Half Bridge Inverter

Figure 11.: Half Bridge Inverter

This topology is composed by a capacitive divider which provides a return path for the load current. Asthe transistors conduct alternately each half-cycle, the resonant load causes alternate polarity half sinewaveswith peak voltages of π/2 ∗ Vdc. The sum of these half sines produces a full sinusoidal wave, that is halveddue to the half bridge capacitors, so the resulting peak to peak voltage is π/2 ∗ Vdc.

The full bridge inverter has two switches instead the capacitive divider, therefore the voltage stress ishalved but the cost of four transistors does not compensate the reduction of the voltage rating.

1.3.2 Push-Pull Inverter

Figure 12.: Push Pull Inverter

As can be seen in the Figure 12, the current-fed push pull inverter has two switches referenced to ground,the switches are driven with 50% duty cycle alternatively, the inductor Lck provides a nearly constant currentsource [16] and the capacitor C across the transformer primary forms a parallel resonant load in combinationwith the primary winding inductance [16].

Figure 13 presents the waveforms of this inverter. In the center tap appears a full rectified sine wave witha theoretical peak amplitude of V p = π/2 ∗ VDC . By operating the inverter at the resonance frequency, thecommutation occurs when v1 and v2 passes through zero, ensuring the zero voltage switching and eliminat-ing the commutation losses.

Since each half-cycle the current flows in opposite direction through the two half windings, in the sec-ondary winding a sinewave is produced with a peak to peak amplitude of 2π ∗ VDC ∗

NsN p .

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1.3 resonant inverters

Figure 13.: Waveforms of Push Pull Inverter

1.3.3 Class E Inverter

Unlike the aforementioned inverters, the Class E inverter has a single switch which allows improving theballast efficiency. A capacitor parallel to the transistor is used to commute the switch at the instant in whichthe voltage is zero, so that the power losses produced by the overlapping of the current and voltage incommutation could be negligible (Zero Voltage Switching - ZVS) [17]. When this is achieved the circuitcould be operated at higher frequencies, with only conduction losses produced by the parasitic elements.The control circuit also takes some power, reducing the efficiency.

The class E inverter will be used to design the ballast in this, therefore the next chapter contains a thoroughexplanation of its design and working principle, but before, the argumentation of the selection of Class EInverter as the most appropriated topology for ballasts powered by batteries will be presented.

1.3.4 Comparison between Inverters

Table 2 presents a summary of the advantages and disadvantages of the resonant inverters previously ex-plained to easily compare them.

The selection of the inverter was based on the following characteristics:The current-fed resonant inverters allow the parallel operation of the lamps, in which the failure of one or

more lamps will not disable the remaining lamps [16].The driven requirements of the inverter can be simplified by reducing the number of switches and elimi-

nating the floating transistors. As can be seen in Table 2 the Half bridge inverter has two transistors and oneof them is floating, whereas the Class E just has one non-floating transistor, therefore it has the benefit ofreduced control and driver circuit complexity.

Although Half Bridge and Push Pull inverters have lower voltage stresses [18], the reduction of activedevices in Class E inverter, results in a considerably higher efficiency (Statistically 20% more efficient thanother classes of inverters [18]). Also, the Class E resonant inverter does not use a transformer, which de-creases the implementation cost.

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1.3 resonant inverters

Table 2.: Comparison between Push-Pull, Half Bridge and Class E Inverters [5]INVERTER ADVANTAGES DISADVANTAGES

Push-Pull• Works at high frequencies• All its switches are referenced

to ground• Low voltage stress

• Has two switches• Uses a transformer

Half Bridge• Works at high frequencies• Low voltage stress

• Has two switches• Has a floating switch• The output amplitude just can

be controlled by an input con-verter

• Uses a transformer

Clase E• Works at high frequencies• Has just one switch• Highest efficiency

• High voltage stress

That said, the Class E inverter was the chosen inverter to design the ballast; the next chapter contains athorough explanation of its design and working principle.

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2A NA LY S I S A N D D E S I G N O F A C L A S S E I N V E RT E R

The basic circuit of Class E resonant inverter is shown in figure 14. This converter has two stages when it isworking in optimum operation mode [19]:

Figure 14.: Class E Inverter

1. MOSFET (M) ON (ideal switch): The equivalent circuit is a LCR series. All the source energy isstored in Li, and does not flow through RL (the load) [19]. The sinusoidal output voltage depends onL1 y C1.

2. MOSFET (M) OFF: The voltage across CT increases from zero to a maximum value and decreases tozero again. In order to achieve a high efficiency, the inverter must have Zero Voltage Switching (ZVS),therefore vCT should be brought back to zero at time of transistor turns-on. The slope of vCT shouldbe zero at the same time, so that ideally there will be no switching losses. Throughout the secondoperation mode the current on CT will be sinusoidal.

Figure 15 shows the waveforms of Class E inverter operating in optimum mode. The current i , is thecurrent that flows through the switch and capacitor CT , it has a DC component due to Li and a sinusoidalcomponent due to the resonant tank.

The sub-optimum operation mode, is presented when the voltage across CT decreases to zero with a finitenegative slope, therefore the internal diode of the MOSFET conducts [19]. This operation mode will not beconsidered in this document.

2.1 steady-state analisys

In order to design Class E resonant Inverter, is necessary to analyse its steady-state behavior. For this analysisit is assumed that [17]:

17

2.1 steady-state analisys

Figure 15.: Waveforms in Class E In-verter in optimum mode

Figure 16.: Detailed waveforms

• The inductance Li has a reactance sufficiently high that the current flowing through it may be regardedas constant (dc input current), and it has no series resistance.

• The Quality Factor (Q) of the resonant tank should be high enough to keep the output current andoutput voltage as a sinusoid at the switching frequency.

• MOSFET acts as an ideal switch, lossless, and the action of the transistor is instantaneous.

2.1.1 Basic Relationships

Figure 16 shows in more detail some of the waveforms of the inverter so that the analysis can be clearlypresented. y is an angle equivalent to the duty cycle and ϕ is defined as the phase angle between the outputvoltage (V0) and the gate-source voltage in the MOSFET ( VGS ). According to this, the output equations ofthe inverter are:

v0 = Vout sin(θ+ ϕ)

i0 =Vout

RLsin(θ+ ϕ) (6)

Where, i0 is the output current, Vout is the amplitude of the output voltage and RL is the load resistance.To begin the analysis, a series reactance jX produced by the difference in the reactances of the inductor

and capacitor of the series tank (X = ωL1 −1

ωC1) is considered [17]; the voltage v1(θ) is defined as the sum

between the output voltage and the voltage across this series reactance ( vx):

18

2.1 steady-state analisys

v1(θ) = v0 + vx

v1(θ) = Vout sin(θ+ ϕ) + L1dI0

dt+

1C1

∫I0dt (7)

Replacing and simplifying:

v1(θ) = Vo1 sin(θ+ ϕ1) (8)

Where,

Vo1 =

Vout

√1 +

( XRL

)2 (9)

ϕ1 = ϕ+ tan( XRL

)= ϕ+ ψ (10)

Now, the voltage across CT is:

vCT =1

CT

∫iCT dt =

1ωCT

∫iCT (θ)dθ =

1ωCT

∫ θ

θ0

IDC − i0(u)du

vCT =1

ωCT

∫ θ

π/2−yIDC −

Vout

RLsin(u + ϕ)du

vCT =1

ωCT

[IDC

(θ −

π

2+ y

)+

Vout

RLcos(θ+ ϕ) − sin(y − ϕ)

](11)

Since the resonant tank has zero resistance at the fundamental frequency, there is no voltage across it. Thismeans that, in the fundamental frequency, the voltage across CT should be the same that the voltage v1. Themagnitude (Vo1) of the fundamental component of the voltage across CT can be calculated through Fourieranalysis [17]. Once the equation of Vo1 is found, is possible to determine the value of Vout through equation(9):

Vout =IDCRL(2y cosϕ1 cos y − 2 sin y cosϕ1 − 2y sinϕ1 + sin y)

2 sin(y − ϕ) sinϕ1 sin y + 12 [− sin(2ϕ+ ψ) sin(2y) + 2y cosψ]

Vout = IDCRLg(ϕ,ψ, y) (12)

Where, g is a function of y (Figure 16), ϕ1 and ψ (defined in equation (10)) that allows simplifying somecalculations.

2.1.2 Power and Efficiency

In order to continue the study of the inverter, is necessary to realize the power analysis. To find the efficiency

of the circuit, the resistance that the inverter shows to the power supply should be determined(

RDC = VDCIDC

).

Firstly, as there is no dc voltage drop across the inductance Li, the relation between the voltage VDC and therest of the parameters is:

19

2.1 steady-state analisys

VDC =1

2π

∫ 2π

0vCT dθ

VDC =IDC

2πωCT

∫ π2+y

π2−y

([y −

π

2+ g sin(ϕ − y)

]+ θ+ g cos(θ+ ϕ)

)dθ

VDC =IDC

2πωCT([2y2 + 2yg sin(ϕ − y)] − 2g sinϕ sin y) (13)

Then, dividing by the input current IDC , the resistance RDC can be expressed as:

RDC =[2y2 + 2yg sin(ϕ − y)] − 2g sinϕ sin y

2πwCT(14)

Relating the output and input power is possible to obtain the efficiency of the inverter

Pin =V2

DC

RDC(15)

Pout =12

V2out

RL=

I2DCg2RL

2= V2

DCg2RL2R2DC (16)

η =g2RL

2RDC(17)

A high efficiency operation is possible if the circuit parameters are chosen to drop the voltage drain tozero at the instant in which the transistor turns on, that can be calculated setting equation (11) equal to zeroat ( θ = π/2 + y) [17],

0 = 2y − 2g cosϕ sin y

g =y

cosϕ sin y(18)

ξ represents the normalized slope of the voltage across CT at the time of turn-on:

ξ =1

VDC

dvCT

dθ

∣∣∣∣∣θ= π

2+y(19)

ξ =1

RDCωCT[1 − g cos(y + ϕ)] (20)

Replacing equation (19) into equation (14), followed by an expansion of the trigonometric functions ispossible to determine an expression for ϕ :

tanϕ =

sin yy − cos y

ξyπ cos y − (1 + ξ

π ) sin y(21)

2.1.3 Final Considerations

In [17] some performance curves with variations in the slope of the voltage VCT ( ξ) and variations in theduty cycle (y) are shown. According to that curves, the determined values were: ξ = 0 and y = π

2 . The first

20

2.2 resonant tank

condition does not allow negative voltages over CT (Optimum mode), and the second one (50 % duty cycle)produces the peak power output.

According to these values, and assuming a 100% efficiency is possible to find the parameters of the circuit.From equation (21):

tanϕ =

sin π2

π2− cos π

2

− sin π2

=−2π

→ ϕ = 32, 4816 = −0, 5669rad (22)

With the calculated ϕ value and the equation (18):

g =π2

cos(−0, 56) sin π2→ g = 1, 8621 (23)

Considering an ideal efficiency in equations (17) and (23):

1 =g2RL

2RDC→ RDC =

g2RL

2= 1, 7337RL (24)

12

V2out

RL=

V2DCg2RL

2R2DC

→ Vout =VDCgRL

1, 7337RL=

VDCgRL

1, 7337RL=

1, 8621VDC

1, 7337= 1, 074VDC (25)

The final parameter to determine is X, from the equation (10) is known that X and ψ are related. In [17]a detailed explanation of the mathematical procedure to find ψ is presented.

ψ = arctan[π

8

(π2

2− 2

)]→ ψ = 49, 052 = 0, 85613rad (26)

X = RL tan(ψ)

→ X = 1, 1525RL (27)

2.2 resonant tank

In order to accomplish all the ballast requirements, mentioned in chapter 1, this section presents the selectionof the appropriate resonant tank.

Although the series resonant tank of the basic Class E inverter (Figure 17.a) can limit the output currentdue to its impedance in series with the lamp (represented by RL), it can not reach the ignition voltage, sinceat the preheating stage the lamp resistance is considered to be extremely high so the load of the resonantcircuit is represented by an open circuit, therefore there is no current flowing through the tank.

On the other hand, using the tank of the Figure 17.b the capacitor Cr2 has no effect when the lamp ( Rlamp)behaves as an open circuit, whereas Cr1 forms the series resonant circuit with L1. There is no resistanceto damp resonance in this circuit, other than the ESR of these components, and the MOSFET that excites

21

2.3 final design

Figure 17.: Resonant Tanks: a) Series Tank LC, b) Parallel Tank LCC

them. So this circuit in the ignition stage has a very high Q, limited only by parasitic elements, which allowsreaching the elevated ignition voltage.

The equations relating the two resonant tanks are given by [20]:

RL =RlampXCR1

2

Rlamp2 + (XCR1 + XCR2)

2 (28)

XC1 =XCR1[Rlamp

2 + XCR2(XCR1 + XCR2)]

Rlamp2 + (XCR1 + XCR2)

2 (29)

Once the resonant tank has been chosen, and taking under consideration the equations governing the basicClass E inverter ((6)-(26)) it is possible to design the ballast, but there is a last restriction to consider: Somesolutions of equations (28) and (29) can result in complex numbers. In order to avoid this, the circuit mustmeet the following inequality [20]:

(1 −

RL

Rlamp

)(1 +

R2L

(QRL − X)2

)< 1 (30)

2.3 final design

Based on the equations and constraints presented in this document, a Class E resonant inverter for a 32WT8fluorescent lamp was designed by a calculative spreadsheet. The input voltage, desired efficiency, workingfrequency and load resistance are specifications of the system that were used as the input parameters todesign the inverter.

The input voltage was set to 12 V, the chosen efficiency was 90%, the selected frequency was 130 kHzin order to increase the luminous efficacy [21] and the lamp (load) was replaced by its equivalent resistance,

according to [22] the nominal voltage of that lamp is 135 Vrms, thus RLamp =V2

lampPlamp

= 580Ω.In order to calculate the components, the equations and analysis of the inverter with the series resonant

tank were used at first, then the equations (28), (29), (30) allow the calculation of the equivalent values inthe selected tank.

By manipulating equations (16) and (17) is possible to find Vout in terms of known parameters. As theresistance of the series resonant tank RL is not the same RLamp (Figure 17).

Vout =VDCgRL

g2RL2η

=2VDCη

g(31)

With (16) and Vout, RL can be found. Once it has been obtained RDC is determined with equation (17).

22

2.3 final design

RL =V2

out

2Pout(32)

RDC =g2RL

2η(33)

The capacitor CT is calculated according to equation (14), where the value of ϕ was given by equation(22):

CT =[2y2 + 2yg sin(ϕ − y)] − 2g sinϕ sin y

2πwRDC(34)

The final components calculations were made taking under consideration the minimum allowed Q (equa-tion (30)). From the definition of quality factor in a series resonant tank Q = wL1

RL, so L1 can be solved.

L1 =QRL

ω(35)

As X = ωL1 −1

ωC1= 1, 1525RL, is possible to find C1 and finally solve the equations (28) and (29) to

obtain CR1 and CR2.The spreadsheet was used to select the components as close as possible to standard values. Figure 18

shows the designed Class E resonant inverter.

Figure 18.: Final Design Class E Inverter

In order to assemble the circuit the following considerations were taken:

1. The MOSFET should be able to withstand voltages greater than the peak voltage over CT . In [17] thedevice stress is determined by finding the time at which that peak occurs ( θmax) through:

vCT =1

ωCT

Ic

(θ −

π

2+ y

)+

Vout

RL(cos(θ+ ϕ) − sin(y − ϕ))

(36)

θmax = arcsin(1g

)− ϕ (37)

Replacing equation (37) in (36) it is possible to find stress voltage in the MOSFET.

2. Li should be high enough to keep IC constant with a negligible superimposed ripple current.

3. The large sinusoidal current flowing through L1, along with the high switching frequency producesskin effect and a small skin depth that can be mitigated by using Litz wire.

4. In order to avoid major losses in the circuit, all the components were selected with their ESR as lowas possible.

23

3

C O N T RO L C I R C U I T

3.1 control circuit design and analysis

The input impedance of the resonant circuit is a function of the switching frequency. When the switchingfrequency varies, the energy transferred to the output will also change. Thus, the output power can becontrolled by changing the switching frequency.

Figure 19.: Lamp Voltage in different resonant states [23]

The control circuit regulates the control variable and assures that all the operation phases are completed.The first stage is the preheating, although it is not indispensable, it is desirable to include this phase in orderto increase the lamp lifetime [23]. In this work, preheating phase is performed at a frequency above theresonance, as shown in Figure 19.

Once the filaments have been preheated, the lamp is prepared to ignite the discharge. The output voltageis increased by lowering the switching frequency until the lamp resistance breaks down abruptly and theresonant circuit shows strong damping.

3.1.1 Block Diagram

In order to achieve the aforementioned behavior, the control circuit shown in Figure 20 is proposed. Inthis figure the two possibles variables of control are clearly state: the light intensity or lamp current. Eachvariable passes through a block to obtain a dc voltage equivalent to its measure. The selection between thetwo control variables is manually performed.

24

3.1 control circuit design and analysis

Figure 20.: Control Block Diagram

When the circuit is turned on, the block "Switched control for operation phases" keeps in its output avoltage equal to Vre f , so that the voltage error is zero and the VCO (Voltage Controlled Oscillator) inputremains constant during the necessary time to heat the filaments.

Then, the input of the error amplifier switches to the output of the selected sensing circuit; as the lampis still off, the measurement is zero, and because of the controller, the frequency decreases until the outputvoltage is high enough to ignite the lamp. At that time the resistance of the lamp changes, and the controlreaches the nominal frequency.

The description of each block will be presented below, and the complete schematic can be seen in theAppendix A.

3.1.1.1 Sensing Circuits

The current sensing circuit, obtains a voltage equivalent to the output current through a resistance in serieswith the lamp (Rsense). This circuit has two stages, the first stage inverts the input signal and applies a gain,then in the second stage a half wave rectifier in series with a low pass filter gets an equivalent value of thepeak current in the lamp. Figure 21 shows the topology used.

Figure 21.: Topology Current Sensing Circuit

In order to sense the light intensity, the light sense circuit uses the integrated circuit OPT101 [24]. It is amonolithic photodiode with on-chip transimpedance amplifier, the output of the amplifier increases linearlywith light intensity, so the gain of the amplifier is adjusted to accomplish the desired luminosity (Figure 22).

25

3.1 control circuit design and analysis

Figure 22.: Topology Light Sensing Circuit

3.1.1.2 Switched Control for Operation Phases

In order to commute the circuit between the preheating stage and free-running stage, the multiplexer CD4052Bwas used. One of the two binary control inputs of the integrated circuit commutes the output of the "SwitchedControl for Operation Phases Block" between a reference voltage (equal to the reference of the error ampli-fier) and the output of the respective sensing circuit.

The state of the control input was realized through a timer which controls the preheating time.

3.1.1.3 Controller

The error amplifier along with the PI controller were designed to be able to ignite the lamp and then regulatethe variable of control. The principal limitation to design this stage was the closed loop gain, since when thecontrol is released (immediately after the preheating stage) the lamp is off, therefore the error signal will behigh and the response of the controller can be so fast that the lamp fails to ignite.

As the limitation in the response of the controller is an unknown parameter, the controller was manuallyadjusted to guaranty the ignition of the lamp.

3.1.1.4 Voltage Controlled Oscilator (VCO) and MOSFET Driver

Considering the high operating frequency of the ballast (130kHz), the IC CD4046 (Phase Locked Loop -PLL) was selected to generate the switching signal. Apart from reaching the operating frequency, the VCOof the PLL allows the programming the minimum and maximum working frequencies. This programming isdone by selecting the values and relationships of two resistors and a capacitor.

As can be seen in Figure 19 all the operation phases are performed at higher frequencies than the resonance.If the control circuit tries to drive the lamp below the resonance, the control loop will have a positive feedbackand therefore it can not be controlled. To prevent this, the VCO was adjusted with a frequency offset equalto the resonance of the inverter and a maximum frequency equal to the preheating frequency.

Finally to complete the explanation of the block diagram , the chosen MOSFET Driver was the IR2110.As it requires a certain voltage amplitude in its input (10 - 20V) a comparator was used between the VCOoutput and the Driver.

26

4

S I M U L AT I O N S A N D E X P E R I M E N TA L R E S U LT S

4.1 simulations results

Due to the replacement of the lamp by its equivalent resistance (580Ω), the output voltage and the outputcurrent (Figure 23) are completely sinusoidal. Instead of the equivalent resistance model, the electricalbehavior of the lamp can be modeled more precisely as a non linear function of the power, as was presentedin the theoretical framework; therefore in the experimental results the lamp signals will show some distortion.

Figure 23.: Simulations Results of the Voltage and Current on the lamp equivalent resistance.

As can be seen in Figure 24, the ZVS operation was achieved since the waveform of vC1 drops to zero atthe instant the transistor turns-on ( vGS ).

Figure 24.: Simulations Results of the Voltage across the MOSFETvC1 and the Switching Signal vGS

27

4.2 experimental results

Figure 25 shows the frequency response of the inverter during the preheating stage (blue curve) and inthe steady-state stage (red curve), according with the simulation, the resonance frequency occurs at 130 kHz,which corresponds to the switching frequency chosen for the inverter. In this figure is possible to see thedifference between the quality factor of the two stages in order to ignite the lamp with a high enough voltagein the preheating.

Figure 25.: Simulation of the frequency response of the inverter in the preheating (BLUE) and steady-state(RED).

4.2 experimental results

Before the final assemble of the ballast in the printed circuit board (PCB), a prototype was implemented. Inthis section will be shown the experimental results of both circuits.

4.2.1 Prototype Results

As a result of the series resistance and tolerance of the components, the Zero Voltage Switching was notachieved with the theoretical values calculated. It is possible to adjust the network load for nominal ClassE operation (ZVS) by observing the vCT waveform [25]. Figure 26 illustrates the effect of adjusting thecomponents and Table 3 presents the final values.

Figure 26.: Effects in VCT waveform by adjusting the component values[25].

After the tuning procedure, an efficiency of 87% (without the control circuit) was achieved.As seen in Figure 27, with the aforementioned adjustments, the inverter operates with ZVS when it has a

12V input and a 32W output power.

28

4.2 experimental results

Table 3.: Final Components Values

Component Value

Li 800 µ H

L1 65 µ H

C1 139 n F

Cr1 22 n F

Cr2 2.1 n F

Figure 27.: Voltage on CT and Vgs. Scale : 1.2 µsdiv ; 10 V

div

Figure 28 shows the output waveforms and the power over the lamp. In steady-state the ballast presents apeak voltage of 198 V and peak current of 340 mA consistent with the nominal power specifications [22].

Figure 28.: Voltage, Current and Power in the lamp. Scale 2 µsdiv ; 164 V

div and 320 mAdiv

The losses distribution among the components is presented in Table 4. The total sum of losses in thecircuit corresponds to the efficiency of the ballast mentioned before. In Appendix C it is possible to find thevoltage and current waveforms along with the dissipated power of the components showed in this table.

In figure 29 the operation phases of the control are shown 1. During the preheating time (approximately2 seconds) the inverter maintains constant the output voltage, after that, the control begins the regulation ofthe chosen variable (current or light intensity).

1 For the prototype, the control was assembled only with the current sense circuit.

29

4.2 experimental results

Table 4.: Distribution lossesComponent Dissipated Power (W) Dissipated Power (%)

L1 0.82 2.27Li 0.2 0.55CT 0.3 0.83Cr1 1.1 3.05Cr2 0.2 0.55

Rsense 0.021 0.058Control 0.8 2.21Mosfet 1.6 4.43

TOTAL 5.05 14

Figure 29.: Operation phases (Voltage and Current). Scale .2 sdiv ; and 340 V

div and500 mAdiv

Finally, the circuit measurements were taken varying the supply voltage in open and close loop, tables 5and 6 present the obtained results. According to table 5 the control regulation is under ± 1% .

Table 5.: Output power of the circuit in open loopVin(V) Iout(Apk) Pout(W) ∆Pout(%)

10.5 0.277 27.7 -13.511 0.297 29.3 -8.43

11.5 0.310 30.58 -4.4412 0.330 32 013 0.362 34.32 +7.25

13.8 0.378 35.8 +11.88

Table 6.: Output power with current controlVin(V) Iin(A) Pout(W) η ∆Pout(%)

10.5 3.67 32.1 0.83 +0.3111 3.45 32.12 0.85 +0.38

11.5 3.45 31.8 0.85 -0.6312 3.09 31.9 0.86 -0.3113 2.86 32 0.859 0

13.8 2.73 32.3 0.857 +0.94

4.2.2 PCB Results

Once all the adjustments and measurements of the prototype were performed, it was possible to designthe printed circuit board (Appendix D). Although the components in the two circuits were the same, thefirst evaluation of the PCB showed strong differences with the prototype in the efficiency and ZVS. Thedifferences between the circuits may be justified by the resistance of the traces and the connectors used bythe lamp and the inductor L1.

30

4.2 experimental results

In order to obtain commutation at zero voltage and improve the efficiency, the tuning procedure wasrealized once again. As a result of the components adjustment, when the ZVS was achieved the efficiencywas not the expected (80%), however the efficiency was improved to 84% without ZVS.

Figure 30.: Voltage on CT and Vgs without ZVS Scale 2 µsdiv ; 10 V

div and5 Vdiv

Figure 31.: Voltage on CT and Vgs with ZVS. Scale 2 µsdiv ; 10 V

div and5 Vdiv

Figure 30 shows the vCT and vgs curves with the highest efficiency obtained and Figure 31 shows thesame waveforms when the ballast was tuned in order to obtain the ZVS. These results may seem inconsistentwith the high efficiency analysis of the class E since the commutation losses were increased, but as thehighest efficiency is obtained by minimizing the total power dissipated [25], in this case the increment in thecommutation losses allows having a reduction in the dissipation of other components in larger amounts.

Figure 32.: Stabilization time of the lamp resistance

Due to the intrinsic parameters of the lamp, once the lamp have been ignited, it takes some time to stabilizethe electric discharge and keep its equivalent resistance constant. Figure 32 shows this behavior with two

31

4.2 experimental results

different lamps, as can be seen the settling time is about 10 minutes. With this graphic is also concluded that,the settling time is not depending on the hours of use of the lamp, but the final power in the older lamp islower than in the new one.

4.2.3 Comparison between Sensing Circuits

To be able to select the best condition for operate the circuit (High efficiency or ZVS), the measurements ofthe control regulation varying the supply voltage with both sensing circuits were taken (Tables 7 - 10).

Vin Iin(A) Pout(W) Iout(mA) η

10.7 3.79 32.47 356.8 0.80

11 3.65 32.33 355.3 0.80

11.5 3.47 32.31 355.5 0.80

12 3.31 32.31 355.5 0.81

12.5 3.18 32.31 355.8 0.81

13 3.06 32.35 355.8 0.81

13.5 2.49 32.37 355.8 0.81

Table 7.: Current Regulation (With ZVS)

Vin Iin(A) Pout(W) Iout(mA) η Lum

10.5 3.88 31.94 350.5 0.78 9030

11 3.65 31.94 350.5 0.79 9050

11.5 3.47 31.92 350.8 0.79 9050

12 3.31 31.94 350.5 0.80 9070

12.5 3.17 31.94 350.3 0.80 9060

13 3.05 31.92 349.8 0.80 9060

13.5 2.94 31.96 349.5 0.80 9070

Table 8.: Light Intensity Regulation (With ZVS)

Whereas in the ZVS operation (Tables 7 and 8) both circuits can regulate all the supply voltage rangewithout exposing to majors power dissipation the components, when the circuit is not operating with ZVS(Tables 9 and 10) neither the current control nor the light control regulate at low voltages. The lack ofregulation is caused by the abruptly decreasing of the efficiency due to the worsening of the ZVS at thosevoltages. According to these reasons the ZVS operation was chosen to realize the measurements and analysisof the PCB ballast.

Vin Iin(A) Pout(W) Iout(mA) η

10.5 3.71 27.29 293.2 0.70

11 3.74 31.01 339.5 0.75

11.5 3.44 32.14 353.8 0.81

12 3.23 32.14 354.8 0.83

12.5 3.07 32.12 354.8 0.83

13 2.94 32.1 354.8 0.83

13.5 2.83 32.1 354.5 0.83

Table 9.: Current Regulation (High efficiency)

Vin Iin(A) Pout(W) Iout(mA) η Lum

10.5 3.76 28.33 304.5 0.71 8200

11 3.79 30.6 320 0.72 8750

11.5 3.42 32 352.3 0.81 9100

12 3.20 32 352.5 0.83 9100

12.5 3.05 32 352.8 0.83 9110

13 32.29 31.98 353 0.84 9110

13.5 2.81 32 353 0.84 9110

Table 10.: Light Intensity Regulation (High efficiency)

From Tables 7 and 8 it can be concluded that the regulation of each control variable is appropriatelyperformed (±1%) 2. The table 8 also shows the directly relation between the light intensity and the outputpower, therefore, if one of the control variables is regulated, the other one is also regulated.

Figure 33 shows how the performance of the current control is limited by the settling time, and only afterthat time the power remains constant.

2 This measurements were taken after the 10 minutes needed for the lamp stabilization.

32

4.2 experimental results

Figure 33.: Current sensing circuit performance

On the other hand, when the control is working with the light sense circuit, the variation of the resistancein the lamp does not affect the light regulation, as can be seen in Figure 34 the control takes less than fourminutes in regulate the light intensity, while the output power is still taking longer than 10 minutes to beregulated.

Figure 34.: Light sensing circuit performance

4.2.4 Comparison with others implemented Ballasts

In this subsection will be presented the measurements and results of the instant-start electronic ballast (ILTECIT232I120EN) powered by the line voltage, in order to compare its performance with the ballast presentedin this document.

In Figure 35 the stabilization time of a commercial ballast is shown. As was expected, the settling time is10 minutes.

The output voltage and current in steady state along with the lamp power can be seen Figure 36 and theignition stage is shown in Figure 37

33

4.2 experimental results

Figure 35.: Stabilization Time of Commercial Ballast ILTEC

Figure 36.: Output waveforms of Commercial Ballast ILTEC

Figure 37.: Ignition Stage of Commercial Ballast ILTEC

The output power of the ballast was 27,83 W and its efficiency was 83%. Accordingly with this efficiencyand the results presented in [20] and [26], with a efficiency of 85% and 83% 3 respectively, can be concludedthat the designed ballast achieved an optimal efficiency.

3 The output power of the ballast in [20] is 25 W and in [26] is 18 W

34

5

C O N C L U S I O N S

• From the simulated and experimental results it was concluded that, although the fluorescent lampmodels presented in [12] and [13] are very complete, a fixed resistance allows a faster simulation ofthe ballast without convergence errors and consistent with the experimentation.

As the ballast is regulated, neither in the power nor in the equivalent resistance of the lamp are changes,consequently, the negative resistance behavior of the lamp can be neglected, so the differences betweenthe simulations with the fixed resistance and the experimental results are given just by the non linearityof the lamp.

• During the Class E inverter design and through the tuning procedure, the circuit sensitivity was clearlyshown by the effects of the series resistances of the components. High resistances in the circuit apartfrom decreasing the efficiency, can affect the lamp ignition, since the quality factor of the resonanttank in the preheating stage may not be high enough.

• Usually, the highest efficiency in the inverter is achieved guarantying the ZVS. But when the seriesresistances are large enough, the ZVS should be infringed in order to minimize the total power dissi-pated. As the increase of the commutation voltage reduces the RMS current in the resonant tank, thedecrease in the i2R power losses can outweigh the increase of the commutation loss.

Whereas a high efficiency was accomplished in the prototype by reducing the losses in the converterthrough the ZVS (86%), ensuring the ZVS in the printed circuit, the efficiency was only 80%. In thiscase, was possible to increase the efficiency to 84%, loosing the ZVS 1.

• Although it is possible to obtain an efficiency of 84% (in the PCB) with an input voltage of 12V andwithout ZVS, this operation is not viable, since by varying the input voltage, the regulation can not beassured. When the input voltage decreases, the commutation voltage rises sharply and the efficiencydecays so far that it is impossible to regulate the expected output current with the designed resonanttank.

Additionally, exposing the components to so low efficiencies, produces high temperatures in the com-ponents and consequently a reduction on their lifetime.

• One of the tested physical characteristics of fluorescent lamps, is the time that the lamp takes to sta-bilize the electric discharge after the ignition (10 minutes approximately when the lamp is completelycold). During the stabilization, the equivalent resistance of the lamps is changing, therefore, no matterif the output current is being regulated, the lamp power will be changing. In consequence, the currentcontrol is limited by the settling time of the lamp.

• As fluorescent lamps gradually lose their initial performance as they age, the current control can notregulate every lamp at the same output power. Even if the lamps had the same time of use, the

1 The measures of the efficiency were taken with a 12V input voltage

35

conclusions

differences between them do not allow that by regulating the current, the lamps present the samepower and light intensity.

• The light intensity as the variable of control instead of the lamp current allows controlling the circuitwithout waiting for the stabilization of the lamp. Additionally, while current based control fails overtime as the lamp performance decays, the light based controller maintains its performance, by applyinga higher power to the lamp.

• Both sensing circuits execute all the phases of operation (preheating, ignition, free-running), and withvariations in the supply voltage, both present a good regulation (± 1%).

36

B I B L I O G R A P H Y

[1] Comisión Nacional para el uso eficiente de la energía. Proyecto nacional de eficiencia energéticaen alumbrado público municipal. http://www.conuee.gob.mx/wb/CONAE/alumbrado_publico/,2012. [Accesado 10-Septiembre-2012].

[2] L. Halonen, E. Tetri, and P. Bhusal. Guidebook on Energy Efficient Electric Lighting for Buildings.Aalto University School of Science and Technology, 2010.

[3] S. Thongkullaphat, P. Liutanakul, and V. Chunkag. Improvement of self-oscillating electronic ballastwith high power factor: A combination of charged-pump and valley-fill. Power Electronics and DriveSystems (PEDS), 2011 IEEE Ninth International Conference on, pages 1090–1093, Dec.

[4] S. Y R Hui, W. Yan, H. Chung, P.W. Tarn, and G. Ho. Energy efficiency comparison of dimmableelectromagnetic and electronic ballast systems. Industry Applications Conference, 2005. Fourtieth IASAnnual Meeting. Conference Record of the 2005, 4:2775–2781, Oct.

[5] Noé Márquez Avendaño. Diseño y construcción de un balastro electrónico alimentado con cd paraencender una lámpara fluorescente de 21 watts, 2005.

[6] Alma E. F. Taylor. Illumination Fundamentals. Lighting Research Center - Rensselaer, 2002.

[7] M. C Ndinechi, A. Oluwaseyi Ogungbenro, O. C Nwadiuko, and Igboebisi Ikechukwu. Reliability as-sessment of incandescent light bulbs in nigeria market and case for energy saving alternative. AcademicResearch International, 2, 2012.

[8] National Lighting Product Information Program. Specifier reports: Electronic ballast, 2000.

[9] E. Ferreira and E. Hammer. F40 fluorescent lamp considerations for operation at high frequency. Jour-nal of the Illuminating, 15:63–74, 1985.

[10] Allings W.R. Important design parameters for solid state ballast. Journal of the Illuminating, 25:203–207, 1989.

[11] M. C.and Cosby R. M. Nelms. A resonant inverter for electronic ballast applications”,. IEEE Transac-tions On Industrial Electronics, 41:418–425, 1994.

[12] B. Hesterman and N. Sun. Pspice high frequency dynamic fluorescent lamp model. IEEE APEC’96,2:641–647, 1996.

[13] Jin-Chyan Hung Wu Tsai and Te-Hung Yu. A pspice circuit model for low-pressure gaseous dischargelamps operating at high frequency. IEEE Transactions On Industrial Electronics, 44:428–431, 1997.

[14] Philips. The ABC’s of Electronic Fluorescent Ballasts. Philips Lighting Electronics N.A., 2009.

[15] Some basics facts and some advanced informationon ballast for fluorescent lamps. http:

//ecospecifier.com.au/media/7231/European%20Copper%20Institute%20-%20Ballasts%

20for%20Fluorescent%20Lights.pdf, 2006.

37

Bibliography

[16] Phillips Semiconductors. Fluorescent Lamp Control. Power Semiconductor Applications Philips Semi-conductors, 579-589.

[17] F. Raab. Idelized operation of the class e tuned power amplifier. IEEE Transactions on Circuits andSystems, 12:725–735, 1977.

[18] A. Ekbote and D.S. Zinger. Comparison of class e and half bridge inverters for use in electronicballasts. Industry Applications Conference, 2006. 41st IAS Annual Meeting. Conference Record of the2006 IEEE, 5:2198–2201, Oct.

[19] H.R Muhammad. Electrónica de potencia, circuitos, dispositvos y aplicaciones. Prentice hall, México,2003.

[20] M. Ponce, J. Arau, J.M. Alonso, and M. Rico-Secades. Electronic ballast based on class e amplifierwith a capacitive inverter and dimming for photovoltaic applications. IEEE APEC 98’, 2:1156–1162,1998.

[21] E.E. Hammer and Terry K. McGowan. Characteristics of various f40 fluorescent systems at 60 hz andhigh frequency. Industry Applications, IEEE Transactions on, IA-21(1):11–16, Jan.

[22] Sylvania fo 32wt8 865, datasheet. Havells Sylvania, 2010.

[23] Icb1fl02g smart ballast control ic for fluorescent lamp ballasts. Infineon Technologies AG, 2007.

[24] Burr-Brown Corporation. Monolithic photodiode and single-supply trasimpedance amplifier. OPT101Datasheet.

[25] N.O Sokal. Class e high-efficiency rf/microwave power amplifiers: Principles of operation, designprocedures, and experimental verification. IEEE Life Fellorw Design Automation, Inc, Lexington,M.A, 2007.

[26] V.G. Krizhanovski, D.V. Chernov, and M.K. Kazimierczuk. Low-voltage electronic ballast based onclass e oscillator. Power Electronics, IEEE Transactions on, 22(3):863–870, 2007.

38

AP S P I C E M O D E L S C O D E

non linear model

.SUBCKT ModeloNoLineal 1 10

*Parámetros Datos Experimentales

+ PARAMS: A1=10468, A2=-431.85, A3=21.641, A4=-0.30272

+ PARAMS: B1=700.7, B2=11.521, B3=-1.1311, B4=0.016615

Rsense 1 2 1; Current sensing resistor

EL 2 10 VALUE=V(1,2)*V(5,10)+V(6,10)*V(6,10)*V(6,10); Lamp Voltage

EP 3 10 VALUE=V(1,10)*V(1,2); Instantaneous lamp power

RP 3 4 100; Resistor time constant

CP 4 10 1uf IC=5V; Average lamp power

EA 5 10 VALUE=A1/LIMIT(V(4,10),4,50)+A2+V(4,10)*(A3+V(4,10)*A4); Calculates A

EB 6 10 VALUE=(B1/LIMIT(V(4,10),4,50)+B2+V(4,10)*(B3+V(4,10)*B4))*V(1,2); Calculates

B*I

Rgnd 10 0 1G; Provides a DC path to ground

.ENDS ModeloNoLineal

Figure 38.: Schematic Diagram of the non Linear Model

linear model

.SUBCKT ModeloLineal 1 10

*Parámetros Datos Experimentales

+ PARAMS: Rs=-190 VH=177.5

Rsense 1 2 1; Current sensing resistor

EL 2 10 VALUE=Rs+VH/(V(5,10))*V(1,2); Lamp Voltage

EI 3 10 VALUE=V(1,2)*V(1,2); Instantaneous lamp current squared

RI 3 4 100; Resistor time constant

39

pspice models code

CI 4 10 1uf IC=.1; Capacitor time contast

EE 5 10 VALUE=sqrt(V(4,10)); Calculates the RMS Lamp Current

Rgnd 10 0 1G;

.ENDS ModeloLineal

Figure 39.: Schematic Diagram of the Linear Model

40

BF U L L BA L L A S T S C H E M AT I C

Figure 40.: Schematic of the Electronic Ballast

41

CE X P E R I M E N TA L WAV E F O R M S O F T H E I N V E RT E R I N T H E P ROT OT Y P E

Figure 41.: Lamp waveforms.Scale 2 µs

div ; 164 Vdiv and320 mA

div

Figure 42.: vCT and vGS waveforms.Scale 1, 24 µs

div ; 160 Vdiv and10 V

div

Figure 43.: Capacitor CT waveforms.Scale 2 µs

div ; 20 Vdiv and320 mA

div

Figure 44.: Inductor Li waveforms.Scale 2 µs

div ; 10 Vdiv and1 A

div

Figure 45.: Inductor L1 waveforms.Scale 2 µs

div ; 20 Vdiv and5 A

div

Figure 46.: Inductor Li waveforms.Scale 2 µs

div ; 2 Vdiv and200 mA

div

42

DP R I N T E D C I R C U I T B OA R D

Figure 47.: PCB Design - Bottom Layer

Figure 48.: PCB Design - Top Layer

43

printed circuit board

Figure 49.: Picture of the Electronic Ballast

44