University of Victoria
Department of Electrical and Computer Engineering
Elec 499B – Group #7
Fixed Frequency Series-Parallel Resonant ZVT DC-DC Converter
Final Project Report
Submitted on: April 8, 2005
Group Members:
Tim Gordon (0220973)
Devin Krenz (0226341)
Nouredine Mahdar (0228583)
Rico Luc (0229785)
Table of Contents
List of Figures and Tables ...... iii
Summary ...... iv
1.0 Introduction ...... 1
2.0 DC-DC Converter Stages ...... 2
2.1 AC-DC Conversion ...... 2
2.2 Control Circuit ...... 2
2.3 Driver Circuit ...... 7
2.4 Power Circuit ...... 7
2.4.1 Power Circuit Calculations ...... 8
2.4.2 Winding of the Transformer ...... 12
3.0 Test Procedure and Results ...... 12
3.1 Testing Procedure ...... 12
3.2 Final Results and Analysis ...... 15
4.0 Recommendations ...... 18
5.0 Conclusions ...... 19
Appendix A (Block Diagram of Control IC)
Appendix B (Pin Descriptions for Control IC)
List of Figures
Figure 1 - DC to DC Converter 2
Figure 2 - AC to DC setup (transformers are not shown) 3
Figure 3 - Schematic of Mosfet Bridge (part of power circuit) 4
Figure 4 - Mosfet gate signal outputs from the control circuit 4
Figure 5 - Error amplifier circuit (internal to control IC) 6
Figure 6 - Schematic of the control circuit 6
Figure 7 - Oscillator schematic and frequency behavioural graph 7
Figure 8 - Control circuit 7
Figure 9 - Current Driver PCB 8
Figure 10 - Power Circuit 9
Figure 11 - Voltage and Current at output of Mosfet Bridge 14
Figure 12 - Gate signals Q1 and Q3 from the control circuit 15
Figure 13 - Plot of Vout vs. Load for Vin = 27V and Vin = 56V 17
Figure 14 - Plot of Iout vs. Load for Vin = 27V and Vin = 56V 18
Figure 15 - Plot of phase vs. Load for Vin = 27V and Vin = 56V 19
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Summary
Over the course of the term the project that was designed, built, and tested was a fixed frequency, DC to DC resonant converter. The goal of the design is to produce an extremely efficient, small, light weight, and cost effective DC to DC converter. These are some of the properties that are made possible by using a ZVT resonant type converter. The converter built in this project has 4 main stages: AC to DC conversion, control circuit, driver board and power board. When the project was completed and ready to be tested it was not able to operate at full power when the load was reduced from 100%. All the results were then collected at a low voltage, where the converter worked quite well. At high voltage when the load was reduced and the control circuit was adjusted to compensate the output voltage the circuit failed. At that point enough data had been collected to put together some detailed results showing that the system was generally operating as expected allowing a constant output voltage with varying load.
1.0 Introduction
Resonant DC to DC converters are a very efficient way of converting DC voltages. By soft switching mosfet transistors and using Zero Voltage Transition (ZVT) techniques, it becomes possible to reduce or boost voltage with hardly any losses. DC to DC converters are used in many power applications including UPS (Uninterruptible Power Supply) systems. There are many different ways to implement a DC to DC converter, but this project focuses on the Fixed Frequency pulse width modulated series-parallel resonant LCC type Converter. To provide pulse width modulation we used the UC1875 IC to control the gating signals of the four power Mosfets (Bridge configuration). The converter also has a capacitive output filter.
The goal for this project is to be able to supply a 120 volt output to a load with a maximum power of 500 watts. The input voltage is variable from 110Vdc to 130Vdc and by manually adjusting the phase delay of the mosfet gate signals it is possible to maintain the output voltage at a constant 120Vdc. Note that the IC used is capable of using feedback to keep the output signal constant with a certain amount of fluctuation in input voltage or load resistance, but due to time restraints this was not implemented in this project.
The DC to DC converter has 4 main stages to its operation. These include AC to DC conversion, control circuit, driver circuit and power circuit. Each of these stages will be discussed in great detail in the following sections. A block diagram of the DC to DC converter is shown in figure 1.
Figure 1 - DC to DC Converter
2.0 DC-DC Converter Stages
2.1 AC-DC Conversion
The AC to DC conversion stage is the simplest part of the project. It consists of a large kVA step down transformer, a variac transformer (to vary input voltage during testing), a full-wave bridge rectifier, and a large filter capacitor. The input to the step down transformer is 208V three-phase supplied from the lab. The output is a 120V three phase signal that is put into the variac transformer so one is able to vary the voltage to the DC to DC converter from zero to 120Vac. The full wave rectifier consists of 3 sets of power diodes that rectify the three phase signal before it is input to the DC to DC converter across a large filter capacitor. There is also a circuit breaker placed between the filter capacitor and the converter for safety and an ammeter to monitor the input current. A picture of the setup is shown in figure 2.
Figure 2 - AC to DC setup (transformers are not shown)
The DC signal supplied by the AC to DC conversion stage is the input to the DC to DC power circuit across the four switching mosfets. This will be talked about in more detail in the power circuit section and is shown in the power schematic.
2.2 Control Circuit
The purpose of the control circuit is to provide pulse width modulation by supplying a square wave pulses to the gates of the mosfets so that they are turned on and off in the correct sequence and at the correct frequency. Referring to the Mosfet Bridge shown in the schematic in figure 3 and the capture of the gate signals shown in figure 4, one can see that the gate signals on Q1 and Q3 are out of phase and the same goes for Q2 and Q4. There is also some delay between when Q1 goes low and Q3 goes high. This is to prevent both series mosfets from turning on at the same time and shorting the DC input. The delay is in the order of a few hundred nanoseconds (about 250ns in this case). There is also a phase shift between the signals on Q1 and Q3 to Q2 and Q4. This phase shift needs to be adjustable in order to vary the output voltage of the DC to DC converter.
Figure 3 - Schematic of Mosfet Bridge (part of power circuit)
Figure 4 - Mosfet gate signal outputs from the control circuit
The switching frequency of the gate signals is designed to be 100 kHz, but the frequency is adjustable in order to fine tune for resonance when connected to the power circuit. This frequency is also a critical piece of information in the design of the power circuit which will be discussed later. Figure 6 shows the schematic of the control circuit while figure 8 shows a picture of the PCB for the control circuit.
By looking at the schematic in figure 6 and the block diagram of the IC given in appendix A, one can see that the control circuit is quite a complicated setup. For this project however all that is required is to calculate the values of the resistors and capacitors placed on the pins of the IC. These components set up the IC to output the gate signals to the mosfets with the correct frequency, phase, delay and to ensure that the IC is protected from noise spikes. In the schematic, C10, C7, C6, C11, C5, and C1 are all noise bypass capacitors. Most of them are selected to be 0.1uF or 0.01uF. The capacitance on the input voltage is selected to be larger for filtering the lower frequency variations. The delay pins (pin 7 and pin 15) also have resistances connected to them, R1 and R8. These resistances set up the delay time for the transition between when Q1 goes low and Q3 goes high. The same goes for Q2 and Q4. They are calculated as follows.
Typical Delay Time (from data sheets) = 250ns, which translates to a duty cycle of approximately 47.5%.
Delay time = 62.5e-12/Idelay => Idelay = 0.25e-3
Idelay = Delay set voltage/ Rdelay, Delay set voltage typically = 2.4V
=>Rdelay = 2.4/0.25e-3 = 9.6kΩ
=>R1 = R8 = 9.6 kΩ
In order to calculate the ramp capacitor (C14) and the slope resistor (R12) one must refer to the pin descriptions in the data sheets for the IC. The pin descriptions are included in Appendix B. The equation that is used for determining these values is shown below.
dV/dT = Sense Voltage/(Rslope*Cramp) = (3.8V–1.3V)/10us
The sense voltage is the difference between the typical ramp peak clamping level and the offset voltage to the input of the PWM comparator (inside IC). 10us is the period of the square wave that is trying to be obtained at the outputs. This design is intended to be for a 100 kHz switching frequency so the period of the waveform is simply the inverse of 100 kHz. The equation now becomes the following.
Rslope*Cramp = 10us
Rslope was selected to be 10kΩ and Cramp was selected to be 1nF to satisfy the above equation.
Figure 5 shows a simplification of the error amplifier circuit and that it is the second input, along with the ramp capacitor, into the PWM comparator. Note that this can also be seen by looking at appendix A. In order for the PWM comparator to toggle, the input voltage to the non-inverting error amplifier input must exceed 1.3V. A voltage divider with a 10k potentiometer and 5k resistor were used to be able to vary the voltage input to the error amplifier. Using these values the input voltage can be varied from 0V to about 3.3V. When adjusting the pot the phase will change between the outputs A, B and outputs C, D.
The switching frequency is set by a parallel capacitor and resistor being connected to the freqset pin (pin 16). Once again by using a potentiometer one is able to vary the switching frequency by adjusting the resistance. Referring again to figure 6 one can see that R9, P2, and C12 make up the configuration.
Figure 5 - Error amplifier circuit (internal to control IC)
Figure 6 - Schematic of the control circuit
In order to set the switching frequency of the gate signals one must refer to the data sheets of the IC to obtain values for the resister and capacitor that have to be placed in parallel and to the frequency set pin. Figure 7 shows a simplified oscillator schematic inside the IC and a graph that is used in determining the values for the RC circuit.
Figure 7 - Oscillator schematic and frequency behavioural graph
The goal for this converter was to obtain a switching frequency of 100 kHz. The resistor and capacitor were then selected according to the above graph. Choosing a capacitor of 4700pF and a resistor of about 11kΩ will reveal the 100 kHz switching frequency. The potentiometer (P2) was then put in place to be able to vary the resistance from 5kΩ to 15kΩ. This allows the switching frequency to be adjusted from about 80 kHz to about 200 kHz in order to fine tune the circuit later when the power board is connected.
Figure 8 - control circuit
Figure 8 shows the layout of the control PCB including the switching frequency adjustment potentiometer and the gate signal phase adjustment potentiometer. Both of these adjustments are needed to fine tune the switching frequency when the power circuit is connected and to vary the phase between the gate signals on Q1, Q3 and Q2, Q4. When the input voltage to the power circuit is within the specified range of 110Vdc to 130Vdc the phase adjustment can be varied to adjust the output to remain constant at 120Vdc. This is the specification for the design; however in practice the potentiometer used has enough range to keep the output constant when the input varies even more. This translates to the phase being adjusted from 0 to 180 degrees between mosfets Q1, Q3 and Q2, Q4. The phase is represented by the symbol, φ in figure 3.
2.3 Driver Circuit
The driver circuit is simply a current amplification stage. The four output signals from the control circuit stage are not capable of supplying very much current (<1A). The power mosfets used on the power circuit have quite large gate-drain and gate-source capacitances. When switching these mosfets on and off at high frequencies such as 100 kHz they draw quite a large amount of ‘in rush’ current instantaneously. The current required to turn on one of these mosfets without delay is greater than 1 amp which is larger than the control circuit IC is capable of sourcing.