ECE 556 Power Electronics: DC-DC Converters
Lab 8 Procedure
In this lab we complete the design of our buck-boost regulator and then test the regulator at high
power. This lab has three parts: (1) Design of a low power negative power supply using a charge pump,
(2) design of the feedback components, and (3) testing of the power supply.
Design of a -12 V Charge Pump
The complete circuit for our buck-boost regulator is shown in Figure 1. The LM324 OP-AMP used in
the feedback path requires a low power negative supply. We will generate this supply using an inverting
charge pump. The circuit below uses a 555 timer configured as an astable multivibrator to drive an
inverting charge pump:
12V
+
R8
C11
100u
-12v
3
+
D7
DSCHG
GND
5
+
CV
1
+
C15
THR
TRG
C13
11DQ04
LM555C
6
2
D8
C12
OUT
+
7
VCC
R9
RST
4
U6
8
11DQ04
C16
0.01u
Design this circuit to produce an output of approximately -9 volts at about 100 mA. The LM 324 does
not draw very much current, so when you use this supply with the LM324, its output will be closer to
-12 volts. Test your supply for various loads and show that the supply meets your design specifications.
Feedback Design
We must now complete the design of the remaining feedback components.
o Choose R1, R2, R4, and R5 to produce 2.5 volts at Vout when the output voltage is -25 volts.
Low power Testing
o Verify the operation of the inverting charge pump.
o Verify the operation of the OPAMPs in the feedback loop.
Remaining Connections
There are a few connections that we need to remember to connect or remove before we start testing.
o Hook the output of the gate drive to the MOSFET (connect G1 and S1).
o Remove the jumper between G1 and S1.
o Remove the black static protective stuff we placed on the MOSFET.
o Remove the jumper between pin 1 of the SG3525A and ground.
o Remember to make the connection between pin 7 of the LM324 and pin 1 of the SG3525A. This
is labeled Vout in the circuit.
High Power Testing
o Use your protective eyewear when testing.
o Do not first test this supply unless I am present.
o Set the current limit of the input power supply to about 1 amp.
o With no-load hooked to the output of your supply, turn on the power, and see if the output goes to
-25 volts.
o If your supply works at no load:
1. Increase the current limit to 5 amps.
2. Add the load resistor and test your supply with an output current up to 2 amps.
High Power Measurements
1. For VIN = 12 volts, measure VIN, VOUT, IIN, and IOUT as you change the output power from zero
to full power. Obtain the following plots:
o Output voltage versus output current.
o Efficiency versus output power.
o The ratio of input current to output current versus output power.
2. For VIN = 14 volts, measure VIN, VOUT, IIN, and IOUT as you change the output power from zero
to full power. Obtain the following plots:
o Output voltage versus output current.
o Efficiency versus output power.
o The ratio of input current to output current versus output power.
3. With an oscilloscope measure the output ripple using the AC setting of the oscilloscope.
Measure the ripple at full load for VIN = 12 V and VIN = 14 V.
4. Measure the voltage at pin 10 of the SG3525A with the oscilloscope and measure the DC
output voltage of your converter with a Multimeter. Slowly increase the load and view these
signals. Once you reach the current limit, the output voltage should decrease as you increase
the output current. Increase the output current beyond 2 A and see if you can observe that the
peak inductor current is limited to about 8 A and that once the peak current limit is reached, the
output voltage of the supply starts to decrease. Reaching the peak current limit consistently
may also cause the soft-start to kick in and you will see the output voltage of your supply cycle
up and down.
5
4
3
2
1
U1
Positive Input
12V
J1
A
B
Input 12 - 14 Volts
Q1
IRF540
-
+
C1
+
C2
0.1u
G1
Poly
CS1050
S1
D
UF1006
J2
D
D2
I_Lim
D1
J3
Output
Input Ground
R3
R
Output
Voltage
L1
270u
C3
C4
R1
+
10000U
Output Voltage: -25 Volts
Max Output Current: 2 A
0.1u
Poly
J4
R2
Ground
Out
12V
R14
C
C
C5
0.1 uF
+
5
-
6
11
7
1
R4
LM324
+
3
-
2
11
Vout
Signal Ground
U2A
4
4
U2B
LM324
C6
Power Ground
0.1 uF
12V
-12v
R13
R5
+
R8
C11
100u
-12v
RST
THR
TRG
8
GND
C15
1
+V
0V
Vin
6
2
4
3
3
+
D7
C13
11DQ04
CV
5
+
2
OUT
DSCHG
LM555C
+
GND
U3
NKE1215S
D8
C12
+
7
VCC
R9
1
4
11DQ04
U6
C16
0.01u
B
B
12V
C7
100u
+
C8
0.1 uF
U4
1
8
R6
2
R
5
CT
7
DISC
VCC
5
RD
C14
+
R10
RT
SD
Vout
4
D4
31DQ04
31DQ04
+
C10
0.1 uF
PS9634
C9
100u
Poly
VREF
16
SYNC
3
S1
SG3525A
1
-V
9
COMP
CSS
OSC
D3
Vref
8
RC Filters
Close to
SG3525
6
10
13
A
+V
2
12
14
11
I_Lim
VCC
GND
OUTB
OUTA
RT
CSS
CT
A
15
U5
G1
R7
7
6
R11
+
ECE Department
5500 Wabash Avenue
Terre Haute, IN 47803
Ph: (812) 877-8512
FAX: (253) 369-9536
Figure 1
+
C17
R12
Name:
Marc E. Herniter
Size
Document Name
Class: ECE 556
Rev
1
12 Volt to -25 Volt Buck-Boost Converter
Date:
5
4
3
2
Sheet
Monday, January 19, 2004
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