Switching-Mode Regulators
Convert an unregulated DC voltage
into a regulated DC voltage
ECE 442 Power Electronics
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Buck Regulator
Output Voltage is Less Than the Input Voltage
BJT Chopper
Control Voltage
L-C Filter
Load
Freewheeling Diode
ECE 442 Power Electronics
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Mode 1 Operation – Q1 switched ON
Input current flows through L, C, and the Load – rises linearly
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Mode 2 Operation – Q1 switched OFF
Inductor current continues to flow through L, C, the Load,
and Dm, falling linearly until Q1 is turned ON again.
ECE 442 Power Electronics
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Waveforms for the Buck Regulator
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Summary for the Buck Regulator
• The peak-to-peak ripple current is
Vs k (1  k )
I 
fL
• The peak-to-peak ripple voltage is
Vs k (1  k )
Vc 
8LCf 2
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Summary for continuous current case
• The critical value of the inductor is
(1  k ) R
Lc  L 
2f
• The critical value of the capacitor is
1 k
Cc  C 
2
16 Lf
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Boost Regulator
Output Voltage is Greater Than the Input Voltage
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Mode 1 Operation – M1 switched ON
Input current flows through L and M1 (linearly)
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Mode 2 Operation – M1 switched OFF
Current that flowed through the transistor now flows
through L, C, the Load, and diode Dm until transistor
M1 is turned ON again.
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Waveforms for the Boost Regulator
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Summary for the Boost Regulator
• The peak-to-peak ripple current is
Vs k
I 
fL
• The peak-to-peak ripple voltage is
Iak
Vc 
fC
ECE 442 Power Electronics
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Summary for continuous current case
• The critical value of the inductor is
k (1  k ) R
Lc  L 
2f
• The critical value of the capacitor is
k
Cc  C 
2 fR
ECE 442 Power Electronics
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Buck-Boost Regulator
The output voltage could be less than or greater than the
input voltage
The polarity of the output voltage is opposite to the polarity
of the input voltage Vs – Inverting Regulator
ECE 442 Power Electronics
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Mode 1 Operation – Q1 turned ON
Input current flows through inductor L and Q1 linearly.
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Mode 2 Operation – Q1 switched OFF
Current flowing through L now flows through L, C, Dm
and the Load.
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Waveforms for the Buck-Boost Regulator
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Summary for the Buck-Boost Regulator
• The peak-to-peak ripple current is
Vs k
I 
fL
• The peak-to-peak ripple voltage is
Iak
Vc 
fC
ECE 442 Power Electronics
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Summary for continuous current case
• The critical value of the inductor is
(1  k ) R
Lc  L 
2f
• The critical value of the capacitor is
k
Cc 
2 fR
ECE 442 Power Electronics
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Converter with a highly inductive Load
Input Current is “pulse”
shaped
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Connect an input filter
Filter out the converter-generated harmonics from the
supply line.
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Equivalent circuit for the convertergenerated harmonic currents
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Example 5.10
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Buck Chopper for PSpice Simulation
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Results from PSpice
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Fourier Coefficients of the input current
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Example 5.10 in MultiSim
L1
681.82uH
Vy
Q1
11
0V
12
5
L2
40.91uH
6
4R1
2 Ohm
BJT_NPN_VIRTUAL*
8
R2
250 Ohm
XFG1
Vs
110 V
7
D1
DIODE_VIRTUAL
1
R3
1 Ohm
C1
8.33uF
3
Vx
0V
0
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Fourier Coefficients of the Input Current
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