Philips 7629, Service Manual

Page: 89 / 374

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.
Over Temperature protection
When the junction temperature exceeds the thermal shutdown temperature (typ. 140º
C), the IC will disable the driver. When the V
CC
voltage drops to UVLO, the V
CC
capacitor will be recharged to the V
(start)
level. If the temperature is still too high, the V
CC
voltage will drop again to the UVLO level (Safe-Restart mode). This mode will persist
until the junction temperature drops 8 degrees typically below the shutdown
temperature.
Mains dependent operation enabling level
To prevent the supply from starting at a low input voltage, which could cause audible
noise, a mains detection is implemented (Mlevel). This detection is provided via pin 8,
that detects the minimum start-up voltage between 60 and 100 V. As previously
mentioned, the controller is enabled between 60 and 100 V.
An additional advantage of this function is the protection against a disconnected buffer
capacitor (C
IN
). In this case, the supply will not be able to start-up because the V
CC
capacitor will not be charged to the start-up voltage.