Mosfet failures in this differential piezo drive circuit

[core7916]

TL;DR

why short between drain and source ?

Hello, i am using a switching circuit with the power mosfets to actuate the piezo loads, but while working on this circuit, my mosfet is getting bad, means i am observing short between mosfet source and drain. ( happened in 4 mosfets)
i will attach the circuit daigram of my circuit.
switching frequency will be 10hz with 2ms charging and rest is discharging the 3uf capacitor (ref to load and have 200kohm resistance).
in circuit c1 ref to load. and all components are rated for more than 300v. (part numbers are different in real time circuit.)
what wrong with my circuit? and what to do to avoid the shorting of mosfets.

 

[berkeman]

Are you sure you are not overlapping the two drive signals? Maybe there is a momentary short from Vcc to ground through the two MOSFETs at the crossover points. You may need to generate your two control signals so there is a dead time between them. Can you show the full circuit with the signal generation and FET gate drive circuits?

 

[Baluncore]

When one MOSFET fails, it will probably take out the other.
A fast fuse, in the power supply or drain, may protect the second MOSFET from the first failure. That will help identify which MOSFET is failing first.

It is normal practice to include about ten ohms of series resistance in the gate drive; Rg = 10. That has two effects. It limits the switching slew rate; di/dt, and it can prevent a destructive parasitic oscillation between the driver and the MOSFET gate during switching.

 

[DaveE]

This is a 250V MOSFET in a 200V circuit. If you have ringing it would be easy to overvoltage them. OTOH, the load is well damped and turn off is benign at zero current. So I wouldn't expect much. In any case, you could try an RC snubber right at the FETs if the load is far away. But, the load is an RC snubber, so there's no point if you have good layout.

My rough calculations show that the thermal stress should be OK.

There are really only 3 ways to kill a MOSFET. Overheating, or overvoltage at the drain or gate.

 

[core7916]

Are you sure you are not overlapping the two drive signals? Maybe there is a momentary short from Vcc to ground through the two MOSFETs at the crossover points. You may need to generate your two control signals so there is a dead time between them. Can you show the full circuit with the signal generation and FET gate drive circuits?

sir, to mofets never gets on at same time since one signal is inverted input of another. and i am generating these signal from fpga controller. the controller signal will be fed to ir2110 gate driver which will drive the circuit.

 

[core7916]

This is a 250V MOSFET in a 200V circuit. If you have ringing it would be easy to overvoltage them. OTOH, the load is well damped and turn off is benign at zero current. So I wouldn't expect much. In any case, you could try an RC snubber right at the FETs if the load is far away. But, the load is an RC snubber, so there's no point if you have good layout.

My rough calculations show that the thermal stress should be OK.

There are really only 3 ways to kill a MOSFET. Overheating, or overvoltage at the drain or gate.

sir, the part number i have mentioned in circuit is not the same component which i ma using, i am using mosfet (STB45N40DM2AG) which has high rating (400v , 38A).

 

[berkeman]

sir, to mofets never gets on at same time since one signal is inverted input of another

Of course they can. And if the ON FET doesn't immediately shut off while the other FET is turning ON, you have a window of vulnerability for a momentary short circuit. Do that often enough and you will heat up the FETs and cause a failure.

i am generating these signal from fpga controller.

So instead of generating one signal and inverting it, generate two control signals that have a dead time (adjustable in your FPGA) between them.

 

[Baluncore]

The IR2110 gate driver provides independent low-side and high-side drivers. It does not prevent overlapping conduction, nor does it introduce dead time.

 

[core7916]

thank you for your replies.

 

[berkeman]

@core7916 -- Were you able to figure this out and get your circuit working reliably?

 

[core7916]

@core7916 -- Were you able to figure this out and get your circuit working reliably?

yes sir, i have kept the sufficient time between 2 signals. but this doesn't solved my issue, the actual issue is with my capacitor c2. i have replaced it, after that my circuit worked.
Thank you.

 

[techn0]

If the charge pulse and the discharge pulse gates are left floating while the 100 to 200v power is present then the gates can float to those voltages as well instantly killing the mosfets. I have seen this on a circuit I am working with right now at only 24 volts vcc.

 

[DaveE]

If the charge pulse and the discharge pulse gates are left floating while the 100 to 200v power is present then the gates can float to those voltages as well instantly killing the mosfets. I have seen this on a circuit I am working with right now at only 24 volts vcc.

Yes! This is important. The gate drive circuit should always be able to source or sink current to maintain the desired voltage. Also, the Drain-Gate capacitance will act as negative feedback and couple any dv/dt at the drain back to the gate. This is dominant during switching, a huge fraction of the gate drive current flows through C_DG. This is amplified in HV circuits from the Miller Effect. But there shouldn't be any DC "floating" in a healthy MOSFET, unless it comes through the other circuitry, like diode leakage and such.