In the Quasi-Resonant mode each period can be divided into four different time
intervals, in chronological order:
• Interval 1: t0 < t < t1primary stroke at the beginning of the first interval, the
MOSFET is switched ‘on’ and energy is stored in the primary inductance
(magnetization). At the end, the MOSFET is switched ‘off’ and the second
interval starts.
• Interval 2:t1 < t < t2 commutation time In the second interval, the drain voltage
will rise from almost zero to V
IN
+n•(V
OUT
+V
F
). V
F
is the forward voltage drop of
de diode that will be omitted from the equations from now on. The current will
change its positive derivative, corresponding to V
IN
/L
P
, to a negative derivative,
corresponding to -n•V
OUT
/L
P
.
• Interval 3:t2 < t < t3 secondary stroke In the third interval, the stored energy is
transferred to the output, so the diode starts to conduct and the inductive current
I
L
will decrease. In other words, the transformer will be demagnetized. When the
inductive current has become zero the next interval begins.
• Interval 4:t3 < t < t00 resonance time In the fourth interval, the energy stored in
the drain capacitor C
D
will start to resonate with the inductance L
P
.The voltage
and current waveforms are sinusoidal waveforms. The drain voltage will drop
from V
IN
+n•V
OUT
to V
IN
-n•V
OUT
.
Frequency Behavior
The frequency in the QR-mode is determined by the power stage and is not influenced
by the controller (important parameters are L
P
and C
D
). The frequency varies with the
input voltage V
IN
and the output power P
OUT
.If the required output power increases,
more energy has to be stored in the transformer. This leads to longer magnetizing t
PRIM
and demagnetizing t
SEC
times, which will decrease the frequency. See the frequency
versus output power characteristics below. The frequency characteristic is not only
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