What's the purpose of this third winding in this pc power supply circuit diagram?

[neonwarrior]

Hello everyone, please can you help me answer this two questions about a PC power supply with half bridge topology. Please see the image I highlighted the components.

1 - Whats the purpose of the third winding in the driver transformer??

2 - Whats the purpose of the pull up resistor (330 k) in half bridge low side npn transistor?

Thanks in advance
Neonwarrior

 

[betwixt]

I'm guessing that as there is no start-up supply to the TL494 and there is therefore no way it can run until there is already a voltage at the output terminals, the third winding is a 'weak' feedback path so the switcher transistors can oscillate by themselves until the much higher drive power of the TL494 takes over. The two 330K resistors would also be necesary if that was the case so some residual bias current could flow to give them enough gain to start the oscillator runing.

Most PC PSUs have either a brief 'kick start' pulse derived from the incoming rectified line voltage to do this or have a smaller independant low voltage supply 'stand by' which always running to provide power to the main switcher stages.

Brian.

 

[neonwarrior]

Hey thanks for reply.
But if high side npn transistor is on, would the low side transistor turn on a little because of the base pull up resistor to +Vdcbus? and kaboom:lol:

 

[betwixt]

Yes, if the current was high enough but 330K in the base of a low gain transistor isn't going to make it conduct very much. If my theory is correct, it would start to oscillate almost immediately and the much higher drive level from the previous stage would then become the dominant signal.

Brian.