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					             University of Victoria
            Faculty of Engineering
            Elec 499B Final Report




DC-DC Boost Converter for Wind
     Energy Application



              Alternative Energy Solutions
               Victoria, British Columbia




                     David Walden
                        0320460
                 Electrical Engineering
                   dwalden@uvic.ca

                   Jamie Hargreaves
                        0328025
                 Electrical Engineering
                   jhargrea@uvic.ca

                      April 7, 2006




     in partial fulfillment of the requirements of the
                       B.Eng. Degree
                                      TABLE OF CONTENTS

LIST OF FIGURES .......................................................................................................... 3
LIST OF TABLES ............................................................................................................ 3
SUMMARY ....................................................................................................................... 4
1.0 INTRODUCTION....................................................................................................... 5
   1.1 INRODUCTION AND APPLICATION .................................................................. 5
   1.2 PROJECT SPECIFICATION ................................................................................... 5

2.0 HARDWARE DESIGN .............................................................................................. 6
   2.1 BASIC BOOSTER CIRCUIT................................................................................... 6
   2.2 ZERO VOLTAGE SWITCHING ............................................................................. 6
   2.3 TIMING AND OPERATION OF DC TO DC BOOSTER ...................................... 8
   2.4 DESIGN IMPLEMENTATION ............................................................................. 14

3.0 SOFTWARE IMPLEMENTATION ...................................................................... 18
   3.1 MICORCONTROLLER ......................................................................................... 18
   3.2 PULSE WIDTH MODULATION .......................................................................... 18
   3.3 ANALOG TO DIGITAL CONVERSION ............................................................. 19
   3.4 MICROCONTROLLER PROGRAM DESCRIPTION ......................................... 20

4.0 CLOSED LOOP CONTROL................................................................................... 22
   4.1 CLOSED LOOP IMPLEMENTATION ................................................................. 22
   4.2 OPEN LOOP TO CLOSED LOOP OPERATION ................................................. 22

5.0 RESULTS .................................................................................................................. 23
6.0 CONCLUSIONS ....................................................................................................... 26
7.0 RECOMMENDATIONS.......................................................................................... 26
REFERENCES ................................................................................................................ 27
APPENDIX A – OSCILLOGRAMS
APPENDIX B – PROGRAM CODE
APPENDIX C – CALCULATIONS




                                                                                                                               2
LIST OF FIGURES
FIGURE 1 – BASIC BOOSTER CIRCUIT1.................................................................................................. 6
FIGURE 2 – POWER LOSS DUE TO HARD SWITCHING2 ...................................................................... 7
FIGURE 3 – SOFT SWITCHING, VSW GOES TO ZERO BEFORE TRANSISTOR IS TURNED ON2..... 7
FIGURE 4 – THE BOOSTER CIRCUIT WITH AUXILIARY CIRCUIT HIGHLIGHTED IN RED1 ........ 8
FIGURE 5 – ACTIVE COMPONENTS AT EACH STAGE OF CIRCUIT OPERATION .........................13
FIGURE 6 – TIMING DIAGRAM OF ZVT DC TO DC BOOST CONVERTER1 .....................................14
FIGURE 7 – TCCR1A (TOP) AND TCCR1B (BOTTOM) .........................................................................19
FIGURE 8 – ADMUX (TOP) AND ADCSRA (BOTTOM) ........................................................................20
FIGURE 9 – MICROCONTROLLER PROGRAM FLOW CHART ...........................................................21
FIGURE 10 – ILR (CHANNEL 1 X 10) AND VD1 (CHANNEL 2) ..............................................................23
FIGURE 11 – ILR (CHANNEL 1 X 10) AND VDR (CHANNEL 2) ..............................................................24
FIGURE 12 – ILR (CHANNEL 1 X 10) AND VD2 (CHANNEL 2) ...............................................................24
FIGURE 13 – VGS(MAIN) (CHANNEL 1) AND VDS(MAIN) (CHANNEL 2) ....................................................24
FIGURE 14 – ILR (CHANNEL 1 X 10) AND VDS(AUX) (CHANNEL 2) .......................................................25
FIGURE 15 – ILR (CHANNEL 1 X 10) AND VDF (CHANNEL 2)...............................................................25



                                              LIST OF TABLES
TABLE 1 – CALCULATED COMPONENT VALUES ..............................................................................17
TABLE 2 – TIMER/COUNTER 1 REGISTER VALUES ...........................................................................18
TABLE 3 – MEASURED AND CALCULATED RESULTS IN OPEN LOOP CONTROL ......................23




                                                                                                                                   3
SUMMARY

DC to DC Boost converters utilizing soft switching techniques offer a viable alternative
to traditional hard switched converters. Soft switching techniques implementing Zero
Voltage Transition (ZVT) have been shown to operate at efficiencies of around 97%
while their hard switched counterparts operate at around 91%.

This project included the design, implementation and testing of a ZVT DC-DC Boost
Converter. The converter specifications were as follows:

      Input Voltage Range: 100 – 150V
      Output Voltage: 280V
      Switching Frequency: 100KHz

During testing, the converter yield efficiencies of 96.5% at full load, in agreement with
previously reported results by [1]. Oscillograms illustrated ZVT at the full load, half load
and quarter load for the entire input voltage range.

After open loop testing at these voltages closed loop control was implemented. To safely
implement closed loop control the input voltage range was changed to 8.5 – 20V while
the output target voltage was changed to 31.5V. Closed loop control was successfully
demonstrated at the student project demonstration night on Friday, March 31 at these
lower voltages.

While the project implementation and testing of the ZVT DC-DC boost converter was
there are several aspects of the project that may be improved upon. These include:

              Reprogram the microcontroller to allow for tighter closed loop with faster
               response times.
              Implement an analog opto-isolator at the boost converter output to provide
               protection for the ADC0 microcontroller input in closed loop control. The
               current circuit only has isolation from the PWM channels to the switches
               T1 and T2 (gating pulses)
              Investigate the possibility of powering the microcontroller and booster
               circuit from the DC input voltage. This would be a significant
               improvement over the current design allowing greater portability and
               bring the converter one step closer to becoming a marketable product.

These three recommendations provide ample opportunity for future Elec 499 project
work.




                                                                                           4
1.0 INTRODUCTION

1.1 INRODUCTION AND APPLICATION

Wind turbines offer a viable alternative to traditional means of power generation where
there is a good wind resource. High efficiency power electronics are essential to the
overall operation of any wind turbine system that is used as a generating source for a
power system (i.e. DC bus). After the conversion from AC to DC an efficient DC to DC
boost converter is needed to bring the input potential up to the DC bus potential.
Traditional DC to DC converters utilizing hard switching techniques usually operate at
efficiencies of around 91%. With the advent of newer technologies more efficient
converters are becoming an expectation. One such technology is the Zero Voltage
Transition (ZVT) soft switching technique.

1.2 PROJECT SPECIFICATION

This project will focus on the design and implementation of a DC to DC converter which
will take an input of 100 – 150V and output a constant 280V. As previously mentioned,
to increase the efficiency of the converter ZVT soft switching techniques will be
implemented. The frequency of operation will be 100 KHz. Both open loop control and
closed loop control will be demonstrated. Closed loop control will only be demonstrated
at 8 – 20V input and output a constant 31.5V. The lower voltages for this mode of
operation will protect the microcontroller board. These lower voltages will also allow
safe demonstration of the overall system to the general public at the Elec 499 project
demonstration night.




                                                                                          5
2.0 HARDWARE DESIGN

2.1 BASIC BOOSTER CIRCUIT

Shown in figure 1 is the basic booster circuit (with ZVT circuit omitted). There are two
modes of operation of this circuit. The first mode is the charging interval. During this
mode the switch T1 is on and the voltage across the inductor LF is equal to V1 and current
I1 = iT1 is given by,


                                  iL( t)  
                                                1
                                              Vin t  IL( 0) 0  t  DT
                                            L
where D = Duty Cycle of T1
and T = Period = (1/f)

During the second mode of operation the switch T1 is turned off and the inductor current
becomes,


                        iL( t)  
                                      1
                                    ( Vin  Vo) ( t  DT)  IL( DT) DT  t  T
                                  L




Figure 1 – Basic Booster Circuit1
Qualitatively, during mode one of the circuit operation the inductor is charging and the
diode, DF does not conduct. During mode two of the operation, the diode is forward
biased and the capacitor is changed by the stored energy in the inductor, until the
capacitor voltage becomes greater then V1 and the diode stops conducting. This cycle is
then repeated and the boosting action is observed.

2.2 ZERO VOLTAGE SWITCHING

The main benefit of Zero Voltage Switching (ZVS) is the elimination of transistor turn on
losses. When a transistor is turned with voltage across it there is power loss, this is
because the voltage across the switch Vsw takes time to die down as the current through


                                                                                           6
the switch Isw builds up. As a result of this there is an overlap of voltage and current
which gives way to power loss when turning on the switch, this is called hard switching
and is shown in the figure 2.




Figure 2 – Power Loss Due to Hard Switching2
To eliminate the power loss due to hard switching, the ZVS circuit must bring the voltage
across the transistor to zero before the transistor is turned on. If the voltage across the
transistor is brought to zero before it is turned on there is no overlap of voltage and
current and therefore no power loss2. This is illustrated in figure 3.




Figure 3 – Soft Switching, VSW goes to zero before transistor is turned on2
The elimination of power loss when turning on the transistor increases the efficiency of
the circuit but also increases the upper limit of switching frequency that can be used. An
increase in circuit frequency allows the use of smaller inductors and capacitors which
lowers the cost of building the circuit reduces circuit size and increases reliability and
power density2. Due to these benefits there is a strong drive to design and build DC to DC
booster converters with soft switching techniques, specifically the ZVT technique.

The auxiliary circuit used in this project to achieve ZVS is an active snubber cell that
overcomes the draw backs of hard switching with minimal circuitry and high efficiency1,
figure 4 shows an overview of the booster circuit with the ZVS circuit highlighted in red.




                                                                                           7
Figure 4 – The booster circuit with auxiliary circuit highlighted in red1
The auxiliary circuit has two very important benefits. The first is that the circuit
eliminates the voltage across the main transistor (T1) by sending this voltage to the load
instead of eliminating the voltage through a resistor, which increases the efficiency.
Secondly, the auxiliary transistor (T2) turns on with ZVS which eliminates any turn on
power loss in this transistor. Finally, the ZVS action of the auxiliary circuit does not incur
any extra voltage or current stresses on the T1 and main diode (Df)1.

2.3 TIMING AND OPERATION OF DC TO DC BOOSTER

The following section goes over the details of circuit timing for the complete booster
circuit; this information is taken directly from [1]. All calculations are completed with the
following assumptions:

               a) Input voltage Vi is constant
               b) Output voltage Vo is constant or output capacitor Cf is large enough
               c) Input current Ii is constant or main inductor Lf is large enough
               d) Resonant circuits are ideal
               e) Main inductor Lf is much larger than snubber inductor Lr
               f) Voltage drops and parasitic capacitors of semiconductor devices are
                  ignored
               g) Reverse recovery times of all diodes except the main diode Df are
                  ignored

1. Stage 1 (t0 < t ≤ t2):

        Initial Conditions

            The main transistor T1 and the auxiliary transistor T2 are off
            The main diode Df is in the on state and conducts the current Ii of the main
             inductor Lf
            Output voltage Vo is across main transistor T1 and resonant capacitor Cr



                                                                                            8
        t = t0

           The auxiliary transistor T2 is turned on
           The devices Dr and T2 are turned on under near Zero Current Switching (ZCS)

        t0 < t ≤ t1

           The rise rate of the current through Dr and T2 is limited by the snubber
            inductor Lr
           T2 current rises and Df current falls simultaneously and linearly
           The following equations apply




           T2 current reaches Ii and Df current falls to zero

        t1 < t ≤ t2

           T2 current continues to rise and Df current continues to fall
           At t = t2 the Df current equals –Irr and Df turns off under ZVS
           The following equations apply




           At t = t2 the T2 current equals Ii plus Irr

2. Stage 2 (t2 < t ≤ t3)

        Initial Conditions

           The main transistor T1 and the main diode Df are off



                                                                                       9
           The auxiliary transistor T2 is on and conducts the current Ii + Irr.
           The voltage across T1 equals output voltage Vo

        t = t2

           Parallel resonance between Lr and Cr starts to resonate via the resonant path
            Cr – Dr – Lr – T2 under the input current Ii and with the initial current ILr2

        t2 < t ≤ t3


           The following equations apply for the resonance




           At t = t3 the transfer of energy is complete between Cr and Lr and the voltage
            across Cr and T1 is zero
           The following equations apply for the maximum current and energy levels of
            Lr




3. Stage 3 (t3 < t ≤ t4)

        Initial Conditions

           Auxiliary transistor T2 is on and conducting the maximum current of Lr
           The voltage across the main transistor T1 is zero

        t = t3

           The antiparallel diode DT1 of the main transistor T1 is turned on

        t3 < t ≤ t4

           The antiparallel diode DT1 conducts the excess of the inductor Lr current from
            the input current Ii


                                                                                             10
           The following equations apply




           It is during this stage that ZVS is available for the main transistor T1

4. Stage 4 (t4 < t ≤ t5)

        Initial Conditions

           The main transistor is off and the current through the auxiliary transistor is the
            maximum current through the inductor Lr
           The voltage across T1 is zero and the antiparallel diode is conducting current

        t = t4

           The main transistor T1 is turned on under ZVS
           The auxiliary transistor T2 is turned off with nearly ZVS
           The auxiliary diode D1 is turned on with ZVS

        t4 < t ≤ t5

           The current through T1 is Ii
           A serial resonance between Lr and Cb starts by way of Lr – D1 – Cb – Dr under
            the maximum inductor current of Lr
           The following equations apply for the resonance




           The energy stored in the inductor Lr is transferred to capacitor Cb
           It is assumed that the Cb is charged from exactly zero to Vo with the transfer
            of energy
           The following equations apply




           At t = t5 the resonance stops and Dr and D1 are turned off under near ZCS



                                                                                             11
5. Stage 5 (t5 < t ≤ t6)

        Initial Conditions

           The main transistor T1 is on and conducts current Ii
           The auxiliary capacitor Cb is charged with the output voltage Vo across it

        t5 < t ≤ t6

           The snubber circuit is not active
           The length of this stage is dependant on the duty cycle of the main transistor
            T1 and the required boost
           The following equation applies




6. Stage 6 (t6 < t ≤ t7)

        Initial Conditions

           The main transistor T1 conducts current Ii
           The snubber circuit is not active
           The voltage across the auxiliary capacitor Cb is Vo

        t = t6

           The main transistor T1 is turned off under near ZVS
           The auxiliary diode D2 is turned on with ZVS

        t6 < t ≤ t7

           The capacitor Cr is charged and capacitor Cb is discharged
           The capacitor Cb reduces the rate of rise of voltage across the main transistor
            T1
           At t = t7 the voltage on Cr equals output voltage Vo and the voltage on Cb
            equals zero
           At t = t7 the main diode Df is turned on with ZVS and the auxiliary diode D2 is
            turned off with ZVS
           The following equations apply




                                                                                         12
7. Stage 7 (t7 < t ≤ t8)

           The inductor Lf discharges with current Ii through the main diode Df
           The snubber circuit is not active
           The duration of this stage depends on the duty cycle of the main transistor and
            the required boost
           The following equation applies



           At t = t8 one switching cycle is complete

Figure 5 shows the active components of the converter and the applicable currents during
each stage of operation. Stage (a) – (b) corresponds to (1) – (7) in the above circuit
analysis. Figure 6 illustrates the timing diagram for one complete boosting cycle.




Figure 5 – Active Components at each stage of Circuit Operation


                                                                                        13
Figure 6 – Timing Diagram of ZVT DC to DC Boost Converter 1


2.4 DESIGN IMPLEMENTATION

The following formulas detail the design calculations for the circuit elements.

1. Main Booster Circuit Elements

           a. Compute output current

                                                        Po
                                                Io 
                                                        Vo

           b. Compute required load

                                                        Vo
                                                RL 
                                                        Io



                                                                                  14
           c. Compute required duty cycle

                                                             Vin
                                             D  1 
                                                             Vo

           d. Compute average input current

                                                            Po
                                               Iin 
                                                            Vin
           e. Compute maximum ripple current

                                                      ∆I

           f. Compute the main inductor Lf

                                                       Vin D
                                              Lf 
                                                       fs I

           g. Define maximum output ripple voltage

                                                     ∆Vo

           h. Compute main capacitor Cf

                                                            D
                                            Cf 
                                                    Vo RL fs

2. Auxiliary Circuit Elements

           i. Define fall time tf1 of main transistor and reverse recovery time trr of main
              diode
                                                      tf1
                                                      trr

           j. Compute resonant capacitor Cr

                                                       Iin t f1
                                              Cr 
                                                            Vo

           k. Compute resonant inductor Lr

                                                     Vo  3 t rr
                                             Lr 
                                                      ILmax




                                                                                         15
           l. Compute maximum reverse recovery current
                                                            Vo
                                               Irrmax           t rr
                                                            Lr

           m. Compute the energy stored in the inductor Lr

                                      1                      2 1        2
                                 Eb   Lr ( ILmax Irrmax   Cr Vo
                                                           )
                                      2                        2

           n. Compute the required capacitance Cb

                                                          Eb 2
                                                 Cb 
                                                               2
                                                          Vo

           o. Compute time required for Df current to fall to zero

                                                          Lr Iin
                                                t 01 
                                                            Vo

           p. Compute reverse recovery time

                                                       Lr
                                             t 12          Irrmax
                                                       Vo

           q. Compute time required to dissipate voltage from Cr into Lr

                                     t 23  Lr Cr at an              
                                                                     Vo
                                                             Lr
                                                                         
                                                                 Irrmax
                                                             Cr         

           r. Compute total time for auxiliary circuit to be active so that there is ZVS
              for main transistor

                                            t 3  t 01  t 12  t 23

The calculated values for components are tabulated in table 1.




                                                                                           16
Table 1 – Calculated Component Values

                           Circuit Element Name   Value   Units
                                      RL          313.6    Ω
                                      Lf          1.286    mH
                                      Cf          1.025    µF
                                      Cr          1.161    nF
                                      Lr          18.56    µH
                                      Cb          4.426    nF

See Appendix C – Calculations for details on calculations and results.




                                                                         17
3.0 SOFTWARE IMPLEMENTATION

3.1 MICORCONTROLLER

The Atmel atmega8 microcontroller was used to provide the gating signals to the main
transistor (T1) and the auxiliary transistor (T2). This chip is a reduced instruction set
microcontroller with peripherals for pulse width modulation (PWM) and analog to digital
(ADC) conversion. The Atmel chips analog to digital converter was also used for closed
loop implementation. An external oscillator was used to increase the microcontroller
instruction execution time. The frequency of operation of the microcontroller was set to
8 MHz in order to meet the specified frequency of operation of the PWM.

3.2 PULSE WIDTH MODULATION

Two PWM channels (port B, PB1 and PB2) were used on the microcontroller. Channel 1
was used for T1 while channel 2 was used for T2. The following specifications were
adhered to:

       Frequency of operation: 100 KHz
       Channel 1 duty cycle: variable from 20% – 80%
       Channel 2 duty cycle: set at 5%

In order to meet these specifications the 16-bit Timer/Counter 1 module on the atmega8
chip was used. This timer module was configured for fast PWM. In this mode the
frequency and duty cycle of each channel may be varied or held constant depending on
the program application. In this mode the output is set high when the Timer/Counter
Register (TCNT1x, TCNT1H – high byte and TCNT1L – low byte) equals the Output
Compare Register (OCR1Ax, OCR1AH – high byte and OCR1AL – low byte) and is set
low when TCNT1x reaches the maximum value as set by the Input Capture Register
(ICR1x, ICR1H – high byte and ICR1L – low byte), when TCNT1x equals ICR1x it
resets and beginnings up-counting from 0000hex. OCR1Ax was used for channel 2 and
OCR1Bx (OCR1Bx has the same functionality as OCR1Ax) was used for channel 1.

Figure 1 shows the control registers for timer/counter 1. The registers shown are
Timer/Counter 1 Control Register A (TCCR1A) and Timer/Counter 1 Control Register B
(TCCR1B). Table 1 shows the hexadecimal value that was set in each register in order to
meet the required specifications.
Table 2 – Timer/Counter 1 Register Values

                           Register Name Hexadecimal Value
                             TCCR1A             F2
                             TCCR1B             19
                              ICR1x            00F9
                             OCR1Ax            0095
                             OCRIBx          variable



                                                                                       18
The resolution of the PWM is set by the frequency of operation of the PWM (i.e. ICR1x).
The following calculation details the minimum PWM step size:

                         Maximum Duty Cycle 100% = 00F9hex
                          Minimum Duty Cycle 0% = 0000hex
                                  F9hex = 249decimal
                           Step Size = 100% / 249 = 0.4%

In Table 1 OCR1Bx is shown to have a variable value. In open loop control the value
was set to 005D which corresponds to a duty cycle of 60%. In closed loop control this
value was used as the starting point and was varied depending on the input voltage (see
section 4.0 for details on the closed loop control algorithm and operation).




Figure 7 – TCCR1A (top) and TCCR1B (bottom)


3.3 ANALOG TO DIGITAL CONVERSION

The microcontroller analog to digital (ADC) converter was used to implement close loop
control. The output voltage of the booster circuit was brought back to the microcontroller
through a voltage divider circuit at the output. The following specifications were adhered
to for the ADC:

      Reference voltage: 5V
      Input Voltage range: 0-5V
      Required Resolution: 8 bits

The ADC was set up for continuous conversion mode. Continuous conversion mode runs
as a background program and continually digitizes the analog signal. Figure 2 shows the
two control registers for the ADC. The ADC Multiplexer Selection Register (ADMUX)
selects the analog input channel that the ADC will perform the digital conversion on.
The ADMUX register was set to input the analog signal on ADC0 (ADC channel 0).
The ADC Control and Status Register (ADCSRA) sets the mode of operation for the
ADC. This register also determines the source for the reference voltage, VREF.




                                                                                          19
Figure 8 – ADMUX (top) and ADCSRA (bottom)


With an 8 bit resolution the maximum and minimum ADC conversion results are FFhex
and 00hex, respectively. Based on the specifications of the ADC the minimum step size of
can be calculated:

                            STEP SIZE = VREF / 2N-1         (1.1)
                                   39.1mV = 5V / 27

where N = number of bits
and VREF = externally applied reference voltage

The following formula shows the analog to digital conversion (result in decimal):

             DIGITAL RESULT = ANALOG VOLTAGE / STEP SIZE                   (1.2)

where ANALOG VOLTAGE = voltage on ADC0

To illustrate, an input analog voltage of 2.5 volts would result in an analog to digital
conversion of 80hex.

3.4 MICROCONTROLLER PROGRAM DESCRIPTION

The microcontroller was programmed to operate in two modes – open loop and closed
loop. In both modes timer/counter 1 (as described in section Pulse Width Modulation)
was used. In closed loop control the ADC was implemented (see section Analog to
Digital Conversion for details) to digitize the feedback signal from the output of the
booster circuit. This value is then compared with the set point and the duty cycle of the
main transistor is adjusted if needed. The set point value was determined as per the
output voltage specification (see Closed Loop Implementation Section). Figure 3 shows
the flow chart for the microcontroller program. The code listing is attached as an
appendix in Appendix B – Program Code. The program goes into closed loop mode
when an external switch, connected to port B, pin 3, is set.




                                                                                           20
                                     Initialize PORT B for
                                    PWM output, Initialize
                                    PORT C for ADC input


                                    Initialize Timer1 for fast
                                   PWM operation on PORT
                                          PB1 and PB2.


                                      Initialize ADC for
                                    continuous conversion,
                                       right shift result


                                  Set PB1 (T1) for 100 KHz
                                  operation, 60% duty cycle.
                                  Set PB2 (T2) for 100 KHz
                                  operation, 5% duty cycle.




                                                           OPEN LOOP




                                         Is the mode
                                         of operation
                                         open loop or
                                         closed loop?
    Increase Duty Cycle of                                         Decrease Duty Cycle of
         PB1 by 0.4%                                                   PB1 by 0.4%
                                                 CLOSED LOOP


                                            Is output
                                        voltage greater
                                        or less then the
            LESS THAN                      set point?              GREATER THAN




Figure 9 – Microcontroller Program Flow Chart



                                                                                            21
4.0 CLOSED LOOP CONTROL

4.1 CLOSED LOOP IMPLEMENTATION

For demonstration and application purposes closed loop control was implemented. To
demonstrate closed loop control the specifications of the DC to DC boost converter were
changed. The input voltage range was changed to 8 – 20V from 100 – 150V and the
output voltage specification was set to 31.5V from 280V. These changes were necessary
to be able to safely couple the microcontroller and the boost circuit to bring the ADC
analog voltage signal back to the microcontroller board. In order to bring the output
voltage within the ADC range of 0 – 5V a voltage divider circuit was used. The voltage
divider circuit consisted of a 100KΩ resistor in series with a 10KΩ resistor. The voltage
across the 10KΩ was brought back to the microcontroller to be digitized. With this
divider circuit an output of 31.5V corresponded to an input to the ADC of 2.86V, well
within the 0-5V ADC range.

4.2 OPEN LOOP TO CLOSED LOOP OPERATION

A procedure must be followed in order to safely switch the microcontroller to closed loop
from open loop control. First, with the microcontroller initially off, power is applied to
the microcontroller and booster circuit. Second, the input voltage should be adjusted
until the output voltage is 31.5V. Once the output voltage equals 31.5V the mode of
operation of the microcontroller may be changed, by setting the switch connected to PB3
high. Failure to bring the output to 31.5V before switching to closed loop control would
result in the duty cycle of the T1 aggressively changing to drive the output to 31.5V.
Switching between open loop and closed loop at this voltage results in no change to the
duty cycle of T1. Once in closed loop, the input voltage may be adjusted within the
specified range (8 – 20V) and the output will hold constant at 31.5V (~ +/- 5%).




                                                                                       22
5.0 RESULTS

The dc-dc booster circuit was tested at full load, half load, and quarter load for minimum
input voltage (~100V), maximum input voltage (~150V) and mid range input voltage
(~125V). Table 3 details the results of these tests.

Table 3 – Measured and Calculated Results in Open Loop Control

              Input                                    Output            Load   Efficiency
Voltage      Current       Power       Voltage         Current   Power    (Ω)      (%)
  (V)          (A)          (W)         (V)             (A)       (W)
 142.7         1.81        258.3        280             0.89     249.2    316      96.5
 138.8         0.99        137.4        280             0.47     131.6    600      95.8
 133.8         0.54         72.3        280             0.25       70    1200      93.0
 99.6          2.67        265.9        279             0.92     256.9    316      96.5
 95.9          1.45        139.1        280             0.47     131.6    600      94.6
 90.8          0.79         71.7        279             0.24      67.0   1200      93.3
 123.0         2.14        263.2        281             0.90     252.9    316      93.6
 118.8         1.16        137.8        280             0.46     128.8    600      93.5
 113.6         0.62         70.4        280             0.23      64.4   1200      91.5

Oscillograms were obtained for full load at 100V input. Figures 10 – 15 shows these
graphs. The original oscillograms are included in Appendix A – Oscillograms.




Figure 10 – ILr (channel 1 x 10) and VD1 (channel 2)




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Figure 11 – ILr (channel 1 x 10) and VDr (channel 2)




Figure 12 – ILr (channel 1 x 10) and VD2 (channel 2)




Figure 13 – VGS(main) (channel 1) and VDS(main) (channel 2)




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Figure 14 – ILr (channel 1 x 10) and VDS(aux) (channel 2)




Figure 15 – ILr (channel 1 x 10) and VDf (channel 2)


From the above oscillograms it can be seen that the main switch T1 is turned on with ZVS
and turned off near ZVS. The auxiliary switch T2 is turned on under near ZCS and turned
off under near ZVS. Additionally, the devices DF, Dr, D1 and D2 operate with soft
switching.

While this project did not investigate the efficiencies associated with hard switching, hard
switching efficiencies have been reported by [1] of about 91%, significantly less than the
efficiencies reported in Table 3. For comparison, in the soft switching scheme, the main
switch loss is about 27%1 and the total circuit loss is about 36%1 of that in its counterpart
hard switching converter.




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6.0 CONCLUSIONS

The objective of this project was to design and implement a DC to DC Boost Converter
using soft-switching techniques. The specifications that were followed were:

      Input voltage range of 100 – 150V
      Output voltage 280V
      Frequency of operation 100KHz

The circuit was tested at full load, half load and quarter load. Table 1 details the results.
All of the devices in this converter switched on and off at or near ZVS and/or ZCS as
shown in Figures 10 – 15. Additionally efficiencies approaching 97% were observed at
full load for maximum input, minimum input and mid range input voltages. These
reported efficiencies represent approximately a 6% increase over previously reported
hard switching schemes.

7.0 RECOMMENDATIONS

While the design and implementation of the DC to DC boost converter yielded acceptable
results there is room for improvement. Areas for future work include:

              Reprogram the microcontroller to allow for tighter closed loop with faster
               response times.
              Implement an analog opto-isolator at the boost converter output to provide
               protection for the ADC0 microcontroller input in closed loop control. The
               current circuit only has isolation from the PWM channels to the switches
               T1 and T2 (gating pulses)
              Investigate the possibility of powering the microcontroller and booster
               circuit from the DC input voltage. This would be a significant
               improvement over the current design allowing greater portability and
               bring the converter one step closer to becoming a marketable product.

These three recommendations provide ample opportunity for future Elec 499 project
work.




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REFERENCES

[1]   H. Bodur, A. F. Bakan, “A New ZVT-PWM DC-DC Converter,” IEEE
      Transactions on Power Electronics, vol. 17, pp. 40-47, January 2002.

[2]   D. Gautam (private communication), 2006.

[3]   I. Batarseh, Power Electronic Circuits. Massachusetts: John Wiley and Sons,
      2004.




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