Adding output overvoltage shutdown to UC3854 Boost PFC?

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Hi,
We wish to add output overvoltage shutdown to our UC3854 Boost PFC. Vout is 390V nom and we wish to do Overvoltage shutdown at 440VDC.

The UC3854 datasheet says that the enable pin cant be used to get fast cessation of the gate drive output, so we cant use that. The soft start cap on UC3854 is around 4.7uF (and charged up to 7v5) as a minimum so pulling that to ground suddenly would overcurrent any small transistor used.

So do you agree that pulling the IAC pin to ground is the best way? This would result in zero multiplier output current, which in turn means zero current demand.

UC3854 datasheet
 

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Why do you need so fast gate drive disable that the INA pin can't be used?
An overvoltage situation can probably not occur as fast as a short circuit or similar.
 
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Hi,

I think it´s a good idea to use the soft start, because it ensures soft power up.
Some smps controllers may become upset when they start while the ouput is powered up yet.

The softstart capacitor is charged usually with current in the microamperes range.
Thus a bjt with or without current limiting resistor should work.

Klaus
 
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