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Old 28th March 2008, 05:26 PM   #1
thefish is offline thefish  United States
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Default 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.

Click the image to open in full size.

High and Low side gate signals with power supply caps:
Click the image to open in full size.

High and Low side gate signals without power supply caps:
Click the image to open in full size.

There is also a good deal of hissing when the input is muted (this was not in revision 1).
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Old 28th March 2008, 09:52 PM   #2
thefish is offline thefish  United States
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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.
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Old 28th March 2008, 10:07 PM   #3
Pafi is offline Pafi  Hungary
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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.
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Old 28th March 2008, 10:22 PM   #4
thefish is offline thefish  United States
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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.
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Old 28th March 2008, 10:28 PM   #5
Pafi is offline Pafi  Hungary
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Then you can try decreasing Rg.
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Old 29th March 2008, 01:54 AM   #6
thefish is offline thefish  United States
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I swapped the 0.1uF caps with 1uF caps and the traces look much nicer now but it still had the heating.

Click the image to open in full size.

Searching through my resistors I dont have any smaller ones for the gate I am thinking maybe 10ohm should do it.
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Old 29th March 2008, 03:08 AM   #7
fredos is offline fredos  Canada
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Simply more dead time...You did'nt see the overlap on your scope???
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Old 29th March 2008, 03:30 AM   #8
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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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Old 29th March 2008, 03:45 AM   #9
thefish is offline thefish  United States
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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.
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Old 29th March 2008, 03:46 AM   #10
thefish is offline thefish  United States
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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...
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