Philips 7629 Service Manual page 80

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
de diode that will be omitted from the equations from now on. The current will
change its positive derivative, corresponding to V
corresponding to -n•V
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 will decrease. In other words, the transformer will be demagnetized. When the
L
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
and current waveforms are sinusoidal waveforms. The drain voltage will drop
from V +n•V
IN
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
input voltage V and the output power P
IN
more energy has to be stored in the transformer. This leads to longer magnetizing t
and demagnetizing t
versus output power characteristics below. The frequency characteristic is not only
+n•(V
IN
/L .
OUT P
will start to resonate with the inductance L
D
to V -n•V
OUT IN OUT
times, which will decrease the frequency. See the frequency
SEC
+V ). V
OUT F F
IN
.
and C ). The frequency varies with the
P D
.If the required output power increases,
OUT
is the forward voltage drop of
/L , to a negative derivative,
P
.The voltage
P
PRIM