LME49830 burning - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th April 2009, 10:58 PM   #1
Domid31 is offline Domid31  France
diyAudio Member
 
Join Date: Aug 2007
Default LME49830 burning

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 ...
  Reply With Quote
Old 29th April 2009, 07:44 AM   #2
Dxvideo is offline Dxvideo  Turkey
diyAudio Member
 
Dxvideo's Avatar
 
Join Date: Nov 2003
Location: Turkey
Send a message via Yahoo to 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?
__________________
Best regards,
Ozgur
  Reply With Quote
Old 29th April 2009, 03:03 PM   #3
Domid31 is offline Domid31  France
diyAudio Member
 
Join Date: Aug 2007
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.
  Reply With Quote
Old 29th April 2009, 03:28 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
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.
__________________
regards Andrew T.
  Reply With Quote
Old 29th April 2009, 03:29 PM   #5
Dxvideo is offline Dxvideo  Turkey
diyAudio Member
 
Dxvideo's Avatar
 
Join Date: Nov 2003
Location: Turkey
Send a message via Yahoo to 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..
__________________
Best regards,
Ozgur
  Reply With Quote
Old 29th April 2009, 03:33 PM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Default 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.
__________________
regards Andrew T.
  Reply With Quote
Old 30th April 2009, 06:32 AM   #7
Dxvideo is offline Dxvideo  Turkey
diyAudio Member
 
Dxvideo's Avatar
 
Join Date: Nov 2003
Location: Turkey
Send a message via Yahoo to Dxvideo
Default Re: FET bias

Quote:
Originally posted by AndrewT
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..
__________________
Best regards,
Ozgur
  Reply With Quote
Old 30th April 2009, 07:30 AM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I prefer to accept Borbely rather than National's application note.

I trust him more than the design team.
__________________
regards Andrew T.
  Reply With Quote
Old 30th April 2009, 08:42 AM   #9
Dxvideo is offline Dxvideo  Turkey
diyAudio Member
 
Dxvideo's Avatar
 
Join Date: Nov 2003
Location: Turkey
Send a message via Yahoo to 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.
__________________
Best regards,
Ozgur
  Reply With Quote
Old 30th April 2009, 08:52 AM   #10
Dxvideo is offline Dxvideo  Turkey
diyAudio Member
 
Dxvideo's Avatar
 
Join Date: Nov 2003
Location: Turkey
Send a message via Yahoo to Dxvideo
Default Re: LME49830 burning

Quote:
Originally posted by Domid31
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.
__________________
Best regards,
Ozgur
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
The Burning Amp Festival-Burning Gadgets Preview Zen Mod Everything Else 115 27th October 2007 05:50 PM


New To Site? Need Help?

All times are GMT. The time now is 04:32 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2