LNK306 input review

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Hi Team
We are designing one over voltages Protector ckt. Please let me know following
1) Why we are making pi filter after rectification?
2)Does Voltage rating of filter capacitor will vary depending upon if we are choosing Half wave, Full wave or bridge rectifier ? If yes why ?
3) What is minimum voltage at which we will be able to get 12 V regulated output ?
4) Why inrush limiting resistor is required ? what will happen if we are not using that ?


1. The pi filter at the input side after the bridge rectifier has two functions: one, it makes the full/half-wave output of the rectifier into a pulsating/relatively smoother DC voltage and two, it helps filter EMI.
2. Depending on whether you would use a half-wave or full-wave bridge, the value of the input capacitor would be chosen via a rule of thumb: 2uF/W (full-bridge/universal input) or 4-5uF/W (half-bridge/universal input).
3. The minimum input DC voltage may be found in the datasheet. It may depend on what device you plan to use.
4. The RF1 or fusible resistor serves as a protection from unwanted in-rush/surge currents at the input side during fault conditions. It serves as a first line of protection for the power supply.