SWITCH-MODE POWER SUPPLIES AND SYSTEMS

1
SWITCH-MODE POWER SUPPLIES AND SYSTEMS
Lecture No 4

• Silesian University of Technology
• Faculty of Automatic Control, Electronics
• and Computer Sciences
• Ryszard Siurek Ph.D., El. Eng.

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Linear regulator versus switching regulator
Basic functional diagrams
Linear regulator
Switching regulator
Uce
Io
Uin
Io
T1
Uout (Uo)
Uout (Uo)
Ku
T1
R1
CC
t
T
fs
RL
amp
RL
Uin
U1
-
e
fc

1 T
fs
R2
Cross-over frequency
Vr
U1
Uo Uin Uce const. Po Uo.Io
PLoss Uce.Io
Uin
t T
U1av Uin gUin
U1av
efficiency
t
Po Pin
Uo.Io Uin.Io
Uin - Uce Uin
T
g duty cycle
h
U1 U1av SUisin(2iPfpt)
i1
Uce gt Ucemin (2,5V) 0,3 lt h lt 0,5 typical
values
eliminated by low-pass filter
Power losses in T1 0 Filter power losses (LC)
0
h 1 (100)
Power loss in T1 - high
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STEP-DOWN (BUCK) SWITCHING REGULATOR
L
T
IL
Io
IT
ID

IC
UC
Uin
U0
U1
RL
UC
D
C
t
U0
T

• Assumptions
• Diode D and transitor T are perfect (ideal)
  switches
• Series resistance of the choke L is negligible
  (rL 0)
• Capacitance C is very large (DUc ltlt Uo)

EL
EL
T
T
I cycle
II cycle
Io
Io
D
D
Uin
Uin
EC
EC
T open, D closed
T closed, D open
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Basic waveforms in step-down switching regulator
U1
I cycle - equivalent circuit 0 lt t lt t
,
L

IL
rL 0
ILmin
I0
t
t
T

IT
UC
Ro
U0
ILmin
U1 Uin
t
ILmax

IL
UC
ltlt U0
,
Calculation of IL superposition method
ILmin
t
ID
ILmax
t
t
IC
t
ltlt1

UC
t
Uc(0)
inductor current swing
5

U1
II cycle - equivalent circuit t lt t lt T
L

rL 0
IL
ILmax
I0
t
t
T

IT
UC
Ro
U0
U0
ILmin
t

ILmax

UC
ltlt U0
IL
I0
Calculating IL - superposition method
ILmin
t
ID
ILmax
Similarly as before
ILmin
t
IC
inductor current swing
t
in steady state

UC
Uc(t)
t
Step-down regulator transfer function
6
Inductor capacitor selection exemplary
calculations for step-down regulator
Design specification UIN 10 15V, Uo
5V, Io 10A, f 100kHz Inductor current
amplitude (general rule) DIL lt 0,1
0,2 I0max we assume
DIL lt 1A
hence
for tmin for tmax
selected L 3 mH 4 mH
The AC voltage component across the capacitor is
approximately described by the equation
Przyjmijmy
Selected C 1000 mF