University of VictoriaFaculty of Engineering

ELEC 499B Report

The ZVT-PWM DC/DC Boost Converter

Submitted to: Dr. A. K. S. BhatDate: April 8, 2005

From Group #6 – Direct Current InnovationsJen MagdalenichStephen Spratt

Lauren Woolstencroft

In partial fulfillment of the requirements of the UVic. B.Eng. degree

Dr. A. K. S. BhatProfessorDepartment of Electrical and Computer EngineeringUniversity of VictoriaVictoria, British Columbia

April 8, 2005

Dear Dr. Bhat,

Please accept the accompanying report entitled “The ZVT-PWM DC/DC Boost Converter.”

This report is the result of work completed in the course ENGR 499 during the spring semester 2005. The ZVT-PWM DC/DC Boost Converter consists of a standard PWM boost converter, a snubber cell used to implement soft-switching, and a control circuit which generates the pulse trains, and implements a feedback loop. A snubber cell is used in combination with a conventional PWM boost converter to decrease switching losses thus increasing efficiency. The converters semiconductor devices are turned on and off under near zero voltage transition (ZVT) and / or zero current transition (ZCT). Because of this, there are no additional voltage and current stresses on the main switch and main diode. Additional benefits of this active snubber cell are its simple structure, relatively low cost, ease of control, and the stresses on the auxiliary components stay at allowable levels for operation.

Through the course of the term, much was learned seeing the project through from the initial design stages to completion. A website with descriptions and color photos was created in addition to the following report and can be viewed at: http://www.engr.uvic.ca/~lwoolste/index.htm. Testing and troubleshooting the circuit as well as gathering results proved helpful in deepening our understanding and knowledge of electronics. As a group, we feel this understanding and knowledge will be helpful in the future and in our careers. We would like to thank you, Dr. Bhat, for your supervision. We would also like to thank the technicians Lynn, Rob, and Paul for their support. Finally, we would like to thank Dr. Zielinski for his organization of ELEC 499B.

Best regards,

Jen MagdalenichStephen SprattLauren Woolstencroft

Table of Contents

List of Tables and Figures.............................................................................................................. iv

Abstract........................................................................................................................................... v

1.0 Introduction........................................................................................................................ 6

2.0 Project Goal and Specifications.........................................................................................6

3.0 ZVT PWM DC/DC Boost Converter Subsystems...............................................................63.1 Boost Converter.......................................................................................................6

3.1.1 Theory of Circuit Operation.............................................................................73.1.2 Design Calculations........................................................................................8

3.2 Snubber Cell...........................................................................................................103.2.1 Assumptions.................................................................................................103.2.2 Theory of Circuit Operation...........................................................................103.2.3 Design Calculations......................................................................................12

3.3 Control Circuit.........................................................................................................133.3.1 Design Calculations:.....................................................................................16

4.0 Experimental Results.......................................................................................................18

5.0 Costs................................................................................................................................ 21

6.0 Conclusion.......................................................................................................................22

7.0 Recommendations...........................................................................................................22

8.0 References.......................................................................................................................23

9.0 Appendix.......................................................................................................................... 249.1 Appendix A: Progress Report #1............................................................................249.2 Appendix B: Progress Report #2............................................................................269.3 Appendix C: Microcontroller Code..........................................................................329.4 Appendix D: Printed Circuit Board Layouts............................................................349.5 Appendix E: Data Sheets.......................................................................................36

List of Tables and Figures

Figure 1: Conventional PWM Boost Converter...............................................................................7Figure 2: Boost Converter Stage 1.................................................................................................7Figure 3: Boost Converter Stage 2.................................................................................................7Figure 4: PWM Boost Converter...................................................................................................10Figure 5: Conventional Boost Converter with Snubber Cell..........................................................11Figure 6: ZVT-PWM DC/DC Boost Converter...............................................................................13Figure 7: Control Circuit................................................................................................................14Figure 8: Microprocessor Flow Diagram.......................................................................................15Figure 9: Waveforms Representing Operational Stages of ZVT-PWM Boost Converter..............16Figure 10: Gating Pulses..............................................................................................................18Figure 11: Efficiency vs. Input Voltage for Various Resistive Loads.............................................19Figure 12: Waveforms with Resistive Load of 30..........................................................................20Figure 13: Waveform across Inductor Lb Vin=17.3V and Load = 30Ω...........................................20Figure 14: Waveform across Inductor Lb where Vin=22.8V and Load = 30Ω................................21Figure 15: Waveform across Inductor Lb where Vin=28V and Load = 30Ω...................................21

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AbstractThe project undertaken was to design and build a direct current (dc) to dc boost converter. More specifically, a zero-voltage-transition step-up dc-dc converter. With variable input voltages ranging from 18V dc to 30V dc, this converter has potential uses with fuel cell or solar panel applications.

For the design solution, the implementation of a ZVT-PWM DC/DC boost converter contains 3 subsystems – a conventional PWM boost converter, a snubber cell, and a control circuit. An Atmel microcontroller is used to send the gating signals to a driver which drives the Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET), allowing the converter’s output to be kept steady at 48V and 250 W through pulse width modulation, even with a fluctuating input voltage. A switching frequency of 50 kHz was achieved, and the method of soft switching implemented was zero voltage transition (ZVT). The use of a PWM boost converter allows for a variable input and constant output. The output is regulated by the control circuit which adjusts the duty cycle of the gating pulse to maintain a constant output.

With the energy markets moving towards more environmentally friendly energy sources, the applications for dc/dc boost converters is increasing. Some of the typical applications of this converter can be found in the auxiliary power supplies of hybrid vehicles. Fuel cell–powered electric vehicles (FCPEV) require an energy storage device to start up the fuel cells and to store the energy captured during regenerative braking. Low-voltage batteries are preferred as the storage device to maintain compatibility with the majority of today’s automobile loads. A dc/dc converter is therefore needed to interface the low-voltage batteries with the fuel cell powered higher voltage dc bus system, because the present fuel cell technology lacks energy storage capability.

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1.0 IntroductionPulse width modulated (PWM) DC/DC converters are widely used in a variety of applications due to their ease of control and modification, however their use in higher frequency applications are limited due to their the significant amount of noise interference and losses that occur. Because of this, soft-switching techniques have become popular to reduce these losses at higher frequencies.

This report documents a student project where the goal is to design and build a zero voltage transition, pulse width modulated DC/DC boost converter with a fixed output of 48VDC. The snubber cell used to implement the soft switching techniques is relatively new where the main transistor is switched under zero voltage and the auxiliary transistor is switched under zero current. The snubber cell is also relatively low cost.

By implementing this snubber cell, the increase in efficiency allows for use in applications such as fuel cell–powered electric vehicles. A dc/dc converter is needed to interface the low-voltage batteries with the fuel cell powered higher voltage dc bus system.

This report details the design method used to meet the project specifications. It also analyzes the functionality, efficiency, and cost of the implemented converter, and recommends changes for future implementations.

2.0 Project Goal and SpecificationsThe objective of this project is to design and build a Zero Voltage Transition, Pulse Width Modulated DC/DC boost converter in the laboratory with the following ideal specifications:

Input Voltage Range: 18-30VDC

Output Voltage: 48VDC

Output Power: 250W

Switching Frequency: 200kHz

3.0 ZVT PWM DC/DC Boost Converter SubsystemsThe proposed design solution of the ZVT PWM DC/DC boost converter consists of 3 main subsystems; the boost converter, the snubber cell, and the control circuit. The following sections detail the 3 subsystems in terms of the theory of their circuit operation, and the design calculations to meet the project specifications.

3.1 Boost ConverterA conventional PWM boost converter provides the framework for the ZVT PWM DC/DC boost converter and provides a controlled output of 48VDC with an input of 18-30VDC.

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3.1.1 Theory of Circuit OperationA conventional PWM boost converter is shown below in Figure 1.

Figure 1: Conventional PWM Boost ConverterA power MOSFET (M1) is used as the switching device. A rectangular pulse train sent from the microprocessor closes and opens the switch, and output voltage control is attained by varying the duty cycle of the pulse.

There are two stages of operation for the boost converter; stage 1: the switch is on, and stage 2: the switch is off. These two stages are shown below in Figures 2 and 3.

Figure 2: Boost Converter Stage 1

Figure 3: Boost Converter Stage 2When MOSFET M1 is turned on in stage 1, current flows from the input source Vin through inductor Lb and M1 and energy is stored in the inductor’s magnetic field. There is not current through Db at this stage and load current is supplied by the charge in Cor. During this charging interval, the voltage across the inductor Lb is Vin, and iL(t) is given by:

for (1)

Where D is the duty cycle and T is the switching period.

When MOSFET M1 is turned off in stage 2, inductor Lb opposes any drop in current by immediately reversing its electromotive force (EMF) so that the inductor’s voltage adds to the source voltage. The inductor voltage is now Vin-Vout and iL(t) is given by:

for (2)

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By solving equations (1) and (2) at t=DT and t=T respectively, and assuming , the duty cycle, or ratio between input and output voltage can be found as follows:

(3)

By assuming an ideal converter with no losses, the average input and output powers must be equal. In other words:

or .

Also, (4)

By solving equations (1) and (2) for minimum and maximum current values, the minimum inductor value that will keep the boost converter in continuous conduction mode (CCM) can be calculated using equation (4) as follows:

Setting , (5)

3.1.2 Design CalculationsAs discussed in section 2.0: Project Goal and Specifications, the ZVT-PWM DC/DC Boost converter has the following specifications:Vin = 18V – 30V DCVout = 48V DCfs = 200kHzP=250WGiven the input voltage range of 18 – 30 V, the duty cycle D can be calculated as follows:

, and

Using the max output power specification of 250W, the minimum resistive load can be calculated as follows:

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Initial calculations are done using a switching frequency of 100 kHz which leads to a period T=1/fs=1e-5s. The inductor Lb can be calculated using the following inequality:

for D=0.625

for D=0.375

Let which is 5 times the initial value of Lcrit.

An inductance of L=33.75uH can be realized given:

Where N represents the number of turns around the core. Using a core of material TDK PQ5050 PC44, and the fact that with 13 turns the inductance is measured in the laboratory to be 75.3uH, N can be calculated using the ratio:

Using an inductance value of , the maximum load can be calculated as follows:

In theory, the load should be no higher then to ensure the circuit remains in CCM, however in reality load should be kept at a lower value due to the potential of parasitic resistances present in the circuit.

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The resulting boost converter schematic is shown below in Figure 4.

Figure 4: PWM Boost ConverterThe components used for the boost converter, as shown in Figure 4, are as follows:

Power MOSFET: IRF640 (International Rectifier) Schottkey Diode Db: BR10100 Zener Diode Caps/Resistors – values as shown on circuit diagram Inductor – Wound with 9 turns using a core of material TDK PQ5050 PC44.

3.2 Snubber CellThe snubber cell is used in combination with the conventional PWM boost converter, discussed in section 3.1: Boost Converter, to decreases switching losses. The converter’s semiconductor devices are turned on and off under near zero voltage transition (ZVT) and / or zero current transition (ZCT). Because of this, there are no additional voltage and current stresses on the main switch and main diode. Additional benefits of this active snubber cell are its simple structure, relatively low cost, ease of control, and the stresses on the auxiliary components stay at allowable levels for operation. Furthermore, the converter behaves like a conventional PWM converter during the majority of the time because the time period the snubber cell is active is very short.

3.2.1 AssumptionsThe following assumptions are made to simplify the steady state analysis of the circuit during one switching cycle:

i) Input Voltage Vin is constant.ii) Output voltage Vo is constant or output capacitor Cor is large enough.iii) Input current Iin is constant or main inductor Lb is large enough.iv) Resonant circuits are ideal.v) Main inductor Lb is much larger than the snubber inductor Lr.vi) Voltage drops and parasitic capacitors of semiconductor devices are ignored.vii) Reverse recovery time of all diodes except the main diode Db is ignored.

3.2.2 Theory of Circuit OperationA simple snubber cell implemented with the conventional boost converter discussed above is shown below in Figure 5. The operational stages of the ZVT-PWM boost converter can be split up into six basic stages over one switching cycle as described below.

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Figure 5: Conventional Boost Converter with Snubber Cell

Stage 1This stage begins with MOSFETs M1 (main) and M2 (auxiliary) turned off. Db is on and is conducting current Iin of the main inductor Lb. The turn on signal is sent to transistor M2 from the microprocessor at which time Dr and M2 are turned on at near ZCS. The rate of rise of the current through Dr and M2 is limited by the snubber inductor Lr. As the current in M2 reaches Iin, Db current falls to zero, and thus Db turns off under ZVS.

Stage 2Diode Db and transistor M1 are now both in the off state. VCr=Vout. A parallel resonance between Lr and Cr begins to resonate through the path Cr–Dr–Lr–M2 under the input current Iin and with the initial current of ILr of the inductor Lr. When the transfer of energy stored in capacitor Cr to the inductor Lr is completed, and the current and energy values of the inductor Lr reach their maximum level at the same time. Now only transistor M2 is on and is conducting the maximum current of the inductor Lr. Also, the VCr=0.

Stage 3Transistor M1 receives a turn on signal and at the same time the turn on signal is removed from transistor M2. So, M1 turns on under ZVS and conducts current Iin and M2 is turns off under near ZVS through Cb. Serial resonance between Lr and Cb starts to resonate through Lr-Da-Cb-Dr under the maximum inductor current. Thus, throughout this stage the energy stored in the inductor Lr is transferred to capacitor Cb. As soon as the inductor current drops to zero, auxiliary diodes Dr and Da are turned off under ZCS through Lr, and Cb is charged to Vo.

Stage 4Transistor M1 continues to conduct input current Iin, and the snubber circuit is not active. The duration of this stage is the ‘on’ time of the MOSFET M1 as a traditional normally operating boost converter and is determined by PWM control.

Stage 5The gate signal of the main transistor M1 is removed and M1 turns off under ZVS. Auxiliary diode Dc turns on with ZVS because of capacitor Cb being charged to Vo. During this stage Cr is charged and Cb is discharged. When Cr voltage reaches Vo and Cb goes to zero simultaneously, diode Db is turned on with ZVS and the auxiliary diode Dc is turned off with ZVS. Thus, Cb restricts the rise rate of transistor M1 voltage and M1 is turned off under near ZVS.

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Stage 6

Main diode Db continues conducting the input current Iin and the snubber circuit is not active. This stages duration is the ‘off’ time of the transistor M1 as in a conventional PWM boost converter. At the end of stage 6, stage 1 would again begin starting another switching cycle.

3.2.3 Design CalculationsThe value of Lr can be calculated using the following equation:

Where represents the reverse recovery time of the main diode Db. Db is a

schottkey diode BR10100 with a very fast recovery time of 5ns.

Let

The value of Cb can be calculated using the following equation:

Where represents the fall time of the auxiliary MOSFET M2.

Let

The value of Cr can be calculated using the following equation:

Where represents the fall time of the main MOSFET M1.

These values of Cr, Cb, and Lr also satisfy the following equation:

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A reference voltage of 2.5V required by the control circuit is provided through a voltage divider. R12 and R14 can be calculated as follows:

Let and .

The resulting ZVT-PWM Boost Converter Schematic is shown below in Figure 6.

Figure 6: ZVT-PWM DC/DC Boost ConverterThe components used for the Snubber Cell as shown in Figure 6 are as follows:

Power MOSFET IRF640 Schottkey Diodes: BR10100 Zener Diode Capacitors & Resistors (values as shown in Figure 6) Inductor L=75nH (realized with wire)

3.3 Control Circuit

The purpose of the control circuit is to provide the main and auxiliary MOSFETs with gating pulses. The main components of the control circuit are the microprocessor, the opto isolators and the drivers. The microprocessor used was the ATMEGA8.

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The schematic of the control circuit can be seen below in Figure 7.

Figure 7: Control Circuit

The microprocessor was programmed to output two gating pulses. Ideally, the goal was to attain a switching frequency of 200 kHz; however, the circuit was only tested effectively with a switching frequency of 50 kHz. Both pulses are sent at a frequency of 50kHz with an initial duty cycle of the main mosfet set to 50%. The first pulse is sent to the auxiliary MOSFET, with an on time of 200ns. The pulse to the main MOSFET is sent when the pulse to the auxiliary MOSFET ends.

A feedback loop is implemented to ensure a constant 48V is obtained at the output for any input between 18-30VDC. The microprocessor is programmed to automatically adjust the duty cycle according to a comparison made between a reference voltage of 2.5V and a feedback signal from a voltage divider at the output. This comparison makes use of the ATMEGA8 analog to digital converter. The voltage divider (shown in Figure 6 as R12 and R14) is designed such that at the desired output of 48V the feedback signal is 2.5V and no adjustment of the duty cycle is made. If the output voltage is greater than 48V the duty cycle is decreased until the desired output voltage is obtained. If the output voltage is less than 48V the duty cycle is increased until the desired output is obtained. The microprocessor continuously loops through this code adjusting the output voltage according to the variance in the input voltage. After each loop the reference is checked again to ensure the output voltage is maintained.

A 16MHz crystal was used as a clock source for the microprocessor. This crystal was chosen as it is the fastest that can be used with the ATMEGA8, and allows the sending of pulses to be done as quickly as possible. To calculate the number of cycles needed for each pulse, the execution time of each instruction was calculated and the corresponding numbers of instructions were used to make each pulse the appropriate length. The calculation of pulse lengths can be seen below in section 3.3.1 Design Calculations.

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The following flow diagram maps the events programmed to take place in the microprocessor:

Figure 8: Microprocessor Flow Diagram

Initialize microprocessor

Declare Variables

Set Initial Duty Cycle = 50%

Set Frequency to 50 kHz

Output Pulses

Read A to D Converter

Feedback = Desired Output

Keep duty cycle constant

Feedback>Desired Output

Decrease Duty Cycle

Increase Duty Cycle

Initialize Interrupts

Initialize Output and Input Ports

YES

NO

YES

NO

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3.3.1 Design Calculations:

Using the 6 operation stages (as described in 3.2.2: Theory of Circuit Operation), the following calculations were made to determine the required on and off times of the MOSFETs. These stages were chosen based on the waveforms in Figure 9. These calculations were based on the project specifications, with an ideal switching frequency of 200 kHz.

Figure 9: Waveforms Representing Operational Stages of ZVT-PWM Boost Converter1

1 A New ZVT-PWM DC–DC Converter Hacý Bodur, Member, IEEE, and A. Faruk Bakan. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 17, NO. 1, JANUARY 2002

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Stage 1: t0 < t < t2 The following equations are valid:

Using the values calculated in the design section, for this stage we find the time to be:

and

was found to be 5ns, obtained from data sheets. Therefore the time for stage 1 is

Stage 2: t2 < t < t3 The following equations are valid:

Where

The on time of the auxiliary transistor was calculated to be 42.37ns. This is the minimum time the auxiliary MOSFET needs to be on. To ensure the MOSFET was conducting for a long enough period of time, the auxiliary pulse was set at 200ns, as suggested by Dr. Bhat, project supervisor.

The on time for the main MOSFET was calculated using the operation frequency, 50 kHz, and a 50% duty cycle.

For a duty cycle of 50%, the on and off time must be equal. Therefore:

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This can be seen in the oscilloscope print out of the gating pulses below in Figure 10.

Figure 10: Gating Pulses

4.0 Experimental ResultsThe ZVT-PWM DC/DC Boost Converter was built in the laboratory and the actual product resulted in the following specifications:

Input Voltage Range: 18-30VDC

Output Voltage: 48VDC

Output Power: 250W

Switching Frequency: 51.9kHz

All of the specifications where achieved with exception of the switching frequency which was discussed in section 3.3: Control Circuit.

Efficiency, η, of the ZVT-PWM DC/DC Boost Converter can be determined as follows:

where Vout, Vin, Iout & Iin can all be measured in the laboratory.

Table 1 below details the Voltage and Current measurements taken in the laboratory at different resistive loads, and the resulting efficiency calculations:

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Table 1: Voltage and Current Measurements and Resulting Efficiency and Various Resistive Loads

Load (ohm) Vin (V) Iin (A) Vout (V) Iout (A) η (%)30 18.00 4.58 48.00 1.62 94.32  20.10 4.10 48.00 1.63 94.94  24.45 3.34 48.00 1.63 95.81  28.08 2.89 48.00 1.64 97.00

20 18.00 7.04 48.00 2.47 93.56  20.40 6.12 48.00 2.45 94.19  24.80 4.99 48.00 2.46 95.42  28.30 4.30 48.00 2.45 96.64

15 18.24 9.32 48.00 3.25 91.77  20.70 8.10 48.00 3.26 93.33  25.00 6.59 48.00 3.26 94.98  28.54 5.72 48.00 3.26 95.85

10 19.00 13.20 48.00 4.85 92.82  20.15 12.39 48.00 4.84 93.06  25.00 9.80 48.00 4.83 94.63  28.50 8.50 48.00 4.83 95.70        Average η: 94.63

Figure 11 below describes Table 1 in graph format.

Figure 11: Efficiency vs. Input Voltage for Various Resistive LoadsAs shown in Table 1 and Figure 11, as the resistive load decreases the efficiency decreases (and output power increases). It can also be seen that as the input voltage increases the efficiency also increases. Overall, the ZVT-PWM DC/DC Boost Converter resulted in an average efficiency of 94.63%. Figure 12 below shows waveforms resulting from the ZVT-PWM DC/DC Boost Converter with a resistive load addition of 30Ω. Channel 1 represents the main MOSFET (M1) between drain and source channels. Channel 2 represents the main diode Db. Channel 3 represents the pulse from

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the microprocessor to the gate of the main MOSFET (M1), and Channel 4 represents the pulse from the microprocessor to the gate of the auxiliary MOSFET (M2).

Figure 12: Waveforms with Resistive Load of 30As shown in Figure 12, once the pulse is sent from the microprocessor to the main MOSFET (M1), the main MOSFET is turned on under near Zero Voltage Switching (ZVS). Furthermore, once the main pulse is turned off, the main MOSFET turns off under near ZVS. At this point, the main diode (Db) turns on under near ZVS (Please note that while the waveforms of the main MOSFET and main diode may appear to follow each other, they are reverse given the location of the respective ‘ground’).

In Figure 13, 14 and 15 below, Channel 1 represents the voltage across the main inductor (Lb) and Channel 2 represents the pulse from the microprocessor to the main MOSFET (M1). As shown, the pulse train in Figure 13 has a 64.1% duty cycle, the pulse train in Figure 14 has a duty cycle of 52.5% and the pulse train in Figure 15 has a duty cycle of 41.9%.

Figure 13: Waveform across Inductor Lb Vin=17.3V and Load = 30Ω

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Figure 14: Waveform across Inductor Lb where Vin=22.8V and Load = 30Ω

Figure 15: Waveform across Inductor Lb where Vin=28V and Load = 30Ω

For all 3 Figures, the inductor Lb charges for the duration of the pulses ‘on’ time, and discharges into capacitor Cor for the duration of the pulses ‘off’ time.

5.0 CostsThe costs associated with the design and implementation of the ZVT-PWM DC/DC boost converter were relatively low given the simple design of both the conventional boost converter and snubber cell. The per unit costs are detailed below in Table 2:

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Table 2: Per Unit Costs for the ZVT-PWM DC/DC Boost Converter

Component Part Number Unit Price (CAN$) Quantity Total Cost (CAN$)FET Driver UC3710N 8.49 2 16.98Optoisolator HCPL 2601 2.19 2 4.38Power MOSFET IRF640 3.38 2 6.76Schottkey Diode BR10100 1.59 5 7.955V Regulator LM7805C 0.69 2 1.38Atmel Microprocessor ATMEGA8 4.78 1 4.7816 MHz Crystal Oscillator CA-301 16.0000M-C 1.25 1 1.25PCB Board   20.75 2 41.50Miscellaneous Parts Capacitors/Resistors etc.     5.00  Total Per Unit Cost: 89.98

6.0 ConclusionThe 3 subsystems of the ZVT-PWM DC/DC Boost Converter – the conventional PWM boost converter, the snubber cell, and the control circuit were constructed and tested in the laboratory. The ZVT PWM boost converter proposed for this project was tested at a switching frequency of 50 kHz. This switching frequency was lower than the project goal of 200 kHz due to the inability of the microprocessor used to generate fast enough gating pulses to the MOSFETs while implementing the feedback loop. Other factors include the introduction of noise in the circuit as a breadboard was used for part of the control circuit. Theoretical calculations were made and used to select components for each of the subsystems. First each subsystem was test individually and then the subsystems were implemented and tested together.

While the project goal included a switching frequency of 200kHz, through testing, the circuit was confirmed functional at 50kHz. The circuit was also tested with various resistive loads and the circuit was confirmed to be functional at a minimum of 50W, and a maximum of 250W. The overall efficiency of the ZVT-PWM DC/DC Boost converter was calculated though measurements taken in the laboratory to be 94.63%. With this high efficiency and relative low per unit cost of $89.98, this ZVT-PWM DC/DC boost converter is suitable for applications such as fuel-cell powered electric vehicles where a fixed out of 48VDC is required.

7.0 RecommendationsEven though the ZVT-PWM DC/DC converter was implemented and tested at a switching frequency of 50kHz, the following recommendations outline what could be done to achieve the desired switching frequency of 200kHz:

1. Use an Atmel, or other brand such a PIC microcontroller, with a clocking frequency capability higher than 16MHz to accommodate for a higher switching frequency.

2. Use a digital programmable logic device such as an FPGA which has capabilities to accommodate for a higher switching frequency.

3. Use a PCB for the entire control circuit to eliminate some noise inherent in using a breadboard

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8.0 ReferencesI. Batarseh, Power Electronic Circuits, USA: John Wiley & Sons, Inc., 2004.

II. Hacý Bodur, nd A. Faruk Bakan. A New ZVT-PWM DC–DC Converter. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 17, NO. 1, JANUARY 2002

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9.0 Appendix

9.1 Appendix A: Progress Report #1

Progress Report #1Group #6

The project being undertaken by Direct Current Innovations (DCI) is to design and build a zero-voltage-transition step-up dc-dc converter for fuel cell application. Soft switching techniques will be used to reduce the switching losses in the power converter.

DCI consists of 3 members: Jen Magdalenich Email: jetner@hotmail.comStephen Spratt Email: spratter@yahoo.comLauren Woolstencroft Email: laurenjw@uvic.ca

The fuel cell array voltage will be 24 V dc, Power output =250 W, Vout = 48 V dc or higher (if possible). The switching frequency will be 200kHz, and zero voltage transition (ZVT), the method of soft switching will be implemented to reduce switching losses. Preferred choice will be given to a micro-controller for controlling the boost circuit (MOSFET’s); however, should this be unfeasible an integrated circuit (IC) will be used.

DCI proposes to begin work on the converter by obtaining hardware for a hard switching boost circuit and programming the microcontroller to send correct duration impulse (gating) signals. The impulse signal will be fed to a ‘driver’, changing it as needed to drive the MOSFETs. The boost converter and microcontroller will then be tested together (and reconfigured if needed). Once hard switching boost conversion of the dc signal is obtained the soft switching circuit will be integrated giving a zero voltage transition.

DCI will break-out the tasks involved in this project as follows: Pulse signal and microcontroller – Jen PCB – to be obtained from Dr. Bhat Boost Converter & Boost Converter Simulation – Lauren ZVT circuit (Soft Switching) to be implemented in Boost Converter – Stephen Obtaining driver for MOSFET – Stephen Poster – Jen Presentation – Jen, Lauren & Stephen Website – Lauren Progress Reports – Stephen Final Report – to be sectionalized between Jen, Lauren & Stephen at a later date.

The timeline for this project by DCI is seen below:

Month Week ProgressJanuary 1 Research and Design Development

2 ↓ 3 ↓ 4 Hardware Acquisition and continued Design work

February 1 Design & Circuit Simulation Work 2 ↓ 3 Wiring, Waveform Acquisition, Troubleshooting

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4 ↓March 1 ↓

2 ↓ 3 Poster, Website, Report 4 ↓

April 1 Presentation & Report

The progress made to date has been contact with the supervisor of the project, Dr. AKS Bhat. Research on IEEE papers, namely that of “A New ZVT PWM Converter” from Jan 2002, and “High Efficiency Telecom Rectifier Using Soft Switching”, from IEEE Intelec Conference 1991. The microcontroller chip has been ‘reserved’ as Rob in the electronics lab wing has been contacted and the controllers are expected to arrive within the week, possibly early next week (January 31st).

Sincerely,

Direct Current Innovations (Group #6) Jen Magdalenich Stephen Spratt Lauren Woolstencroft

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9.2 Appendix B: Progress Report #2

Progress Report #2 - Group #6

To: Dr. A. K. S. BhatCc: Dr. A. Zielinski, Mr. Kiran Swaroop

Date: March 7, 2005

Group: Jen MagdalenichStephen SprattLauren Woolstencroft

The implementation of a Zero Voltage Transition (ZVT) Pulse Width Modulated (PWM) DC/DC Boost Converter is making progress with much of the design phase completed, and aspects of the testing and implementation in progress. This progress report divides the project into 4 main sections, and comments on the work completed, and any remaining tasks. The 4 sections are; the boost converter, the snubber cell, the control circuit, and a miscellaneous section including website design etc.

1. The Boost Converter

Prior to implementing the snubber cell, a boost converter was designed with the following specifications:Vin = 18V – 30V DCVout = 48V DCfs = 200kHzP=250W

The following design calculations were completed by Lauren.

Figure 1: Boost Converter

Given the input voltage range of 18 – 30 V, the duty cycle D can be calculated as follows:

, and

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The design problem specifies a switching frequency of 200kHz, however initial calculations are done using 100kHz. This leads to a period T=1/fs=1e-5. The design problem also specifies a max output power of 250W.

The inductor Lb can be calculated using the following inequality: (please note that in order for the circuit to always operate in continuous conduction mode (CCM)).

for D=0.625

for D=0.375

Let which is 5 times the initial value of Lcrit.

An inductance of L=33.75uH can be realized given:

Where N represents the number of turns around the core. Using a core of material TDK PQ5050 PC44, and the fact that with 13 turns the inductance is measured in the laboratory to be 75.3uH, N can be calculated using the ratio:

Using an inductance value of , the maximum load can be calculated as follows:

In theory, the load should be no higher then to ensure the circuit remains in CCM, however in reality load should be kept at a lower value due to the potential of parasitic resistances present in the circuit.

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All of the components required for the implementation of the boost converter have been obtained with the help of Dr. Bhat and the lab technicians. Jen, Stephen, and Lauren have all been involved in the acquisition of all of the components. Component information is as follows:

Power MOSFET: IRF640 (International Rectifier) Schottkey Diode Db: BR10100 Zener Diode Caps/Resistors – values as shown on circuit diagram Inductor – Wound with 9 turns using a core of material TDK PQ5050 PC44.

The majority of the components have been soldered to the PCB provided by Dr. Bhat by Jen and Stephen and full testing of the circuit is yet to be completed by Jen, Stephen and Lauren.

2. The Snubber Cell

The snubber cell is used to decreases switching losses. The following design calculations were completed by Lauren and Stephen.

Figure 2: ZVT Boost Converter

The value of Lr can be calculated using the following equation:

Where represents the reverse recovery time of the main diode Db. Db is a

schottkey diode with a very fast recovery time of 5ns.

Let

The value of Cb can be calculated using the following equation:

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Where represents the fall time of the auxiliary MOSFET M2.

Let

The value of Cr can be calculated using the following equation:

Where represents the fall time of the main MOSFET M1.

These values of Cr, Cb, and Lr also satisfy the following equation:

In order to get a reference feedback voltage of 2.5V, R12 and R14 can be calculated as follows:

Let and .

The components for the snubber cell are still in the process of being obtained by Jen, Stephen and Lauren. The following is a list of the components required for the snubber cell that have already been obtained.

Power MOSFET IRF640 Schottkey Diode: BR10100 Cr=4nF & Cb=7nF Resistors (values as per above)

The above components have been soldered to the auxiliary PCB by Jen and Stephen.

The following is a list of components yet to be obtained: Inductor L=75nH

As soon as the above component is realized, testing of the snubber cell will be completed by Jen, Stephen and Lauren.

3. The Control Circuit

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The control circuit uses an ATMEGA8 microprocessor to send pulse trains to both the main and auxiliary MOSFETs, and to adjust the duty cycle of the pulse train depending on the input voltage.

Please see appendix 1 of this report for a schematic of the Control Circuit.

The pulse trains to both the main and aux. MOSFETs have been programmed and tested with the use of the optoisolator and the fet driver. An external Crystal of 16MHz is used to ensure a fast switching frequency. This task was completed by Jen and Lauren.

The use of the ATMEGA8 A/D is required to read a reference voltage at the output and determine the change of pulse width required or a constant 48V output. A voltage divider as shown in Figure 2 is used to provide a reference voltage of 2.5V when the output is indeed 48V. This aspect of the control circuit is still in progress and is being completed by Jen.

All of the components required for the control circuit have been obtained by Jen, Stephen and Lauren. Below is a list of the components used in the control circuit:

Atmel ATMEGA8 microprocessor and development kit. Optoisolator HCPL 2601 FET Driver UC2710 5V Voltage Regulator LM7805C Resistors/capacitors – values as shown on schematic

All of the above components have been soldered to both the main and aux PCB’s by Jen and Stephen.

4. Miscellaneous

Website – Task in progress and being completed by LaurenPoster – to be completed by JenPresentation – April 1, 2005 – to be organized by Jen, Stephen and LaurenProgress Reports – Completed by Stephen, with contributions from Jen and LaurenFinal Report – to be sectionalized by Jen, Stephen and Lauren

Regards,

Direct Current Innovations (499 Group 6)

Jen MagdalenichStephen SprattLauren Woolstencroft

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Appendix 1 – Control Circuit Schematic

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9.3 Appendix C: Microcontroller Code.include "m8def.inc"

;variables.def temp = R16.def temp2 = R17.def outputH = R19.def outputL = R18.def SCR1 = R20.def SCR2 = R21.def OFFT = R22

.org 0x0000rjmp start

start: ;set port B to output ldi temp, 0xFF out DDRB, temp ldi temp, 0x00 ;out DDRC, temp out PORTB, temp

;define variables ldi outputL, 0x00 ldi outputH, 0x02 ldi SCR1, 0x03 ldi SCR2, 0x07 ldi OFFT, 0x09

;start a to dA2D: ldi temp, 0xC0

out ADCSR, temp

;starting pulsespulse1: ldi temp, 0x01 out PORTB, temp mov temp2, SCR1p1loop: dec temp2 brne p1loop rjmp pulse2

pulse2: ldi temp, 0x02 out PORTB, temp mov temp2, SCR2p2loop: dec temp2 brne p2loop rjmp off

off: ldi temp, 0x00

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out PORTB, temp mov temp2, OFFToffloop:dec temp2 brne offloop

rjmp pulse1

ldi temp2, ADCSRcpi temp2, 0x80brne pulse1rjmp OC1A

OC1A: in temp2, ADCHsubi temp2, 0x02breq nextrjmp compute1

next: in temp2, ADCLsubi temp2, 0x00breq finishedrjmp compute2

compute1: in temp2, ADCHsubi temp2, 0x02brlo increasedec SCR1rjmp finished

increase: inc SCR1rjmp finished

compute2: in temp2, ADCLsubi temp2, 0x00brlo increase2dec SCR1rjmp finished

increase2: inc SCR1rjmp finished

finished: rjmp pulse1

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9.4 Appendix D: Printed Circuit Board Layouts

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9.5 Appendix E: Data Sheets

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