Philips 7603 series, Service Manual

Page: 72 / 257

In the Quasi-Resonant mode each period can be divided into four different time intervals, in chronological order:
•
Interval 1: t0 < t < t1 primary 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
VIN+n•(VOUT +VF). VF 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 VIN/LP, to a negative derivative, corresponding to -n•VOUT
/LP.
•
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 IL 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 CD will start to
resonate with the inductance LP. The voltage and current waveforms are sinusoidal waveforms. The drain voltage will
drop from VIN+n•VOUT to VIN-n•VOUT.
Frequency Behavior
The frequency in the QR-mode is determined by the power stage and is not influenced by the controller (important
parameters are LP and CD). The frequency varies with the input voltage VIN and the output power POUT. If the required
output power increases, more energy has to be stored in the transformer. This leads to longer magnetizing tPRIM and
demagnetizing tSEC times, which will decrease the frequency.
See the frequency versus output power characteristics below. The frequency characteristic is not only output power-, but
also input voltage dependent. The higher the input voltage, the smaller tPRIM, so the higher the frequency will be.
Point P1 is the minimum frequency fMIN that occurs at the specified minimum input voltage and maximum output power
required by the application. Of course the minimum frequency has to be chosen above the audible limit (>20 kHz).
Start-Up Sequence
When the rectified AC voltage VIN (via the center tap connected to pin 8) reaches the Mains dependent operation level
(Mlevel: between 60 and 100 V), the internal 'Mlevel switch' will be opened and the start-up current source is enabled to
charge capacitor C2521 at the VCC pin as shown below.