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ECE 556 Power Electronics: DC-DC Converters
Lab 6 Procedure
In this lab we will choose the components for the SG3525A PWM control IC.
Prelab
We will be building a buck-boost supply with the following specifications:
• 12 V ≤ Vin ≤ 14 V
• Output Voltage = -25 V
• Output current less than or equal to 2 A.
• Output voltage ripple: Less than 100 mV.
Make the following calculations:
• Choose the proper switching frequency.
• Verify that the inductor and capacitor values specified in Figure 1 achieve the specifications.
• Calculate the inductor currents (I1 and I2) at full load.
• Calculate the peak diode current and choose a diode.
• Calculate the capacitor RMS ripple current.
Design Procedure
We will start with the circuit shown in Figure 1. Note in this figure that the gate drive output (pin 13) of
the SG3525A is not connected. We will test the waveforms of the control chip before we hook up the
MOSFET switches. We will now choose the components for the SC3525A. Review the datasheet of the
SG3525A while reading this
1. The SG3525A is a pulse width modulation (PWM) feedback controller. It uses negative feedback
to force the voltage at the inverting input (pin 1) of the error amp to be equal to the voltage at the
non-inverting input (pin 2) of the error amp. (See the SG3525A block diagram on page 1 of the
data sheet). Choose R11 and R12 to produce 2.5 volts at pin 2 of the SG3525A.
2. This PWM controller has two complementary outputs called Output A (pin 11) and Output B
(pin 14). Both outputs have a duty cycle limited to less than 50% and both outputs are never high
at the same time. If the frequency of each output is F, then the frequency of the sum of the
outputs is 2F. Example waveforms are shown below:
20V
10V
0V
V(OUTPUT_A)
20V
10V
0V
V(OUTPUT_B)
20V
10V
SEL>>
0V
0s
20us
40us
V(OUTPUT_A)+ V(OUTPUT_B)
60us
80us
100us
120us
140us
160us
180us
200us
Time
The frequency of each output is 10 kHz. However, since we are using both outputs in parallel,
the switching waveform in the circuit will have a frequency of 20 kHz as shown in the bottom
waveform.
This PWM IC also allows you to control a parameter called the dead time. This amount
of time is a fixed length where both outputs are off. The negative feedback can easily force the
PWM portion of the controller to go to 100% duty cycle, which means that the switches will
never turn off. If the switches never turn off, the inductor will never discharge and the inductor
current will become very large. The dead time is used to guarantee that both MOSFET switches
have a guaranteed amount of off time, preventing this problem. You can also think of the dead
time as a method of limiting the duty cycle to some maximum value. In our design, the duty
cycle should be about 50% when the supply is working properly. As a circuit protection, we can
use the dead time to limit the duty cycle to something larger than 50% but significantly less than
100%. This eliminates one failure mode of our supply.
a. Choose Rt, Rd, and Ct to produce your calculated switching frequency and a dead time
that limits the maximum duty cycle to 80%. An equation is given on the bottom of page 3
of the data sheet, and there are graphs on pages 6 and 7 of the data sheet to help choose
the values. Note that the frequency of the ramp is twice the frequency of one of the gate
drive outputs.
3. When you first turn on your supply, the output voltage is much less than − 25 V. The feedback
tells the controller to go to maximum pulse width. Since the output voltage is close to the input
voltage, there is not a large voltage across the inductor when it is discharging. Thus, at start-up,
the inductor does not discharge much and the feedback forces maximum pulse width, which
charges the inductor as much as possible. The result is a large inductor current at start-up. To
prevent this problem, the SG3525A has a soft-start feature. At start-up, the soft-start feature
limits the pulse-width. Initially the pulse-width is limited to zero, and then is slowly allowed to
increase. Once the pulse-width from the soft-start is greater than the pulse width from the
feedback, the feedback takes over. The soft-start only limits the pulse width at start-up or when
an over-current fault is detected.
The rate at which the pulse-width is allowed to increase during start-up is determined by
CSS. This capacitor is charged by a 50 µA current source. The cap voltage starts at zero and then
It
. When the capacitor reaches approximately 2.5
charges positive with the equation ∆V =
CSS
volts, the soft-start is complete, and the feedback takes over. Choose CSS so that the soft-start
takes between 0.5 and 1 second. (You can choose a longer time if you want to see the output
voltage slowly increase. – This is annoying when you want to actually use the supply.)
4. Pin 10 of the SG3525A is the shut down pin. This can be used as an on-off control as well as a
cycle-by-cycle current limit. During a cycle, when the voltage a pin 10 goes above 1 volt, the
output pulse (pins 11 or 14) will be set to zero and the MOSFETs will be turned off for the cycle.
This is the cycle-by-cycle current limit. This mode is used to monitor the switch current and turn
off the switch of the current becomes too large in any cycle.
If the voltage at pin 10 remains above 1 volt for an extended period of time, the SG3525
discharges CSS and initializes a soft-start. This mode can be thought of as an average current
limit. If the current is too high, the supply turns off and then continually attempts to restart.
We will use pin for both purposes. We are monitoring the switch current with the CS1050
current transformer. This is a 50 to 1 transformer. The current through the output winding of the
CS1050 is 50 times less that the current through the switches. This current then flows through R3
producing the voltage called I_Lim.
All switching supplies produce a lot of noise, and pin 10 of the SG3525A is designed to
reach quickly to protect the MOSFET switches. Thus, any noise on pin 10 can falsely trip the
over current limit. To prevent this problem, carefully wiring is needed and a low-pass filter
comprised of R10 and C14 has been added.
a. Choose R3 to limit the switch current to some maximum that is appropriate for your
design.
b. Choose R10 and C14 to choose a cut-off frequency of 100 kHz.
c. Before wiring up this portion, ask me about the layout for this portion of the circuit.
Construction and Testing
Wire the circuit of figure 1. Note that the circuit has a signal ground and a power ground. To keep high
currents from flowing through the signal ground, we make the grounds separate, and then connect them
at a single point. This point is shown on the circuit drawing. Also note that we are wiring up the 12 volt
supply as a two separate networks as well. Each 12 V network should be separate and then connected
together with a single wire connection.
Using the following wire colors for your circuit:
Black – Signal Ground
Green – Floating Gate Drive Connections
Red - 12V
Yellow – (-)12V
Blue – Miscellaneous connections
Before testing, you will need to add the two jumpers and the pull-up resistor as shown in Figure 2. These
items are temporary and will be removed later. The jumper across the gate and source of the MOSFET is
necessary to make sure that the MOSFET is off. The jumper from pin 1 to ground of the SG3525A tells
it that the output voltage is zero and that it should go to maximum duty cycle. The pull-up resistor
connected to pin 13 is necessary since we are using the drive outputs of the SG3525A as open-collector
outputs. If you recall, an open-collector output can only go low and requires a resistor to pull the output
up to a high voltage.
Measure and verify the following:
1. The waveform a pin 5 is a ramp close to your design frequency.
2. The waveform at pin 13 is a square wave at the correct switching frequency with the appropriate
dead time.
5
4
3
2
1
U1
Positive Input
J1
A
B
Input 12 - 14 Volts
-
+
C1
+
C2
0.1u
G1
Poly
Q1
IRF540
CS1050
S1
D
UF1006
J2
D
D2
I_Lim
D1
J3
Output
Input Ground
R3
R
Output
Voltage
L1
270u
C3
C4
+
10000U
Output Voltage: -25 Volts
Max Output Current: 2 A
0.1u
Poly
J4
Ground
Out
Single wire
connection
Single wire
connection
C
C
Signal Ground
Power Ground
B
B
12V
C7
100u
7
DISC
6
RT
VCC
CT
RD
RT
C14
+
R10
Vout
SD
1
-V
9
COMP
CSS
OSC
4
VREF
16
SYNC
3
SG3525A
10
8
RC Filters
Close to
SG3525
13
A
+V
2
12
14
11
I_Lim
VCC
Vref
CSS
CT
A
15
U5
5
GND
OUTB
OUTA
+
C8
0.1 uF
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
Monday, January 19, 2004
Sheet
1
1
of
1
5
4
3
2
1
U1
Positive Input
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
Jumper
Input Ground
R3
R
Output
Output
Voltage
L1
270u
C3
C4
+
10000U
Output Voltage: -25 Volts
Max Output Current: 2 A
0.1u
Poly
J4
Ground
Out
Single wire
connection
Single wire
connection
C
C
Signal Ground
Power Ground
B
B
12V
C7
100u
+
C8
0.1 uF
Pull-Up
470
CT
7
DISC
6
RT
VCC
5
RD
C14
+
R10
4
16
SYNC
3
SG3525A
10
SD
1
-V
9
COMP
CSS
A
+V
2
R11
ECE Department
5500 Wabash Avenue
Terre Haute, IN 47803
Ph: (812) 877-8512
FAX: (253) 369-9536
Figure 2
+
+
Vout
OSC
VREF
Pull-Up
Vref
8
RC Filters
Close to
SG3525
13
12
14
11
I_Lim
VCC
GND
OUTB
OUTA
RT
CSS
CT
A
15
U5
C17
R12
Name:
Marc E. Herniter
Size
Document Name
Jumper
Class: ECE 556
Rev
1
12 Volt to -25 Volt Buck-Boost Converter
Date:
5
4
3
2
Sheet
Monday, January 19, 2004
1
1
of
1