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 -