Hello all,
I am going through power up tests of my hobby Class D audio amp but have hit something very interesting but equally as frustrating.
For your knowledge, I have powered the board up with +-24V as depicted 'without' the MOSFET bridge and the power rails hold up fine (i.e. no short circuit etc..).
I then mount in the MOSFET bridge with the few select other components as depicted. The capacitance in series with the inductance was put there to signify stray capacitances (for simulation).
After mounting in the MOSFET bridge (with each FET rated to Vbrss = 150V, i.e. reverse voltage) the power rails hold up for approximately 3 seconds and then the MOSFET goes short circuit and since my +-24V supplies have over current protection, they come down with it.
I am very confused at how this simple setup keeps blowing my FETs. The only theories I have until now are:
1. Inductance somehow oscillates. I cannot see how this would cause an issue as the +-24V supplies would clamp any oscillatory voltages above this. I tried with and without the bus capacitors of 1000uF thinking this may have been an issue but same problem. Simulations showed no oscillation during pulse power up (as expected).
2. The gates are currently floating and maybe they couple in a pulse and create a short circuit. The FETs do not get warm though, so I do not believe this is the case. If they were failing due to overload, they should be smoking hot.
3. (most likely) The gate to drain/source go partially short circuit after failure (100 ohms to each). The Vgs of these MOSFETs are 20V, so it might be a scenario of the inductor oscillating while the gates are floating which creates a Vgs of > 20V which causes a complete failure of the device. Subsequently perhaps there is a coupling path for +24V onto the lower gate, which then sees a source of -24V. I guess I just figured if it was floating for now there would not be enough power behind these voltages to cause a gate breakdown, but perhaps someone else can reflect a bit.
Another interesting fact is that if I power up the +24V but leave the -24V out, it holds up fine and vice versa for -24V but no +24V. It is only when I turn both on the device decides to die.
The actual MOSFET bridge I am using is the IRFI4019.
Any thoughts would be greatly appreciated, I'm pretty stuck on this one for no and afraid to proceed at the expense of continually blowing up my FETs.
I am going through power up tests of my hobby Class D audio amp but have hit something very interesting but equally as frustrating.
For your knowledge, I have powered the board up with +-24V as depicted 'without' the MOSFET bridge and the power rails hold up fine (i.e. no short circuit etc..).
I then mount in the MOSFET bridge with the few select other components as depicted. The capacitance in series with the inductance was put there to signify stray capacitances (for simulation).
After mounting in the MOSFET bridge (with each FET rated to Vbrss = 150V, i.e. reverse voltage) the power rails hold up for approximately 3 seconds and then the MOSFET goes short circuit and since my +-24V supplies have over current protection, they come down with it.
I am very confused at how this simple setup keeps blowing my FETs. The only theories I have until now are:
1. Inductance somehow oscillates. I cannot see how this would cause an issue as the +-24V supplies would clamp any oscillatory voltages above this. I tried with and without the bus capacitors of 1000uF thinking this may have been an issue but same problem. Simulations showed no oscillation during pulse power up (as expected).
2. The gates are currently floating and maybe they couple in a pulse and create a short circuit. The FETs do not get warm though, so I do not believe this is the case. If they were failing due to overload, they should be smoking hot.
3. (most likely) The gate to drain/source go partially short circuit after failure (100 ohms to each). The Vgs of these MOSFETs are 20V, so it might be a scenario of the inductor oscillating while the gates are floating which creates a Vgs of > 20V which causes a complete failure of the device. Subsequently perhaps there is a coupling path for +24V onto the lower gate, which then sees a source of -24V. I guess I just figured if it was floating for now there would not be enough power behind these voltages to cause a gate breakdown, but perhaps someone else can reflect a bit.
Another interesting fact is that if I power up the +24V but leave the -24V out, it holds up fine and vice versa for -24V but no +24V. It is only when I turn both on the device decides to die.
The actual MOSFET bridge I am using is the IRFI4019.
Any thoughts would be greatly appreciated, I'm pretty stuck on this one for no and afraid to proceed at the expense of continually blowing up my FETs.
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