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ECE 4501 Power Systems Laboratory Manual Rev 1.0 8.0 DC/DC SWITCHING CONVERTERS 8.1 DC/DC STEP-DOWN POWER SUPPLY 8.1.1 PRE-LAB DESIGN It is desired to design and build a simple Firing Control Circuit for a Pulse-Width Modulated (PWM) Chopper. The circuit will consist of a 555 Timer biased as an Astable Multi-Vibrator. A bypass diode and potentiometer will provide the ability to maintain a relatively constant frequency of oscillation as the potentiometer is turned to vary the pulse width. FIGURE 8-1 Build this circuit on a prototype board for use in Lab 8. Make sure that all +5 volt and ground connections come from a common rail. The +5 V and ground potentials will be provided in the laboratory. (Your Instructor Probably Has The Parts You Need) Test the circuit to ensure proper function and bring it to lab at your designated time. Only ONE circuit per lab group is necessary. - 1- ECE 4501 Power Systems Laboratory Manual Rev 1.0 8.1.2 OBJECTIVE To gain insight into the components that make up a switching power supply and study methods of building them. 8.1.3 DISCUSSION In AC systems, voltage level is easily and efficiently changed with a transformer. In DC systems, advances in power electronics have made it possible to efficiently “transform” DC levels as well. DC-to-DC conversion can be done quite simply with a Chopper, a device that is, in essence, just a switch that turns on and off the DC source, to raise or lower the average value of DC voltage seen at the load. In the circuit shown in Figure 8-2 below, the source voltage, Vs, is “chopped” to produce an average voltage somewhere between 0% and 100% of Vs. Thus the average value of the voltage applied to the Load, VL, is controlled by closing and opening the “switch”, Q1. To close the switch, a firing signal is delivered to the gate of the MOSFET, causing it to conduct between source and drain. To open the switch, the firing signal is removed and the MOSFET is self-biased to stop conducting. If the switch is opened and closed periodically, the voltage seen at the load will sometimes be Vs and sometimes Zero. The average value seen at the load will lie somewhere in between, related to the amount of time the switch is open and the amount of time it is closed. This is called Pulse Width Modulation (PWM). FIGURE 8-2 To understand PWM, it is useful to examine what happens during one full cycle of closing and opening of the switch, called the modulation period. In discussing the period of modulation, let time be divided into uniform periods of one millisecond each and let a period be called T, the modulation period. During T, there is a time, t0 to t1, during which the MOSFET Q1 is on, and a time, t1 to t2, during which it is off, as indicated in the Figure 8-3 below. This is true for each period and therefore Q1 turns on and off 1000 times every second when T = 1 ms. - 2- ECE 4501 Power Systems Laboratory Manual Rev 1.0 FIGURE 8-3 When Q1 is on, Vs volts are applied to the motor load for t 1 milliseconds. When Q1 is off, zero volts are applied to the load. However, the motor current, I a, is still allowed to circulate through the diode. The magnitude of the motor current will diminish between t 1 and t2 as losses in the motor dissipate energy. The voltage, Vm, seen by the motor load can be expressed in terms of the source voltage, Vs, and the “ON” time, t1, and the period of modulation, T. The equation is: Vm = Vs where = t1 / T The symbol is called the Duty Cycle. As duty cycle is increased from 0% to 100%, the average voltage applied to the motor increases from 0 to Vs volts and the motor speeds up. As seen in Figure 8-3, the output voltage of the PWM Chopper is a square-wave and the output current is saw-toothed. The noisy output of the chopper was acceptable in Lab 9 where the connected load was a DC Motor with an inherently long time constant. However, in general, power supplies must possess certain features to make them safe and useful: Anti-Reverse: This feature minimizes the harmful effects of applying the wrong polarity to the load. A simple anti-reverse mechanism is a power diode in main line of the power supply to prevent reverse current. Overcurrent Protection: Disconnects the power supply from its source if output current exceeds a safe level. A fuse can provide protection from overcurrent. Output Filtering: To minimize the voltage “ripple” seen by the load. In a chopper circuit, a series inductor and shunt capacitor placed between the MOSFET switch and the load can provide effective filtering when properly sized. - 3- ECE 4501 Power Systems Laboratory Manual Rev 1.0 Voltage Regulation: To increase both the accuracy and precision of the output voltage, closedloop control is added. Both voltage feedback and current feedback schemes are used in industry. Either scheme can be complicated. The basic Buck Chopper circuit is shown below: FIGURE 8-4 In the circuit shown in Figure 8-4 above, the source voltage, Vs, is “chopped” to produce an average voltage somewhere between 0% and 100% of Vs. Thus the average value of the voltage applied to the Load, VL, is controlled by closing and opening the “switch”, Q1. To close the switch, a firing signal is delivered to the gate of the MOSFET, causing it to conduct between source and drain. To open the switch, the firing signal is removed and the MOSFET is self-biased to stop conducting. In PWM, the switch is closed and opened every modulation period. The series inductor and shunt capacitor in the above circuit form a low-pass filter that works to limit the rate of change in current and voltage at the source. The result is smoother waveforms on the source side of the MOSFET during chopping. A similar low-pass filter should also be inserted on the load side. For proper ON-OFF switching, the gate of the MOSFET must be biased with respect to its source terminal. A P-Channel MOSFET will start to conduct from source to drain when the gate terminal is biased negatively relative to the source terminal. When the voltage at the gate with respect to the source (Vgs) is about –5 Volts, the MOSFET will be “ON” and will conduct between source and drain with Rds of approximately 0.5 Ohms. Most power MOSFETS cannot withstand a Vgs voltage of greater than +/- 20 Volts. The biasing circuit for the gate must therefore be able to apply a Vgs voltage of 0 Volts (or greater) when it is desired that the MOSFET be OFF, and a Vgs of –5 to –15 volts when the ON condition is desired. A typical biasing circuit for a P-Channel MOSFET is shown below: - 4- ECE 4501 Power Systems Laboratory Manual FIGURE 8-5 8.1.3 INSTRUMENTATION Power Supply Module DC Metering Module Smoothing Inductor Module Resistance Module Capacitance Module Power Diode Module Chopper Circuit Board Oscilloscope Firing Circuit EMS 8821 EMS 8412 EMS 8325 EMS 8311 EMS 8331 EMS 8842 -NA-NA-NA- 8.1.4 PROCEDURE 1) Connect the following circuit: - 5- Rev 1.0 ECE 4501 Power Systems Laboratory Manual Rev 1.0 FIGURE 8-6 2) Select the 7-N meter position on the Lab-Volt power supply. Turn on the oscilloscope. Set it up to trigger on Ch1 and to measure Ch1 – Duty Cycle, Ch1-Frequency, Ch2 – Amplitude (pkpk), Ch2 – Mean (average) 3) Make sure that the Lab-Volt Voltage Control is turned fully counterclockwise and turn on the Power Supply. 4) Slowly turn the voltage control clockwise until the voltage output 7-N is 20 Volts (about 15%). 5) Adjust the potentiometer on the PWM firing circuit (measuring duty cycle on channel 1 of the oscope) to 12.5% duty cycle and use it as input to the Chopper Circuit Board. 6) Read the DC Voltmeter across the load. It should read about 2.5 volts. If it reads Zero Volts, there is a problem somewhere in the circuit. Recheck the wiring and verify that the Firing Control Circuit is working properly. When a non-zero reading is available, record it in the table below. 7) Change the duty cycle to 25% on the Chopper by adjusting the potentiometer on the Firing Control Circuit. It is OK to leave the power supply on. 8) Measure the average load voltage (DC Voltage) for duty cycles of 12.5%, 25%, 50% and 75% and record them in the table provided. - 6- ECE 4501 Power Systems Laboratory Manual Average Load Voltage Signal/Duty Cycle Voltage A / 12.5% B / 25% C / 50% D / 75% Rev 1.0 Vdc Vdc Vdc Vdc 9) Select Firing Signal B as input to the Chopper. Fine-tune the Oscilloscope to display the load voltage waveform. Sketch it below: (A sketch includes volts/div and secs/div!!) Figure 8-7 10) Now change the frequency of the clock signal on the Firing Circuit to approximately 10 Hz. (Putting an additional 10 F in parallel with the existing charging capacitor should do it) No need to turn off the power supply. 11) What is the immediate result in the reading on the DC Voltmeter Module? ___________________________________________________________ Why? ______________________________________________________ ___________________________________________________________ 12) Remove the 10 F capacitor and TURN OFF THE POWER SUPPLY, leaving the voltage control untouched. OUTPUT FILTERING: 13) To smooth the output waveform, it is necessary to store excess energy when the chopper is ON and return it to the load when the chopper is OFF. Add a free-wheeling Diode, a series inductor and a shunt capacitor to the circuit used above to make the following circuit: - 7- ECE 4501 Power Systems Laboratory Manual Rev 1.0 FIGURE 8-8 14) Turn OFF all the Capacitor switches (Zero micro-farads) on the Capacitor Module. Turn ON the power supply at 20 volts. 15) Apply a 50% duty cycle Firing Signal to the Chopper Board and record the value of the DC Voltmeter across the load. This is the average voltage. ______________________________ Vdc. 16) Observe the waveform of the load voltage as seen on the oscilloscope. Record the Peak Value of the resistor voltage, and sketch the waveform on the graph provided. _______________________________ Vpeak - 8- ECE 4501 Power Systems Laboratory Manual Rev 1.0 Figure 8-9 17) Calculate the percent ripple with the following equation: Ripple (%) = Vpeak – Vavg x 100% Vavg Percent Ripple = ________________%, Zero uF 18) Now switch on 2.2 microfarad capacitor on the Capacitor Module. Calculate the percent ripple: Percent Ripple = ________________%, 2.2 F 19) Now switch on 4.4 microfarad capacitor in parallel with the 2.2 microfarad capacitor on the Capacitor Module. Calculate the percent ripple: Percent Ripple = ________________%, 6.6 F 20) Now switch on all three capacitors in the Capacitor Module (for a total of 15.4 F). Calculate the percent ripple: Percent Ripple = ________________%, 15.4 F 21) Compare the load voltage waveform currently on the oscilloscope with your previous sketch. Does it look smoother? __________________ 22) Has the average voltage (as indicated by the DC voltmeter) increased as capacitance increased? _________________ If so, why? _________________________________________ 23) Return the voltage control to Zero percent and turn OFF the power supply. - 9- ECE 4501 Power Systems Laboratory Manual Rev 1.0 8.1.5 CONCLUSIONS 1. Explain the purpose of the free-wheeling diode in the filter: ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 2. What other specific hardware would this power supply need to be a truly useful as a variable power supply? (i.e. what components would provide the additional features of power supplies outlined at the Introduction of this lab?) ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 3. What is the airspeed velocity of an unladen Swallow? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ - 10 -