Download figure 10-1

ECE 4501
10.0
Power Systems Laboratory Manual
Rev 1.0
DC/DC SWITCHING CONVERTERS (LAB 7)
10.1 DC/DC STEP-DOWN POWER SUPPLY
10.1.0 PRE-LAB DESIGN
It 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 10-1
Build the following modules on a prototype board for use in Lab 10. Make sure that all
Vcc and Ground connections come from a common rail.
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.
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 which 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:
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ECE 4501
Power Systems Laboratory Manual
Q3,Q2,Q1,Q0
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Output A:
1
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Output B:
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
Output C:
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
Rev 1.0
Output D:
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
Each 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 period and  = 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.
10.1.1 OBJECTIVE
To gain insight into the components that make up a switching power supply and study
methods of building them.
10.1.2 DISCUSSION
In Lab 9, a Buck Chopper was used to vary the speed of a DC Motor. In essence, the
One-Quadrant Buck Chopper can be considered a crude step-down (bucking) power
supply. The output voltage of the 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.
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Power Systems Laboratory Manual
Rev 1.0
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 10-2
In the circuit shown in Figure 10-2 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, V L, 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 work to limit the rate of
change in current and voltage respectively. The result is smoother waveforms during
chopping.
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 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
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Power Systems Laboratory Manual
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when the ON condition is desired. A typical biasing circuit for a P-Channel MOSFET is
shown below:
FIGURE 10-3
10.1.3 INSTRUMENTATION
Power Supply Module
DC Metering Module
Smoothing Inductor Module
Resistance Module
Capacitance Module
Power Diode Module
Chopper Circuit Board
Oscilloscope
Firing Circuit
10.1.4 PROCEDURE
1) Connect the following circuit:
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EMS 8821
EMS 8412
EMS 8325
EMS 8311
EMS 8331
EMS 8842
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ECE 4501
Power Systems Laboratory Manual
FIGURE 10-4
2) Select the following equipment settings:
Oscilloscope
Option
Chan. 1 Sensitivity:
Vertical Mode:
Display Mode:
Time Base:
Trigger Source:
Trigger Slope:
Trigger Coupling:
Lab-Volt
Voltage Selector Knob
(EMS 8821)
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Setting
5 V / Div.
DC Coupled
Chopped
A
0.5 ms / Div.
External
Positive (+)
HF Rejection
7-N
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ECE 4501
Power Systems Laboratory Manual
Rev 1.0
3) Select Firing Control Signal A as input to the Chopper Circuit Board and as
trigger signal for the Oscilloscope.
4) Make sure that the voltage control is turned fully counterclockwise and turn
on the Power Supply. Turn on the Oscilloscope.
5) Slowly turn the voltage control clockwise until the voltage output 7-N is 20
Volts (about 15%).
6) Read the DC Voltmeter across the load. 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 on the Chopper by removing Signal A and putting
Firing Control Signal B into Chopper Circuit Board. It is OK to leave the
power supply on.
8) Measure the average load voltage for each available signal, A through D,
and record them below:
Average Load Voltage
Signal/Duty Cycle
Voltage
A / 12.5%
B / 25%
C / 50%
D / 75%
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:
Figure 10-5
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Power Systems Laboratory Manual
Rev 1.0
10) Now change the frequency of the clock signal on the firing circuit to
approximately 100 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?
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Why? ______________________________________________________
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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:
FIGURE 10-6
14) Apply Firing Signal B to the Chopper and record the value of the DC
Voltmeter across the load. This is the average voltage.
______________________________ Vdc.
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Power Systems Laboratory Manual
Rev 1.0
15) Observe the waveform of the load voltage as seen on the oscilloscope.
Record the Peak Value of the Voltage, and sketch it on the graph provided.
_______________________________ Vpeak
Figure 10-7
16) Calculate the percent ripple with the following equation:
Ripple (%) = Vpeak – Vavg x 100%
Vavg
Percent Ripple = ________________%, 2.2 F
17) 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
18) 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
19) Look at the load voltage waveform on the oscilloscope. Does it look
smoother? __________
20) Note that the average voltage has increased significantly. One moust lower
the duty cycle to maintain the desired average output voltage.
21) Return the voltage control to Zero percent and turn OFF the power supply.
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Power Systems Laboratory Manual
Rev 1.0
10.1.5 CONCLUSIONS
1.
Try to explain the purpose of the free-wheeling diode in the filter:
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2.
What other specific hardware would this power supply need to be a truly
useful device? (i.e. what components would provide the additional features
of power supplies outlined at the beginning of this lab?)
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3.
What is the airspeed velocity of an unladen Swallow?
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