Philips 7603 series, Service Manual

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Over Voltage Protection
The Over Voltage Protection ensures that the output voltage will remain below an adjustable level. This works by sensing
the auxiliary voltage via the current flowing into pin 4 (DEM) during the secondary stroke. This voltage is a well-defined
replica of the output voltage. Any voltage spikes are averaged by an internal filter.
If the output voltage exceeds the OVP trip level, the OVP circuit switches the power MOSFET 'off'.
Next, the controller waits until the 'under voltage lock out' level (UVLO =
±
9 V) is reached on pin 1 (VCC). This is followed by
a safe restart cycle, after which switching starts again. This process is repeated as long as the OVP condition exists. The
output voltage at which the OVP function trips, is set by the demagnetization resistor R3522.
Over Current Protection
The internal OCP protection circuit limits the 'sense' voltage
on pin 5 to an internal level.
Over Power Protection
During the primary stroke, the rectified AC input voltage is measured by sensing the current drawn from pin 4 (DEM). This
current is dependent on the voltage on pin 9 of transformer 5520 and the value of R3522. The current information is used to
adjust the peak drain current, which is measured via pin ISENSE.
Short Winding Protection
If the 'sense' voltage on pin 5 exceeds the short winding protection voltage (0.75 V), the converter will stop switching. Once
VCC drops below the UVLO level, capacitor C2521 will be recharged and the supply will start again. This cycle will be
repeated until the short circuit is removed (safe restart mode). The short winding protection will also protect in case of a
secondary diode short circuit. This protection circuit is activated after the leading edge blanking time (LEB).
LEB time
The LEB (Leading Edge Blanking) time is an internally fixed delay, preventing false triggering of the comparator due to
current spikes. This delay determines the minimum 'on' time of the controller.