idea on economy bipolar supply
After taking off the lid from another project I noticed a p-channel MOSFET in series with the positive lead from the power connector to the circuit. It's a reverse polarity protection circuit. Although a more elaborate circuit is supposed to be used, this just connects the gate of the MOSFET to ground, the drain to the input and the source to the circuit to be protected. If the power polarity is correct the gate is lower in voltage than the source and the transistor turns on. But if the polarity is reversed the gate is higher in voltage than the source and the transistor is off. In effect this gives the protection of a series diode but with possibly a smaller forward voltage drop, depending on the spec of the MOSFET.
Why not try this in the economy bipolar power supply ?
(see previous blog post: https://wb9kzy.blogspot.com/2022/04/economy-bipolar-supply.html )
A p-channel MOSFET used for the positive rectifier and an N-channel MOSFET for the negative rectifier ?
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| MOSFETs in place of the 1n400x rectifier diodes |
So I did try it, at least with the p-channel MOSFET sub'd for the positive rectifier diode. And it doesn't work, at least not as well as a diode. The transistor gets warm (a bad sign but at least not hot) and the output just looks bad.
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| warm transistor + bad waveform means bad circuit |
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| The diode rectifier no load output (for reference from previous blog posting) |
I then simulated both the MOSFET and diode rectifiers,
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| p-channel MOSFET rectifier |
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| diode rectifier |
the MOSFET simulates as working OK but not really much better than the diode. Of course there is a built-in diode in the MOSFET (aka body diode) and that may be doing the work here even if the transistor isn't ever being switched ON. Or maybe I have the wrong type of MOSFET (depletion mode) ?
When the MOSFET is used in reverse polarity circuit it is fully ON, the gate being well above the threshold switching voltage. But in a rectifier situation the transistor may be off for a fair amount of the cycle and the body diode is pulling the weight.
I'll have to think about it, the warm/hot MOSFET bothers me.
I also tried some Schottky diodes in place of the 1n400x diodes, this does increase the outputs about 1/2 volt, so probably a worthwhile change.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm