LME49830 burning

[Domid31]

Hello,

I am working on a Mos amp using LME49830 and IRFP240/9240
I have already burned many mos, I do not understand why.

Sometimes it burns when powering up, sometimes I can run long time at full power, output is clean, no oscillation.
I am using National application circuit, with my own PCB (there is no mistake on it). Gate resistors are 100 ohms, I did not mount the 12kohm resistor between the 2 gates (Rq in National application), I just add 2 zener diodes between gate and S for mos protection.

When the amp burns, the gate resistors are getting black, the mos and the lme49830 are dead.
I am using a +-50V unregulated power supply, large capacitors (33 000uF).

Any Idea ? Can it be caused by the zener protection diodes ?
Can it be caused by power supply rise (full voltage can be applied on mos before lme49830 because of PCB design) ?
I am a bit lost ...

 

[Dxvideo]

Hi Domid,

Does your output bias current level adjustable like in the datasheet? And did you measure the bias?
Did you hear any "shaking" noise while your MOSses dieing?

 

[Domid31]

Hello,

I have setup to bias current to 70 mA in hot condition.
I am monitoring it: run the amp, wait some minutes, cut input signal, measure and adjust bias.
the bias temp compensation is not used for the moment.

Can this destruction be caused by zener diodes in between gate and source ?
Can it be because I have no resistor between the 2 gates ?
I do not know. I'll replace the burned parts, and test it again.

 

[AndrewT]

Hi,
you must fit a gate stopper resistor to each and every gate lead.
This resistor does it's job better if it is place right on the gate lead with the shortest possible lead length.

 

[Dxvideo]

Dear Domid,

That MOSFETs should have at least ~20v GS voltage limit. So I think, without any input signal its not possible to reach gate voltage limit to kill them.
But; If youre not using a gate resistor for these MOSFETs then you have a big situation! Because, its strongly possible that they will start to oscillate after a short time! I think this is the reason you lost a lot of MOSFETs...
Vbe multiplier solution for bias stability is not a must for this amplifier but its a recommendation. Because as I see in my LM4702+Lateral MOSFET amplifier, they have very good bias stability against temperature..
However I think 70mA is a bit more for IRFPs... 40mA will be sufficient for best distortion results..
Let me know the results after gate resistors added..

 

[AndrewT]

FET bias

for best distortion and sound quality results the higher the better for bias current. just keep on increasing it all the way up to ClassA bias.

Borbely recommends a minimum of 500mA of bias for the whole output stage and a minimum of 150mA for each output FET.

 

[Dxvideo]

Re: FET bias

for best distortion and sound quality results the higher the better for bias current. just keep on increasing it all the way up to ClassA bias.

Borbely recommends a minimum of 500mA of bias for the whole output stage and a minimum of 150mA for each output FET.

Wrong!
Look at the AN-1645... They have already tested 4 diffrerent MOSFET types and determined different bias levels for each.. And for IRFPs the best bias value is 25mA, not 150!
Thats what I meant in my last posting..

 

[AndrewT]

I prefer to accept Borbely rather than National's application note.

I trust him more than the design team.

 

[Dxvideo]

Ok... Thats your choice. But Nationals application note is dedicated for LM4702-LME498XXX series..
So I prefer (and already used) that bias levels and gate stopper values for this series amplifiers only.

 

[Dxvideo]

Gate resistors are 100 ohms, I did not mount the 12kohm resistor between the 2 gates (Rq in National application), I just add 2 zener diodes between gate and S for mos protection.

Ooops... I have missed that you already have gate stoppers! So the oscillation theory is dead.. As mentioned, its too hard to kill your mosfets in this application by the reaching of gate voltage limit.. I would remove the zeners and test the amplifier with inputs muted.

 

[Domid31]

Hi,
you must fit a gate stopper resistor to each and every gate lead.
This resistor does it's job better if it is place right on the gate lead with the shortest possible lead length.

Thank you for your answers.

I already have resistors in the gate signal (100 ohm).
What are gate stopper resistors ? Resistors between gate and source ? Between N mos gate and P mos gate ?

For TDH, I did some measurements at 50 mA bias: les than 0.05 %. That's enough for my application (sono power amp, not very low THD HIFI). THD measured at around 90WRMS on 8 ohms load.
I did measurements with different bias settings. No large improvment (with my measurement tools).
All was before the last MOS explosion !

 

[fab]

Hello,

I am working on a Mos amp using LME49830 and IRFP240/9240
I have already burned many mos, I do not understand why.

Sometimes it burns when powering up, sometimes I can run long time at full power, output is clean, no oscillation.
I am using National application circuit, with my own PCB (there is no mistake on it). Gate resistors are 100 ohms, I did not mount the 12kohm resistor between the 2 gates (Rq in National application), I just add 2 zener diodes between gate and S for mos protection.

When the amp burns, the gate resistors are getting black, the mos and the lme49830 are dead.
I am using a +-50V unregulated power supply, large capacitors (33 000uF).

Any Idea ? Can it be caused by the zener protection diodes ?
Can it be caused by power supply rise (full voltage can be applied on mos before lme49830 because of PCB design) ?
I am a bit lost ...

hi
1) Have you tested about 2V 10KHz square wave on 8ohms in parrallel with 2uf to verify high frequency compensation?

2) Have you mounted the VBE multiplier transistor on the main output heatsink for temperature compensation?

3) you could monitor the power supply drawn current before it burns....

Gate stoppers are the gate resistors.

Maybe you could post a picture...

Good luck

 

[foo]

I had burned some chips
1 is dead because I locked the skrews too hard.
2 are dead for over voltage
another one was dead because of high frequency signal strucked the input satge and destroyed the bias setting stage of the chip.
As a result, a lot of current flaw into the MOSFETS and killed them.
I replaced the MOSFETs and got them destroyed again before I learned that it's caused by the chip.

 

[jackinnj]

When zeners fail, they fail catastrophically.

I hve experienced this only once -- without current flowing across the output pins the chip "railed" and the bias went to the limit of my lab power supply -- which is 400mA.

AN-1645 was referenced to the LM4702 -- this chip couldn't get the bias high enough for the IRFP240/9240, nor did it have sufficient drive capability for multiple sets of output devices. The LME49830 overcame this problem.

All of the driver chips don't like getting hot -- and as they really start to cook you can see their own quiescent current starting to take off -- of course there's shutdown at 150 degrees.

 

[Domid31]

I am still waiting for new MOS.
I have removed the zener diodes, and I also have rerouted the grounds.
I have found out that the output ground (speaker ground) was taken close to the LME. So I have changed it to have a clear separation between low power (input, lme) and high power (mos, speaker) grounds.

I hope I'll be able to do new tests this week-end (including the square on capacitive load for high frequency compensation check).

 

[Domid31]

Hello all,

Thinks are getting better.
New mos, new lme49830, new ground tracks, zener protections removed: 100 W/8 ohms, thd < 0.08%
With square wave load 8 ohms and 2.2 µF: no oscillation. output square is very similar to input signal (which itself is not very good, my PC sound card is not a real function generator).

Thanks for your help !