FIGURE 8-1FIGURE 8-2Figure 8-3FIGURE 8-5FIGURE 8-6Lab-VoltVoltage Selector Knob (EMS 8821)Average Load VoltageSignal/Duty CycleVdcVdcVdcVdcFigure 8-7FIGURE 8-8Figure 8-9ECE 4501 Power Systems Laboratory Manual Rev 1.08.0 DC/DC SWITCHING CONVERTERS 8.1 DC/DC STEP-DOWN POWER SUPPLY8.1.1 PRE-LAB DESIGNIt is desired to design and build a simple Firing Control Circuit for a Pulse-Width Modulation (PWM) Chopper. The circuit will consist of three modules as shown below:FIGURE 8-1Build the following modules on a prototype board for use in Lab 8. Make sure that all Vcc and Ground connections come from a common rail. Vcc and ground potentials will be provided in the laboratory.Clock Module:Use a 555 Timer, resistors and capacitors to construct an Astable Multivibrator, which oscillates at or around 16000 Hz. You may substitute any oscillator circuit of your choice, as long as it can be adjusted easily to run at 600 and 16000 Hz with an output oscillating between Vcc and ground.MOD-16 Counter:Connect a 74LS169 chip to up-count by modulo-16 (0000 – 1111). Use the output of the Clock Module as the clock input. Bias the U/D’, Load’, etc, Pins to produce a standard up-count. Again,any chip will do, as long as it counts N*MOD-8, where N can be 1, 2, 3, etc.Combinational Circuit:Design a combinational circuit that uses the 4 outputs of the MOD-16 counter (or 3 outputs of a MOD-8 counter, etc) as inputs and creates the following output signals:-1 -ECE 4501 Power Systems Laboratory Manual Rev 1.0Q3, Q2, Q1, Q0 Output A: Output B: Output C: Output D:0000 1 1 1 10001 1 1 1 10010 0 1 1 10011 0 1 1 10100 0 0 1 10101 0 0 1 10110 0 0 1 10111 0 0 1 11000 0 0 0 11001 0 0 0 11010 0 0 0 11011 0 0 0 11100 0 0 0 01101 0 0 0 01110 0 0 0 01111 0 0 0 0TABLE 8AEach of the four output signals will represent a different Duty Cycle, . For example, Output A is Logic 1 for 1/8 of the modulation period and thus  = 0.125. Output C is Logic 1 for ½ the periodand  = 0.5. You may use TTL chips or any other, so long as they will work with Vcc = 5 Volts.Test the circuit to ensure proper function and bring it to lab at your designated time. Only ONE circuit per lab group is necessary.8.1.2 OBJECTIVETo gain insight into the components that make up a switching power supply and study methods of building them.8.1.3 DISCUSSIONIn 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 -2 -ECE 4501 Power Systems Laboratory Manual Rev 1.0in 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 8-2To under stand 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.FIGURE 8-3When Q1 is on, Vs volts are applied to the motor load for t1 milliseconds. When Q1 is off, zero volts are applied to the load. However, the motor current, Ia, is still allowed to circulate through the diode. The magnitude of the motor current will diminish between t1 and t2 as losses in the motor dissipate energy. -3 -ECE 4501 Power Systems Laboratory Manual Rev 1.0The 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 / TThe 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.Voltage Regulation: To increase both the accuracy and precision of the output voltage, closed-loop 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 8-4In 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 -4 -ECE