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Class D Switching Power Amplifiers and Power D/A conversion

Class D musical instrument amp trouble
Class D musical instrument amp trouble
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Old 4th November 2004, 06:42 AM   #1
GTC1010 is offline GTC1010  United States
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Default Class D musical instrument amp trouble


I'm having trouble repairing my Class D musical instrument amp. The amp uses four IRFZ34E's, driven in full bridge configuration by a HIP4080A. I blew one of the output MOSFETs by inadvertently shorting the output (Grrrr) .

I bought several IRFZ34's, and also bought several IRFZ44N's thinking that I might be able to make the amp a little more robust by using them instead of the 34's (direct shorts notwithstanding!). I selected four of the 34's and four of the 44's (as in PassDIY) whose Vgs was within 0.100V of each other and tried them out.

The amp now works again, but the MOSFETs run hot. The IRFZ34's didn't last long at all - the heat ate one of them up rather quickly. The IRFZ44's faired much better, but they also run much hotter than the original IRFZ34's. If I use a cooling fan over the heatsinks, the amp is stable and sounds good with the IRFZ44's. Still, the heat is such that it would likely cause a failure if I didn't use a cooling fan. Also, one side of the bridge seems to run a bit hotter than the other.

I want to get the amp back to being reliable w/o having to use a cooling fan, as it was originally. I suspect that my problem is in matching the MOSFETs, but I'm not sure what else to look at in matching devices for this type of amp. What other parameters should I be looking at when matching the devices? Or, is there another aspect that I'm overlooking that is causing the MOSFETSs to run hot? Any/all advice on how to proceed would be greatly appreciated!

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Old 4th November 2004, 07:38 AM   #2
Konrad is offline Konrad  Norway
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Re u shure your driver sirc is still working properly ?
Normaly when repearing, the components next to the broken one neads extra attention. ie here its likely to suspect som cross conduction (heat). There is almost endless possibilities. The litle info given is like : am know letting go my owl into the dark night, do you think you can hit it throwing a rock?
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Old 4th November 2004, 04:42 PM   #3
JohnW is offline JohnW  Czech Republic
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I would suspect that the output stage of the HIP4080A was damaged when then original FETS failed. You should also replace the HIP4080A.

Also, with Class D amps, it’s a very bad idea to change the “part number” of output stage devices, as Gate Charge varies wildly between different FETs – effecting the cross conduction / shoot thru current (and many other circuit variables). Dead-times are set for a particular value / “shape” of Gate charge

Time is my worst enemy!
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Old 4th November 2004, 07:04 PM   #4
GTC1010 is offline GTC1010  United States
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>Dead-times are set for a particular value / “shape” of Gate charge

Ah, I see... I'll replace the HIP4080A, and go with the exact MOSFET replacement. Thanks for your help, JohnW. Considering your advice, and other symptoms that have arisen since my original post, I'm thinking that the issue of dead-times with regard to gate charge is the crux of the problem with my repair.

Konrad, I appreciate your response also, and will check out the driver. Yeah, I know I released the owls into the night, but I was trying not to post a huge amount of text

I'm just looking for avenues to pursue in troubleshooting the amp, whilst at the same time trying to bring myself up to speed with Class-D. I have a lot of experience in electronics, but about nil with Class D amps. I am a fairly quick study, though, and am pouring over as many resources as I can get my hands on, as well as heeding (and appreciating!) the advice of others.

Thanks again for any & all help! (BTW, the amp is an Acoustic Image Clarus 1.)

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Old 11th November 2004, 02:51 PM   #5
km is offline km
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Location: dk
i would think you will have to replace also the hip4080.
are you sure about the fets?
(my records show another type used in this amp) but there may be a couple of versions of them.
deadtime is very important when you replace and even more so when you change the fets, do you know how to adjust the deadtime?
you can run the input section and output at 12v for debugging....
rgds karsten madsen - cadaudio.dk
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Old 12th November 2004, 11:17 PM   #6
dmfraser is offline dmfraser  United States
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Default HIP4080 replacement

At www.schematicsforfree.mattsoft.net, in the automotice section there are the service manuals for the 2003 Kicker brand car amps. Several of them use a Class D LF amp using the HIP4080 driver. The manuals specifically say to replace the HIP4080 whenever you replace an output device.
Dan Fraser
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Old 14th November 2004, 10:38 AM   #7
mhelin is offline mhelin
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Location: Tampere Finland Europe
Default Class D musical instrument amp general

I've been considering to try some class D amp for amplifying the Line6 POD output. Are there any reliable designs that you could recommend? I mean when I look under the pcb of the UcD180 there are millions of small SMD parts, and the whole thing looks a little bit too fragile to be used live.
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