RDR347..Peak charging of Bias Rail capacitor during shorted output?
Posted by: treez
on
On page 12 of RDR347, it says that R17 is to stop the Bias Rail capacitor peak charging when there is a short circuit on the output.
Surely, peak charging is unlikely during output short circuit, because after all, the secondary will have zero volts on it, and the bias winding is coupled to the secondary, so how could peak charging possibly occur?
I understand there will be "some" charging (due to the leakage inductance), but surely nowhere near enough to cause an overvoltage to the LYTswitch?
Peak charging means charging to the peak voltage of the source. Yes it wont charge to 20V or 25V during short circuit because that is not the source voltage available to peak charge C9 during short circuit. The voltage that will peak charge C9 is the voltage spike created by the leakage inductance. So there is nothing wrong with the choice of word of peak charging as long as you understand that peak charging refers to charging of C9 with the available voltage from the source, which in this case is the leakage spike during short circuit condition.
R17 again is used to ensure the driver enters auto restart during short circuit condition. Its definitely not for overvoltage, because you cannot have overvoltage at the otput during output short condition.