Quasi-resonant CoolSET design guide
ICE2QRxx65/80x
Design Guide
7
8 August 2011
85 ~ 265 VAC
Snubber
C
bus
D
r1
~D
r4
R
CS
TL431
Optocoupler
R
b1
R
b2
R
c1
C
c1
C
c2
R
ovs2
R
ovs1
C
VCC
D
VCC
D
O
C
O
L
f
C
f
V
O
C
PS
Startup Cell
CoolSET
®
-Q1
CoolMOS
®
W
p
W
s
W
a
R
VCC
C
ZC
R
ZC2
R
ZC1
Drain
Zero Crossing Block
Power Management
Cycle-by-Cycle
current limitation
Active Burst Mode
PWM controller
Current Mode Control
Protections
VCC
CS
Control Unit
ZC
FB
GND
Figure 2 Typical Application of ICE2QRxx65/80x
The drain-source voltage of the power switch V
ds
will rise very fast after MOSFET is turned off. This is caused
by the energy stored in the leakage inductance of the transformer. A snubber circuit, RCD in most cases, can
be used to limit the maximum drain source voltage caused. After the oscillation 1, the drain-source voltage
goes to its steady value. Here, the voltage v
Refl
is the reflected value of the secondary voltage at the primary
side of the transformer and is calculated as:
n
V
V
V
do
out
Refl
(1)
where n the turns ratio of the transformer, which is defined in this document as:
P
S
/N
N
n
(2)
with N
p
and N
s
are the turns count of the primary and secondary winding, respectively.
After the oscillation 1 is damped, the drain-source voltage of the power switch shows a constant value of
V
bus
+V
Refl
until the transformer is fully demagnetized. This duration builds up the first portion of the off-time
t
off1
.
After the secondary side current falls to zero, the drains-source voltage of the power switch shows another
oscillation (oscillation 2 in Figure 3, this is also mentioned as the main oscillation in this document). This
oscillation happens in the circuit consisting of the equivalent main inductance of the transformer L
p
and the
capacitor across the drain-source (or drain-ground) terminal C
DS
which includes C
o(er)
of the MOSFET. The
frequency of this oscillation is calculated as:
DS
P
OSC2
C
L
2π
1
f
(3)
The amplitude of this oscillation begins with a value of v
Refl
and decreases exponentially with the elapsing
time, which is determined by the losses factor of the resonant circuit. The first minimum of the drain voltage
appears at the half of the oscillation period after the time t
4
and can be apporximated as:
Refl
bus
dsMin
V
-
V
V
(4)
In the quasi-resonant control, the power switch is switched on at the minimum of the drain-source voltage.
From this kind of operation, the switching-on losses are minimized, and switching noise due to dV
ds
/dt is
reduced compared to a normal hard-switching flyback converter.
