SPMS | Page 24 of 32 |
During the primary stroke, the rectified ACinput 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 informationrmation is used toadjust the peak drain current, which is measured via pin I SENSE .
Short Winding Protection
If the ‘sense’ voltage onpin 5 exceeds the short winding protection voltage (0.75 V), theconverter will stop switching. Once V CC drops below the UVLOlevel, capacitor C2521 will be recharged and the supply will startagain. This cycle will be repeated until the short circuit is removed(safe restart mode). The short winding protection will also protectin case of a secondary diode short circuit.
This protection circuit is activated after the leadingedge blanking time (LEB).
LEB time
The LEB (Leading Edge Blanking) time is aninternally fixed delay, preventing false triggering of the comparatordue to current spikes. This delay determines the minimum ‘on’ timeof the controller.
Over Temperature protection
When the junction temperature exceeds the thermalshutdown temperature (typ. 140º C), the IC will disablethe driver. When the V CC voltagedrops to UVLO, the V CC capacitorwill be recharged to the V (start) level.If the temperature is still too high, the V CC voltage will drop againto the UVLO level
Mains dependent operation enabling level
To prevent the supply from starting at a lowinput voltage, which could cause audible noise, a mains detectionis implemented (Mlevel). This detection is provided via pin 8, thatdetects the minimum
An additional advantage of this function is the protection againsta disconnected buffer capacitor (C IN ).In this case, the supply will not be able to
Control
7/8/2004
