ECE 556 Power Electronics: DC-DC Converters Lab 6 Procedure

Transcription

1 EE 6 Power Electronics: - onverters Lab 6 Procedure In this lab we will choose the components for the SG PWM control I. Prelab We will be building a buck-boost supply with the following specifications: V Vin V Voltage = - V current less than or equal to. voltage ripple: Less than 00 mv. Make the following calculations: hoose the proper switching frequency. Verify that the inductor and capacitor values specified in Figure achieve the specifications. alculate the inductor currents (I and I) at full load. alculate the peak diode current and choose a diode. alculate the capacitor RMS ripple current. esign Procedure We will start with the circuit shown in Figure. Note in this figure that the gate drive output (pin ) of the SG is not connected. We will test the waveforms of the control chip before we hook up the MOSFET switches. We will now choose the components for the S. Review the datasheet of the SG while reading this. The SG is a pulse width modulation (PWM) feedback controller. It uses negative feedback to force the voltage at the inverting input (pin ) of the error amp to be equal to the voltage at the non-inverting input (pin ) of the error amp. (See the SG block diagram on page of the data sheet). hoose R and R to produce. volts at pin of the SG.. This PWM controller has two complementary outputs called (pin ) and (pin ). oth outputs have a duty cycle limited to less than 0% and both outputs are never high at the same time. If the frequency of each output is F, then the frequency of the sum of the outputs is F. Example waveforms are shown below: V(OUTPUT_) V(OUTPUT_) SEL>> 0s 0us 0us 60us 80us 00us 0us 0us 60us 80us 00us V(OUTPUT_) V(OUTPUT_) Time The frequency of each output is 0 khz. However, since we are using both outputs in parallel, the switching waveform in the circuit will have a frequency of 0 khz as shown in the bottom waveform.

2 This PWM I also allows you to control a parameter called the dead time. This amount of time is a fixed length where both outputs are off. The negative feedback can easily force the PWM portion of the controller to go to 00% duty cycle, which means that the switches will never turn off. If the switches never turn off, the inductor will never discharge and the inductor current will become very large. The dead time is used to guarantee that both MOSFET switches have a guaranteed amount of off time, preventing this problem. You can also think of the dead time as a method of limiting the duty cycle to some maximum value. In our design, the duty cycle should be about 0% when the supply is working properly. s a circuit protection, we can use the dead time to limit the duty cycle to something larger than 0% but significantly less than 00%. This eliminates one failure mode of our supply. a. hoose R t, R d, and t to produce your calculated switching frequency and a dead time that limits the maximum duty cycle to 80%. n equation is given on the bottom of page of the data sheet, and there are graphs on pages 6 and 7 of the data sheet to help choose the values. Note that the frequency of the ramp is twice the frequency of one of the gate drive outputs.. When you first turn on your supply, the output voltage is much less than V. The feedback tells the controller to go to maximum pulse width. Since the output voltage is close to the input voltage, there is not a large voltage across the inductor when it is discharging. Thus, at start-up, the inductor does not discharge much and the feedback forces maximum pulse width, which charges the inductor as much as possible. The result is a large inductor current at start-up. To prevent this problem, the SG has a soft-start feature. t start-up, the soft-start feature limits the pulse-width. Initially the pulse-width is limited to zero, and then is slowly allowed to increase. Once the pulse-width from the soft-start is greater than the pulse width from the feedback, the feedback takes over. The soft-start only limits the pulse width at start-up or when an over-current fault is detected. The rate at which the pulse-width is allowed to increase during start-up is determined by SS. This capacitor is charged by a 0 µ current source. The cap voltage starts at zero and then It charges positive with the equation V =. When the capacitor reaches approximately. SS volts, the soft-start is complete, and the feedback takes over. hoose SS so that the soft-start takes between 0. and second. (You can choose a longer time if you want to see the output voltage slowly increase. This is annoying when you want to actually use the supply.). Pin 0 of the SG is the shut down pin. This can be used as an on-off control as well as a cycle-by-cycle current limit. uring a cycle, when the voltage a pin 0 goes above volt, the output pulse (pins or ) will be set to zero and the MOSFETs will be turned off for the cycle. This is the cycle-by-cycle current limit. This mode is used to monitor the switch current and turn off the switch of the current becomes too large in any cycle. If the voltage at pin 0 remains above volt for an extended period of time, the SG discharges SS and initializes a soft-start. This mode can be thought of as an average current limit. If the current is too high, the supply turns off and then continually attempts to restart. We will use pin for both purposes. We are monitoring the switch current with the S00 current transformer. This is a 0 to transformer. The current through the output winding of the S00 is 0 times less that the current through the switches. This current then flows through R producing the voltage called. ll switching supplies produce a lot of noise, and pin 0 of the SG is designed to reach quickly to protect the MOSFET switches. Thus, any noise on pin 0 can falsely trip the over current limit. To prevent this problem, carefully wiring is needed and a low-pass filter comprised of R0 and has been added. a. hoose R to limit the switch current to some maximum that is appropriate for your design. b. hoose R0 and to choose a cut-off frequency of 00 khz.

3 c. efore wiring up this portion, ask me about the layout for this portion of the circuit. onstruction and Testing Wire the circuit of figure. Note that the circuit has a signal ground and a power ground. To keep high currents from flowing through the signal ground, we make the grounds separate, and then connect them at a single point. This point is shown on the circuit drawing. lso note that we are wiring up the volt supply as a two separate networks as well. Each V network should be separate and then connected together with a single wire. Using the following wire colors for your circuit: lack Signal Ground Green Floating Gate rive onnections Red - V Yellow (-)V lue Miscellaneous s efore testing, you will need to add the two jumpers and the pull-up resistor as shown in Figure. These items are temporary and will be removed later. The jumper across the gate and source of the MOSFET is necessary to make sure that the MOSFET is off. The jumper from pin to ground of the SG tells it that the output voltage is zero and that it should go to maximum duty cycle. The pull-up resistor connected to pin is necessary since we are using the drive outputs of the SG as open-collector outputs. If you recall, an open-collector output can only go low and requires a resistor to pull the output up to a high voltage. Measure and verify the following:. The waveform a pin is a ramp close to your design frequency.. The waveform at pin is a square wave at the correct switching frequency with the appropriate dead time.

4 - Positive Input U J Input - Volts 0.u S00 G Q IRF0 J UF006 S J Input Ground R R L 70u Voltage 0000U 0.u Voltage: - Volts Max urrent: J Ground Out Signal Ground Power Ground V 8 0. uf 7 00u T R 7 6 U T IS V V OS VREF 6 Vref R Filters lose to SG R0 Vout 0 9 S -V OMP SS 8 SS SG SYN GN OUT OUT V 7 R R Figure EE epartment 00 Wabash venue Terre Haute, IN 780 Ph: (8) FX: () Name: Marc E. Herniter lass: EE 6 Size ocument Name Rev Volt to - Volt uck-oost onverter ate: Monday, January 9, 00 Sheet of

5 - Positive Input U J Input - Volts 0.u S00 G Q IRF0 J UF006 S J Input Ground R R Jumper L 70u Voltage 0000U 0.u Voltage: - Volts Max urrent: J Ground Out Signal Ground Power Ground V 8 0. uf 7 00u Pull-Up T R 7 6 U T IS V V OS VREF 6 Vref 70 Pull-Up R Filters lose to SG R0 Vout 0 9 S -V OMP SS 8 SS SG GN OUT OUT SYN V 7 R R Figure EE epartment 00 Wabash venue Terre Haute, IN 780 Ph: (8) FX: () Jumper Name: Marc E. Herniter lass: EE 6 Size ocument Name Rev Volt to - Volt uck-oost onverter ate: Monday, January 9, 00 Sheet of