Hi-end woofer amplifier problem- Zobel in flame

[androa76]

I have one *hi-end* woofer from my friend on service.
Amplifier is switching HIP2100 and Mosfet bridge combination.
Because of copyright here is partly scheme.
It is second time that ZOBEL CIRCUIT is in flame and mosfets are damaged.
Last year service is just replaced damaged material but not looking for a reason
Output zobel is very strange 6,8R 2W and 47UF !!!! not 47nF /100V...
I have newer seen so deep frequency zobel on amplifier output. What can be reason for burned Zobel... ? High frequency 190khz circuit for hip2100 or hip2100 bootstrap capacitor or to low zobel or any other reason?
Thank you for any help.

 

[mcspero]

Most likely shorted fets have destroyed the zobel network, rather than a zobel network destroying fets. However 47uf does seem high for a class d zobel network.

A commercial class d amplifier I have designed has a 3w 8 ohm resistor in series with a 220nf capacitor.

Is this amplifier you are servicing only designed for subwoofer applications?

 

[androa76]

Hi.
Yes this amplifier is just for 2 pushpull subwoofer drivers.
In front is multiple filter stage, auto power when signal detect...
What could be reason for burning Fets and Zobel network. Yes strange value for zobel.
Thanks D

 

[rayma]

What could be reason for burning Fets and Zobel network.

Strong high frequency oscillation.

 

[androa76]

It is 190khz switching d class amplifier. Doubt that 6000eur expenssive speaker design has oscillation. problem or maybe yes. Output is heavily dampen so must be at hip2100 output. Using bigger bootstrap capacitor can lower frequency but must i redesign the output filter?

 

[luka]

I think it's time to change the fets + drivers it dead, check how the floating point signal of each side of H bridge looks like. Perhaps add small resistor (0.1 non-inductive(metal film)) just under H bridge ground to check who the current is flowing (and it if is in normal range)

And change that 47u for something a lot lower, 1u non elco to start with, if you even need this zobel... I would say no for now, since that 2.2 next to it will be doing all the work
After you know everything else is ok, you could find the correct value for it