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Old 7th October 2014, 02:36 AM   #1
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Default High side drive issue

I'm having issue with a fairly large class D car audio amplifier.I know I'm in the wrong section.I can't find the answers I'm looking for in car audio section.
This amp from what I hear is notorious for output transistor failure due to high side drive voltage and or dead time.The drive voltage is definetly in question and the high side MOSFETs get hot pretty fast.I've heard of modifying the drive but not quite skilled enough to figure it out.
There's quite a few specifics here.If anyone is willing to give me advice I'll go into more detail.
Thanks
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Old 7th October 2014, 02:39 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by Triviumfan4 View Post
This amp from what I hear is notorious for output transistor failure due to high side drive voltage and or dead time.
The drive voltage is definetly in question and the high side MOSFETs get hot pretty fast.
If it uses a charge pump driver IC, the bootstrap capacitor (probably a cheap tantalum) could be bad.

Last edited by rayma; 7th October 2014 at 02:41 AM.
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Old 7th October 2014, 02:45 AM   #3
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I'm not very good with class d yet but...
The amp takes the rail voltage and from what believe uses a voltage doubler to make the drive voltage for the outputs.
The drive voltage is about 10 volts above rail voltage.That seems adequate but every other amp I've worked on was closer to 16-18.
I've read in past posts that this particular amp needed the drive modified to get the high side outputs to run cooler.
The square wave on the gate is perfect just doesn't doesn't have enough voltage and the MOSFETs run to hot.
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Old 7th October 2014, 02:50 AM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by Triviumfan4 View Post
The drive voltage is about 10 volts above rail voltage.That seems adequate but every other amp I've worked on was closer to 16-18.
I've read in past posts that this particular amp needed the drive modified to get the high side outputs to run cooler.
The square wave on the gate is perfect just doesn't doesn't have enough voltage and the MOSFETs run to hot.
OK, that's quite possibly the problem, because you have to have enough gate voltage to enter saturation,
or there will be too much voltage drop across the device, causing high power dissipation. Depending on the device,
you will need at least 12-15V to saturate it. Do you have a schematic?

Last edited by rayma; 7th October 2014 at 02:52 AM.
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Old 7th October 2014, 03:03 AM   #5
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I've never seen a schematic for this amp lol....
Car audio amplifier schematics can be extremely difficult to find.
I been racking my brain trying to figure out how to increase drive voltage.
The repair at this point is just to see if I can accomplish it.I'm still learning class d.
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Old 7th October 2014, 03:13 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by Triviumfan4 View Post
I've never seen a schematic for this amp lol....
Car audio amplifier schematics can be extremely difficult to find.
I been racking my brain trying to figure out how to increase drive voltage.
The repair at this point is just to see if I can accomplish it.I'm still learning class d.
Ok, then there is a small circuit for the high side gate drive. Can you trace it out?
There probably is a small toroidal transformer in this circuit.

Last edited by rayma; 7th October 2014 at 03:15 AM.
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Old 7th October 2014, 03:16 AM   #7
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No transformer.
It takes rectified rail voltage and seems to pass threw capictors creating some kind of voltage doubler circuit right to the collectors of the high side drivers.
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Old 7th October 2014, 03:17 AM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by Triviumfan4 View Post
No transformer.
It takes rectified rail voltage and seems to pass threw capictors creating some kind of voltage doubler circuit right to the collectors of the high side drivers.
I mean for the high side gate drive circuit. Something is used for the level shifting, probably a small transformer.
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