MOSFET heating on my amp...

thefish · 28th March 2008, 05:26 PM

I finished the second revision of my class D amplifier (schematic) but there is a lot of MOSFET heating. If I remove the 0.1uF caps from the power supply there is less heating but more overshoot.

High and Low side gate signals with power supply caps:

High and Low side gate signals without power supply caps:

There is also a good deal of hissing when the input is muted (this was not in revision 1).

thefish · 28th March 2008, 09:52 PM

Anyone have any ideas why this would be happening? Should I use a larger cap on the power supply (maybe 1uF) connection to the FET's? I put a FM radio right next to the amp (withen 6 inches) and get no interference. When I actually rest the FM radio on top of the amp it interfears a lot (even makes the digital dial jump around??).

When the power supply caps are removed it seems that just one of the low side mosfets gets hot fast, the rest are ok.

Pafi · 28th March 2008, 10:07 PM

Do you really use IRF510 as power switch?

Quote:

Should I use a larger cap on the power supply (maybe 1uF) connection to the FET's?

Yes, definitely, as close to FETs as possible, but probably this won't solve this problem.

thefish · 28th March 2008, 10:22 PM

Quote:

Originally posted by Pafi
Do you really use IRF510 as power switch?

Yes, definitely, as close to FETs as possible, but probably this won't solve this problem.

The schematic is in error, they are really IRF540's as the power switch. Also for testing the amp is only running at 13.8V right now, not 50V as designed.

Pafi · 28th March 2008, 10:28 PM

Then you can try decreasing Rg.

thefish · 29th March 2008, 01:54 AM

I swapped the 0.1uF caps with 1uF caps and the traces look much nicer now but it still had the heating.

Searching through my resistors I dont have any smaller ones for the gate

I am thinking maybe 10ohm should do it.

fredos · 29th March 2008, 03:08 AM

Simply more dead time...You did'nt see the overlap on your scope???

lumanauw · 29th March 2008, 03:30 AM

Hi, Thefish,

I see in the picture, your core is green/blue, in magnetics catalog it is type -52, usually for low frequencies (like type -26 yellow/white).
Have you try type -2 core (red/clear) ?

thefish · 29th March 2008, 03:45 AM

Quote:

Originally posted by fredos
Simply more dead time...You did'nt see the overlap on your scope???

I do see the overlap, but on my other amp (revision 1) with the same dead time I get 0 heating with the same load and same input.

thefish · 29th March 2008, 03:46 AM

Quote:

Originally posted by lumanauw
Hi, Thefish,

I see in the picture, your core is green/blue, in magnetics catalog it is type -52, usually for low frequencies (like type -26 yellow/white).
Have you try type -2 core (red/clear) ?

I need to order some new cores, I do have some type 12 cores...

28th March 2008, 05:26 PM	#1
thefish diyAudio Member Join Date: Oct 2006	MOSFET heating on my amp... I finished the second revision of my class D amplifier (schematic) but there is a lot of MOSFET heating. If I remove the 0.1uF caps from the power supply there is less heating but more overshoot. High and Low side gate signals with power supply caps: High and Low side gate signals without power supply caps: There is also a good deal of hissing when the input is muted (this was not in revision 1).

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28th March 2008, 09:52 PM	#2
thefish diyAudio Member Join Date: Oct 2006	Anyone have any ideas why this would be happening? Should I use a larger cap on the power supply (maybe 1uF) connection to the FET's? I put a FM radio right next to the amp (withen 6 inches) and get no interference. When I actually rest the FM radio on top of the amp it interfears a lot (even makes the digital dial jump around??). When the power supply caps are removed it seems that just one of the low side mosfets gets hot fast, the rest are ok.

28th March 2008, 10:28 PM	#5
Pafi diyAudio Member Join Date: May 2004 Location: Budapest	Then you can try decreasing Rg.

29th March 2008, 01:54 AM	#6
thefish diyAudio Member Join Date: Oct 2006	I swapped the 0.1uF caps with 1uF caps and the traces look much nicer now but it still had the heating. Searching through my resistors I dont have any smaller ones for the gate I am thinking maybe 10ohm should do it.

29th March 2008, 03:08 AM	#7
fredos diyAudio Member Join Date: Mar 2005 Location: Quebec	Simply more dead time...You did'nt see the overlap on your scope???

29th March 2008, 03:30 AM	#8
lumanauw diyAudio Moderator Emeritus Join Date: Oct 2002 Location: Bandung	Hi, Thefish, I see in the picture, your core is green/blue, in magnetics catalog it is type -52, usually for low frequencies (like type -26 yellow/white). Have you try type -2 core (red/clear) ?

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