Sunday, 31 December 2017

isolation - Designing flyback converter - multiple isolated outputs


Can I treat 2 secondary coils with dual voltage output as one with double voltage for the purpose of calculations?


This seems to be OK for all formulas based on output power as technically the secondary IS a single coil with tap in the middle.


However for LT converters that use flyback in primary as feedback the formulas for feedback resistor(s) are based on Vout and turns ratio. How do I apply these?


UPDATE:


Allow me to clarify the question. Let's say I used all the right formulas and made myself a nice stable converter that puts out +50V at 100mA from isolated secondary coil and uses flyback voltage on primary as feedback.


Now, what happens if I tap the middle of secondary coil and convert this into dual voltage supply? Will I get a nice stable converter with dual +/-25V at 100mA outputs (minus drop on second diode, of course)? Without any changes in the feedback circuit or re-calculating the whole thing from scratch?


I am about 95% sure tapping output coil will not do anything to the functioning of converter, because the transferred power did not change. Except for that feedback resistor that according to LT8302 datasheet is calculated as:
Rfb = Rref * Nps * (Vout + Vf) / Vref

The Vout part of it is confusing, since now I don't have 50V, I have 2x25V.




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