piclist 2002\10\01\060847a >
Thread: 2-tran SMPS 5v reg now 90% efficicient!
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face BY : Russell McMahon email (remove spam text)



> >Of course you know that you can use a bipolar as buck 'diode' too. Try
it.
> >You need another PNP with E at GND and negative base drive during
flyback.
>
> I think this will definitely reach over spec of reverse voltages over that
transistor during the forward phase.  Unless input voltage is 5V or less.

This is where a MOSFET as a synchronous switch has a definite advantage.

- The back body diode blocks during the "on" phase.
- The back diode starts to handle the current when the pass transistor turns
off.
- The gate is forward biased (unlike a bipolar transistor) even when the
current flows "backwards" through it during the off phase.
- Can use an N channel with good Rdson and low cost.

Driving the rectifier FET on positively during the off phase will probably
require one extra transistor. You would then be up to 4 (pass transistor,
control, rectifier transistor, rectifier controller). Still impressive.

Somewhere about here a pack of low cost comparators may be useful :-) - one
more transistor and an oscillator of sorts and we have boost buck. Then
.......   :-)




       RM

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