Transcript Elektroniczne Układy i Systemy Zasilania

SWITCH-MODE POWER
SUPPLIES AND SYSTEMS
Lecture No 6
Silesian University of Technology
Faculty of Automatic Control, Electronics
and Computer Sciences
Ryszard Siurek Ph.D., El. Eng.
Switch-Mode Power Converters
Application of the switching transformer
- ensures galvanic isolationbetween output and input circuit (safety
regulations, output voltage polarity not restricted)
- small transformer dimensions according to high switching frequency
- output voltage higher or lower than the input voltage independently of
switching regulator configuration
- possibility of efficient operation (with optimum duty cycle) in presence of
high difference between input and output voltage (mains input voltage -
very low output voltage)
- several output voltages easily available
Single-ended forward converter
typical step-down regulator output filter
Ip
T
IS D2
Up
UIN
t T
Uw
Zp
ZS
CIN
L
D1
C
R0
U0
Za
Zp Up

n
ZS US
turns ratio
Transformer model
Ip
Up
rp
Llp
Lls
Lp
FM
rS  rSn2 I*= I /n
w
w
ideal
transformer
U*
p = USn
US
IM
n
Assumptions: Llp, Lls = 0
rp, rs = 0
IS
Forward converter equivalent circuit
0<t<t
I cycle
transistor T - ON , diode D2 - ON, diode D1 - OFF
T Ip = I T
UIN
IM
FM
IS*
IS D
2
Up
US
Lp
IL
D1
L
R0 U0
C
IM
n
IT
IT
ILm in
n
IL
IM
I*w
IMmax
ILmax
IMmax
t
n
IMmax 
ILmax
is (t)  ILm in
ILmin
FM
IT  I*s  IM
FMmax
UIN
t
Lp
Us  U0
L
t
II cycle
t <t <T
Transistor T - OFF , diode D2 - OFF, diode D1 - ON
Da
UIN
D2
dUS
dUa
Za
Z F
UIN p M
Za Lp
UT I
M
Ua=UIN
IM
IT
IT
ILm in
I*w
IM
n
ILmax
IL
IMmax
ILmax
IMmax
t
Up= Uw e
Zp
Za
n
T
I0
ILmin
F M IM
FMmax
current in Zp
t
UT
IMmax
IS=0
I*s  0
Zp dUp
T Ip = 0
current in Za
t1
UIN
UT  UIN  Up
UIN
Zp
Za
Up
n
IL
D1
L
C
R0 U0
US=Up/n
1. When T switches off the overvoltage dUp
appears across Zp and is transformed as
dUS to the secondary side
2. Diode D2 switches off , overvoltage is
transformed to the winding Za and diode Da
switches on
3. The voltage Ua acrosss Za approaches the
value of UIN and can not rise any more
4. Voltage Ua=UIN is transformed to the
primary winding Zp and is limited on the
value Up
5. The core of the transformer is being
demagnetised during t1 (core reset)
Transformer core reset (magnetizing energy recovery)
– detailed analysis
UT
UT
iZa(t)
iZp(t)
T
UIN
UIN
dULL
UT  UIN
Zp
Za
t1
UIN
Zp Lp
Uw e
Za
Zp
t
Zs
Za
 UIN  dULL
real overvoltage
UIN
dULL LL
Zp
T
IZp
IMmax
Za
To keep dULL low, LL should be small
enough - it requires very good magnetic
coupling between the windings Zp and Za
IZa
IMm ax
Zp
Za
In practice Zp = Za and both windings are
bifilary wound
hence UTmax = 2UIN
IMm ax 
FM
FMmax
UIN UIN
t
t  t1  t
Lp
La 1
Full demagnetization of the transformer is possible under the following condition:
Τ  t  t1  t    0,5
Typical magnetizing energy recovery circuits
Da
Zp=Za
D2
Zp
CIN
ZS
L
D1
C
R0
U0
Za
UIN
Cs
T
Ds
Rs
overvoltage dumping circuit snubbar circuit
Disadvantages:
necessity of placing two bifilar windings , difficult construction, high
transformer cost, problems with insulation, duty cycle limited to  < 0,5, snubbar
circuit required to avoid voltage stress across the switching transistor
Advantages:
most of transformer magnetizing energy is recovered (higher efficiency),
only one switching transistor, simple transistor gate drive circuit
D2
Cs
Rs
Up
Zp
ZS
L
D1
C
R0
U0
UIN
CIN
T
UT=Up+UIN
When Uo rises (higher value of Rs), core reset
time t1 decreases and may be shorter than t.
That is why the duty cycle may be higher
( > 0,5)
Disadvantages:
magnetising energy is dissipated in Rs, lower efficiency, high power resistor
(resistors) are required, heating of some components, high coltage stress across the
tswitching transistor ( UT > 2UIN)
Advantages:
simple and cheap transformer, duty cycle not limited to 0,5, no extra voltage
spikes across the transistor – no influence of leakage inductance
This configuration is not used in practice due to excesive power losses
T1
D1
D2
Ip
Up Z
p
UIN
CIN
US
ZS
D1
Up=UIN
 < 0,5
T2
D2
L
C
R0
U0
Disadvantages:
two switching components, complicated drive circuits, higher cost
Advantages:
simple transformer, no excesive voltage spikes – problem of leakage
inductance does not exist, transistor voltahe does not exceed UIN
This configuration often used in power supplies with higher output power – usually
over 300 – 500W
Magnetic core behaviour, magnetising curve ,
core saturation
B T 
B T 
Bs
Bs
H
A
M 
 
H
H  I Z
lg
Core saturation in case of
improper transformer
design
M  B  S
-Bs
-Bs
iM(t)
F M IM
FMmax
magnetising
current
t
demagnetising
current
IMmax
t1
A
M 
 
Ip
FM
IM
t
iS*(t)
t
Core saturation as a result of incomplete core reset
(transformer demagnetisation)
FM
B T 
Bs
S  BS  S
t
H
A
M 
 
Ip
H  I Z
lg
M  B  S
-Bs
t
Output choke (inductance) saturation
B T 
IL
Bs
B0
I1
DB
I0
DH
H
H0(I0)
t
A
M 
 
H1(I1)
Ip
H0 
-Bs
I0  ZL
lg
t