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University of Victoria
Faculty of Engineering
ELEC 499B Report
The ZVT-PWM DC/DC Boost Converter
Submitted to: Dr. A. K. S. Bhat
Date: April 8, 2005
From Group #6 – Direct Current Innovations
Jen Magdalenich
Stephen Spratt
Lauren Woolstencroft
In partial fulfillment of the requirements of the UVic. B.Eng. degree
Dr. A. K. S. Bhat
Professor
Department of Electrical and Computer Engineering
University of Victoria
Victoria, 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 Magdalenich
Stephen Spratt
Lauren 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
4.0
ZVT PWM DC/DC Boost Converter Subsystems ............................................................... 6
3.1 Boost Converter ........................................................................................................ 6
3.1.1 Theory of Circuit Operation ............................................................................. 7
3.1.2 Design Calculations ........................................................................................ 8
3.2 Snubber Cell ........................................................................................................... 10
3.2.1 Assumptions.................................................................................................. 10
3.2.2 Theory of Circuit Operation ........................................................................... 10
3.2.3 Design Calculations ...................................................................................... 12
3.3 Control Circuit ......................................................................................................... 13
3.3.1 Design Calculations: ..................................................................................... 16
Experimental Results ........................................................................................................ 18
5.0
Costs ................................................................................................................................. 21
6.0
Conclusion ........................................................................................................................ 22
7.0
Recommendations ............................................................................................................ 22
8.0
References ....................................................................................................................... 23
9.0
Appendix ........................................................................................................................... 24
9.1 Appendix A: Progress Report #1 ............................................................................ 24
9.2 Appendix B: Progress Report #2 ............................................................................ 26
9.3 Appendix C: Microcontroller Code .......................................................................... 32
9.4 Appendix D: Printed Circuit Board Layouts ............................................................ 34
9.5 Appendix E: Data Sheets ........................................................................................ 36
List of Tables and Figures
Figure 1: Conventional PWM Boost Converter ............................................................................... 7
Figure 2: Boost Converter Stage 1 .................................................................................................. 7
Figure 3: Boost Converter Stage 2 .................................................................................................. 7
Figure 4: PWM Boost Converter.................................................................................................... 10
Figure 5: Conventional Boost Converter with Snubber Cell .......................................................... 11
Figure 6: ZVT-PWM DC/DC Boost Converter ............................................................................... 13
Figure 7: Control Circuit ................................................................................................................. 14
Figure 8: Microprocessor Flow Diagram ....................................................................................... 15
Figure 9: Waveforms Representing Operational Stages of ZVT-PWM Boost Converter .............. 16
Figure 10: Gating Pulses ............................................................................................................... 18
Figure 11: Efficiency vs. Input Voltage for Various Resistive Loads ............................................. 19
Figure 12: Waveforms with Resistive Load of 30 .......................................................................... 20
Figure 13: Waveform across Inductor Lb Vin=17.3V and Load = 30Ω .......................................... 20
Figure 14: Waveform across Inductor Lb where Vin=22.8V and Load = 30Ω................................ 21
Figure 15: Waveform across Inductor Lb where Vin=28V and Load = 30Ω ................................... 21
iv
Abstract
The 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-OxideSemiconductor 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.
v
1.0
Introduction
Pulse 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 Specifications
The 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:
3.0

Input Voltage Range: 18-30VDC

Output Voltage: 48VDC

Output Power: 250W

Switching Frequency: 200kHz
ZVT PWM DC/DC Boost Converter Subsystems
The 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 Converter
A 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.
6
3.1.1
Theory of Circuit Operation
A conventional PWM boost converter is shown below in Figure 1.
Figure 1: Conventional PWM Boost Converter
A 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 2
When 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:
iL (t ) 
1
Vin t  I L (0) for 0  t  DT
L
(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:
iL (t ) 
1
(Vin  Vout )(t  DT )  I L ( DT ) for DT  t  T
L
7
(2)
By solving equations (1) and (2) at t=DT and t=T respectively, and assuming
I L (T )  I L (0) , the
duty cycle, or ratio between input and output voltage can be found as follows:
iL (t  DT ) 
iL (t  T ) 
1
1
Vin DT  I L (0)  I L ( DT )  I L (0)  Vin DT
L
L
1
1
(Vin  Vout )(T  DT )  I L ( DT )  I L ( DT )  I L (0)  (Vin  Vout )(1  D)T
L
L
Vout
1

Vin 1  D
(3)
By assuming an ideal converter with no losses, the average input and output powers must be
equal. In other words:
Vout
I
1
 in 
.
Vin I out 1  D
I
I
V
 Iin (1  D)  L,max L,min (1  D)  out
2
RL
Vin Iin  Vout I out or
Also,
I out
(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:

1
DT 
I L,min  I L (0)  Vin 


2
2L 
 RL (1  D)

1
DT 
I L,max  I L ( DT )  Vin 


2
2L 
 RL (1  D)
RT
2
Setting I L ,min  0 , Lcrit  L (1  D) D
2
3.1.2
(5)
Design Calculations
As discussed in section 2.0: Project Goal and Specifications, the ZVT-PWM DC/DC Boost
converter has the following specifications:
Vin = 18V – 30V DC
Vout = 48V DC
fs = 200kHz
P=250W
Given the input voltage range of 18 – 30 V, the duty cycle D can be calculated as follows:
Vin
Vout
18V
30V
D  1
 0.625 , and D  1 
 0.375
48V
48V
D  1
Using the max output power specification of 250W, the minimum resistive load can be calculated
as follows:
V2
P
RL
8
RL 
V 2 (48V )2

 9.216
P
250W
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:
RT
(1  D) 2 D
2
(9.216)(105 s)
for D=0.625 Lcrit 
(1  0.625)2 (0.625)  4.05 H
2
(9.216)(105 s)
for D=0.375 Lcrit 
(1  0.375)2 (0.375)  6.75 H
2
 Lb  6.75 H
Lb  Lcrit 
Let
LB  33.75 H which is 5 times the initial value of Lcrit.
An inductance of L=33.75uH can be realized given:
N2
l
L
0  r A
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:
L1 N12
75.3uH 132
 N2  9
 2 

33.75uH N 22
L2 N 2
Using an inductance value of LB
 33.75 H , the maximum load can be calculated as follows:
RT
(1  D) 2 D
2
R(105 s)
33.75 H 
(1  0.375) 2 (0.375)
2
R  46.08
Lcrit 
In theory, the load should be no higher then R  46.08 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.
9
The resulting boost converter schematic is shown below in Figure 4.
Figure 4: PWM Boost Converter
The 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 Cell
The 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
Assumptions
The following assumptions are made to simplify the steady state analysis of the circuit during one
switching cycle:
i)
ii)
iii)
iv)
v)
vi)
vii)
3.2.2
Input Voltage Vin is constant.
Output voltage Vo is constant or output capacitor Cor is large enough.
Input current Iin is constant or main inductor Lb is large enough.
Resonant circuits are ideal.
Main inductor Lb is much larger than the snubber inductor Lr.
Voltage drops and parasitic capacitors of semiconductor devices are ignored.
Reverse recovery time of all diodes except the main diode Db is ignored.
Theory of Circuit Operation
A 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.
10
Figure 5: Conventional Boost Converter with Snubber Cell
Stage 1
This 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 2
Diode 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 3
Transistor 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 4
Transistor 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 5
The 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, C b
restricts the rise rate of transistor M1 voltage and M1 is turned off under near ZVS.
11
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 Calculations
The value of Lr can be calculated using the following equation:
Vo
3trr  I in ,max Where trr represents the reverse recovery time of the main diode Db. Db is a
Lr
schottkey diode BR10100 with a very fast recovery time of 5ns.
Vo
3trr  I in ,max
Lr
Vo
P
3trr 
Lr
Vin ,min
48V
250W
3(5ns ) 
Lr
18V
Lr  51.8nH
Let
Lr  75nH
The value of Cb can be calculated using the following equation:

2
Lr CB  t f 2 Where t f 2 represents the fall time of the auxiliary MOSFET M2.

(75nH )CB  36ns
2
CB  7nF
Let
CB  7nF
The value of Cr can be calculated using the following equation:
Cr  CB
Vo  t f 1 Where t f 1 represents the fall time of the main MOSFET M1.
Iin,max
Cr  7nF
(48V )  36ns
13.89 A
Cr  3.418nF
These values of Cr, Cb, and Lr also satisfy the following equation:
1
1
1
Lr ( I in ,max  I rr ,max ) 2  CrVo2  CBVo2
2
2
2
12
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:
R14
Vo  Vref
R14  R12
R14
(48V )  2.5V
R14  R12
Let
R14  1k  and R12  18.2k  .
The resulting ZVT-PWM Boost Converter Schematic is shown below in Figure 6.
Figure 6: ZVT-PWM DC/DC Boost Converter
The 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.
13
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.
14
The following flow diagram maps the events programmed to take place in the microprocessor:
Initialize microprocessor
Initialize Interrupts
Initialize Output and Input Ports
Declare Variables
Set Frequency to 50 kHz
Set Initial Duty Cycle = 50%
Output Pulses
Read A to D Converter
YES
Keep duty
cycle constant
Feedback =
Desired Output
NO
Feedback>Desired
Output
Increase Duty Cycle
NO
YES
Decrease Duty Cycle
Figure 8: Microprocessor Flow Diagram
15
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
16
Stage 1: t0 < t < t2 The following equations are valid:
iT1  0
iT2  0
iDF  I i
vCr  Vo
vCB  0
Using the values calculated in the design section, for this stage we find the time to be:
t01 
Lr
L
78nH
Ii 
9.6 A  15.6ns and t12  trr  r I rr
Vo
48V
Vo
trr was found to be 5ns, obtained from data sheets. Therefore the time for stage 1 is
t02  15.6ns  5ns  20.6ns
Stage 2: t2 < t < t3 The following equations are valid:
iT1  0
iT2  I i  I rr
iDF  0
vCr  Vo
vCB  0
t23  Lr Cr arctg
Where


Vo
48
 (3.418nF )(75nH )arctg 
  21.77ns
Z1I rr
 (4.68)(3.2) 
Z1  Lr Cr  4.68
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.
toff  (0.5)T  10us
ton  tmain  taux  9.8us  200ns  10us
For a duty cycle of 50%, the on and off time must be equal. Therefore:
toff  (0.5)T  10us
ton  tmain  taux  9.8us  200ns  10us
17
This can be seen in the oscilloscope print out of the gating pulses below in Figure 10.
Figure 10: Gating Pulses
4.0
Experimental Results
The 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:

Pout Vout I out

where Vout, Vin, Iout & Iin can all be measured in the laboratory.
Pin
Vin I in
Table 1 below details the Voltage and Current measurements taken in the laboratory at different
resistive loads, and the resulting efficiency calculations:
18
Table 1: Voltage and Current Measurements and Resulting Efficiency and Various
Resistive Loads
Load (ohm)
30
20
15
10
Vin (V)
18.00
20.10
24.45
28.08
18.00
20.40
24.80
28.30
18.24
20.70
25.00
28.54
19.00
20.15
25.00
28.50
Iin (A)
4.58
4.10
3.34
2.89
7.04
6.12
4.99
4.30
9.32
8.10
6.59
5.72
13.20
12.39
9.80
8.50
Vout (V)
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
48.00
Iout (A)
1.62
1.63
1.63
1.64
2.47
2.45
2.46
2.45
3.25
3.26
3.26
3.26
4.85
4.84
4.83
4.83
Average η:
η (%)
94.32
94.94
95.81
97.00
93.56
94.19
95.42
96.64
91.77
93.33
94.98
95.85
92.82
93.06
94.63
95.70
94.63
Figure 11 below describes Table 1 in graph format.
Efficiency vs. Input Voltage
98.00
Efficiency (%)
97.00
96.00
Load = 30ohm
95.00
Load = 20ohm
94.00
Load = 15ohm
93.00
Load = 10ohm
92.00
91.00
16.00
18.00
20.00
22.00
24.00
26.00
28.00
30.00
Input Voltage (V)
Figure 11: Efficiency vs. Input Voltage for Various Resistive Loads
As 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
19
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 30
As 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
20
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
Costs
The 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:
21
Table 2: Per Unit Costs for the ZVT-PWM DC/DC Boost Converter
Component
FET Driver
Optoisolator
Power MOSFET
Schottkey Diode
5V Regulator
Atmel Microprocessor
16 MHz Crystal Oscillator
PCB Board
Miscellaneous Parts
6.0
Part Number
UC3710N
HCPL 2601
IRF640
BR10100
LM7805C
ATMEGA8
CA-301 16.0000M-C
Unit Price (CAN$) Quantity Total Cost (CAN$)
8.49
2
16.98
2.19
2
4.38
3.38
2
6.76
1.59
5
7.95
0.69
2
1.38
4.78
1
4.78
1.25
1
1.25
20.75
2
41.50
Capacitors/Resistors etc.
5.00
Total Per Unit Cost:
89.98
Conclusion
The 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
Recommendations
Even 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
22
8.0
References
I.
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
23
9.0
Appendix
9.1
Appendix A: Progress Report #1
Progress Report #1
Group #6
The project being undertaken by Direct Current Innovations (DCI) is to design and build a zerovoltage-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
Stephen Spratt
Lauren Woolstencroft
Email: [email protected]
Email: [email protected]
Email: [email protected]
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
January
February
Week
1
2
3
4
1
2
3
Progress
Research and Design Development
↓
↓
Hardware Acquisition and continued Design work
Design & Circuit Simulation Work
↓
Wiring, Waveform Acquisition, Troubleshooting
24
4
1
2
3
4
1
March
April
↓
↓
↓
Poster, Website, Report
↓
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
25
9.2
Appendix B: Progress Report #2
Progress Report #2 - Group #6
To:
Cc:
Dr. A. K. S. Bhat
Dr. A. Zielinski, Mr. Kiran Swaroop
Date:
March 7, 2005
Group:
Jen Magdalenich
Stephen Spratt
Lauren 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 DC
Vout = 48V DC
fs = 200kHz
P=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:
Vin
Vout
18V
30V
D  1
 0.625 , and D  1 
 0.375
48V
48V
D  1
26
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.
P
V2
RL
RL 
V 2 (48V )2

 9.216
P
250W
The inductor Lb can be calculated using the following inequality: (please note that
Lb  Lcrit in
order for the circuit to always operate in continuous conduction mode (CCM)).
RT
(1  D) 2 D
2
(9.216)(105 s)
for D=0.625 Lcrit 
(1  0.625)2 (0.625)  4.05 H
2
(9.216)(105 s)
for D=0.375 Lcrit 
(1  0.375)2 (0.375)  6.75 H
2
 Lb  6.75 H
Lb  Lcrit 
Let
LB  33.75 H which is 5 times the initial value of Lcrit.
An inductance of L=33.75uH can be realized given:
N2
l
L
0  r A
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:
L1 N12
75.3uH 132
 N2  9
 2 

33.75uH N 22
L2 N 2
Using an inductance value of LB
 33.75 H , the maximum load can be calculated as follows:
RT
(1  D) 2 D
2
R(105 s)
33.75 H 
(1  0.375) 2 (0.375)
2
R  46.08
Lcrit 
In theory, the load should be no higher then R  46.08 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.
27
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:
Vo
3trr  I in ,max Where trr represents the reverse recovery time of the main diode Db. Db is a
Lr
schottkey diode with a very fast recovery time of 5ns.
Vo
3trr  I in ,max
Lr
Vo
P
3trr 
Lr
Vin ,min
48V
250W
3(5ns ) 
Lr
18V
Lr  51.8nH
Let
Lr  75nH
The value of Cb can be calculated using the following equation:
28

2
Lr CB  t f 2 Where t f 2 represents the fall time of the auxiliary MOSFET M2.

(75nH )CB  36ns
2
CB  7nF
Let
CB  7nF
The value of Cr can be calculated using the following equation:
Cr  CB
Vo  t f 1 Where t f 1 represents the fall time of the main MOSFET M1.
Iin,max
Cr  7nF
(48V )  36ns
13.89 A
Cr  3.418nF
These values of Cr, Cb, and Lr also satisfy the following equation:
1
1
1
Lr ( I in ,max  I rr ,max ) 2  CrVo2  CBVo2
2
2
2
In order to get a reference feedback voltage of 2.5V, R12 and R14 can be calculated as follows:
R14
Vo  Vref
R14  R12
R14
(48V )  2.5V
R14  R12
Let
R14  1k  and R12  18.2k  .
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
29
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 Lauren
Poster – to be completed by Jen
Presentation – April 1, 2005 – to be organized by Jen, Stephen and Lauren
Progress Reports – Completed by Stephen, with contributions from Jen and Lauren
Final Report – to be sectionalized by Jen, Stephen and Lauren
Regards,
Direct Current Innovations (499 Group 6)
Jen Magdalenich
Stephen Spratt
Lauren Woolstencroft
30
Appendix 1 – Control Circuit Schematic
31
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 0x0000
rjmp 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 d
A2D: ldi temp, 0xC0
out
ADCSR, temp
;starting pulses
pulse1: ldi temp, 0x01
out PORTB, temp
mov
temp2, SCR1
p1loop: dec temp2
brne p1loop
rjmp pulse2
pulse2: ldi temp, 0x02
out PORTB, temp
mov
temp2, SCR2
p2loop: dec temp2
brne p2loop
rjmp off
off:
ldi temp, 0x00
32
out PORTB, temp
mov
temp2, OFFT
offloop:dec temp2
brne offloop
rjmp
pulse1
ldi
temp2, ADCSR
cpi
temp2, 0x80
brne
pulse1
rjmp OC1A
OC1A: in
temp2, ADCH
subi
temp2, 0x02
breq
next
rjmp
compute1
next: in temp2, ADCL
subi
temp2, 0x00
breq
finished
rjmp
compute2
compute1:
in
temp2, ADCH
subi
temp2, 0x02
brlo
increase
dec
SCR1
rjmp
finished
increase:
inc
SCR1
rjmp
finished
compute2:
in temp2, ADCL
subi
temp2, 0x00
brlo
increase2
dec SCR1
rjmp
finished
increase2:
inc
SCR1
rjmp
finished
finished:
rjmp
pulse1
33
9.4
Appendix D: Printed Circuit Board Layouts
34
35
9.5
Appendix E: Data Sheets
36