Dear All,

Please refer to the attached picture. The transformer's secondary voltage is a +-15V square wave at the frequency of 100kHz.
I would like to put the breaker that senses the gate current and breaks the circuit in the case of overcurrent.
Could you please recommend any components for this job?

Thank You
BlackMelon

 

A fuse.

 

A fuse.

Yes that would work too. But every time it blows, I have to spend some money lol. Any good circuit that acts as a breaker?

 

hi BM,
Which ever device you use to open the line, you must not leave the Gate floating.
E

 

hi BM,
Which ever device you use to open the line, you must not leave the Gate floating.
E

Forgot to draw a 10k pull-down resistor at the gate. Thank you very much for the warning.

 

Yes that would work too. But every time it blows, I have to spend some money lol. Any good circuit that acts as a breaker?

But every time a circuit breaker opens due to an over-current situation, it means that your MOSFET is dead. Fuses are cheaper and faster than any mechanical circuit breaker. An electronic circuit breaker isn’t going to be very effective if there’s a dead MOSFET in circuit, because it will probably be a dead short to the drain voltage.
.

 

hi BM,
Look thru this link.
E
https://www.google.com/search?client=firefox-b-d&q=resettable+fuses

 

Need to be rated for whatever voltage is on the MOSFET drain. They catch fire in over voltage situations!
Breaking capacity is also quite low.
Also, they are pretty slow, so the MOSFET will fail first.

 

I'm confused, how come there's current flowing into the gate of a MOSFET? They are voltage driven devices.

 

I'm confused, how come there's current flowing into the gate of a MOSFET? They are voltage driven devices.

There won’t be any current flowing until it’s a dead MOSFET. Then it‘S too late.

 

But every time a circuit breaker opens due to an over-current situation, it means that your MOSFET is dead. Fuses are cheaper and faster than any mechanical circuit breaker. An electronic circuit breaker isn’t going to be very effective if there’s a dead MOSFET in circuit, because it will probably be a dead short to the drain voltage.
.

Thank you for the warning! I was trying to come up with electronic circuit breaker. Since it's not very effective, I will drop that idea.

hi BM,
Look thru this link.
E
https://www.google.com/search?client=firefox-b-d&q=resettable+fuses

Wow! That's interesting! I have never used such a thing before. They look almost like a varistor to me, TBH.

Need to be rated for whatever voltage is on the MOSFET drain. They catch fire in over voltage situations!
Breaking capacity is also quite low.
Also, they are pretty slow, so the MOSFET will fail first.

This MOSFET is part of a H-bridge configuration. The DC supply of the bridge has a voltage ripple, which its peak is at 311Volts.

I'm confused, how come there's current flowing into the gate of a MOSFET? They are voltage driven devices.

In the field of power electronics, you have to consider the gate current. Yes, in amplifier, when we do DC biasing, we approximate the gate current to be zero. The fact is when we look into the gate, there are capacitors and they are open at the DC steady state.
However, in PE field, you turn it on-and-off. You keep charging and discharging the capacitors. So, you have to consider the "transient current" of those caps. Please google "Gate Driving Current" or "Gate Driver's Output Current".

Actually, this fuse/circuit breaker is not to mainly protect the MOSFET, but the gate driver itself, TBH

 

I can’t envisage anything that makes the current vary from the predicted value during normal operation. It is a known voltage charging a known gate capacitance through a known resistance at a known frequency.
If the current increases, it is telling me that the MOSFET is faulty. The likely scenario is an overload that makes the MOSFET fail short, which causes the source bond wire to melt. You are then left with a MOSFET with a gate-drain short putting the full supply voltage across your gate drive transformer, which is likely to melt. A simple fuse (or, even better, a fusible resistor as the gate drive resistor) saves the transformer. It’s too late to save the MOSFET.
I can’t think of an electronic solution.
Any mechanical solution is likely to add inductance in the gate circuit.

 

I can’t envisage anything that makes the current vary from the predicted value during normal operation. It is a known voltage charging a known gate capacitance through a known resistance at a known frequency.
If the current increases, it is telling me that the MOSFET is faulty. The likely scenario is an overload that makes the MOSFET fail short, which causes the source bond wire to melt. You are then left with a MOSFET with a gate-drain short putting the full supply voltage across your gate drive transformer, which is likely to melt. A simple fuse (or, even better, a fusible resistor as the gate drive resistor) saves the transformer. It’s too late to save the MOSFET.
I can’t think of an electronic solution.
Any mechanical solution is likely to add inductance in the gate circuit.

Thank you for the advice. This is very informative and helpful. I will put the fuse at the gate then. The thing that I would like to protect is the gate driver (transformer). The MOSFET can easily be protected by just putting a fuse at the + wire of the supply voltage.