Philips 7629, Service Manual

Page: 88 / 374

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
pin4 (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 (V
CC
). 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 pin4 (DEM). This current is dependent on the voltage on pin 9 of
transformer 5520and the value of R3522. The current information is used to adjust the
peak drain current, which is measured via pin I
SENSE
.
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 V
CC
drops below the UVLO level, capacitor C2521
will be recharged and the supply will start again. This cycle will be repeated until the