Back-to-back N Mosfet short-circuit protection

DickCappels

Joined Aug 21, 2008
9,072
I may be getting confused with the term back-to-back. Is that in reference to the two MOSFETs (Q10 and Q20) in this circuit below?
1660833965995.png
Short circuit protection, the simple answer is to use a fuse, which is already there. A better option might be to sense the peak current in Neutral using a resistor or maybe better, a Hall sensor and when the peak current gets larger than a predetermined value, pull down the reset pin (pin 4) of the NE555.
 

crutschow

Joined Mar 14, 2008
30,431
That's not easy to do.
A fast, instrument-type fuse may work but, as experience has shown, the transistor is often faster than the fuse.

Otherwise you would need a circuit to monitor the current (typically using a small shunt resistor in series with the load) to turn off the MOSFETs when the current becomes excessive.
Not sure that can be easily done with a circuit controlling AC.
 

bertus

Joined Apr 5, 2008
22,005
Hello,

@DickCappels , did you read the text as the Q1 and Q2 have a remark sign, that calls to refer to the text.
Also the back to back mosfets are also used in solid state relays as the AQV21:
AQV21_mosfet_opto.png
Bertus
 

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postmanbk

Joined Feb 9, 2012
3
@crutschow,
I did the simulation and see that's good.
Because of the bidirectional current of R-shunt, I used LM324 to amplify the voltage of R-shunt and add to 1/2*VCC. Then using window comparator with latch output to active the 2N3904 to drive the gate of MOSFETs to Ground, if the voltage of R-shunt is outside of window limit.
Here is the picture and the LTspice file. Do you think is it ok?
ThanksShort protection.png
 

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