MOSFET Failure in Class D amplifier kit (silicon chip). - Page 1
Forum section: electronics / projects

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 04, 2013, 10:25:28 am
content: Hi all!I'm having an issue with the 'High-Power class D amplifier kit' that silicon chip published in November 2012.http://www.siliconchip.com.au/Issue/2012/November/High-Power+Class-D+Audio+Amplifier,+Pt.1Initially I built up a kit, used a 40V+40V toroidal transformer which is suggested in the instructions which give +- 57V after rectification. The amp worked fine sitting on the desk. Switched it off, came back to switch and back on and boom! both 5A fuses which were between the capacitor bank and amplifier blew instantly. It turns out Both of the IRBF5615 Mosfets became internally shorted across all three pins. I ordered replacement IRBF5615's from element 14 and put them in. They blew instantly as well.Checked over the WHOLE board, de-soldered all the components that were connected in some way with the Mosfets, and all checked out fine.Purchased another kit, assembled it, put it into a case and once again it worked fine for 4 hours or so.Then... the next day I went to turn it back on, BAM!, blown once again.So that's three sets of MOSFETS that have blown resulting in direct shorts across all three pins!I contacted John Clarke (the author) and the only suggestions I got was to replace the Mosfets (i think i know what will happen) or to reduce the power supply voltage to +-50V (but the setup i'm using was what was recommended in the instructions!?....)TL;DR Does anyone have any idea what might be going wrong or if there's a higher rated MOSFET that I could put in place?Gab

author: c4757p
author_rank: Super Contributor
timestamp: June 04, 2013, 10:29:49 am
content: I'm on my phone so I can't see the schematic (if there even is one), but check whatever drives the FETs. If they are stuck "on" they will burn up almost immediately.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 04, 2013, 10:34:39 am
content: I've taken a photo of the schematic that was included with the kit. Q1 and Q2 are the faults.I'll boot up the amp without the IC's and see what is actually occurring at the gate pins.

author: c4757p
author_rank: Super Contributor
timestamp: June 04, 2013, 10:38:48 am
content: If your first FET was just faulty, the D-G short may have damaged the gate drive output in that controller IC.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 04, 2013, 10:40:11 am
content: Quote from: c4757p on June 04, 2013, 10:38:48 amBut this has happened on two kits.

author: c4757p
author_rank: Super Contributor
timestamp: June 04, 2013, 10:45:12 am
content: Have you actually measured the voltage? It may be significantly high if the transformer is overrated for the application. 60-65V unloaded would not surprise me.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 04, 2013, 10:48:35 am
content: Quote from: c4757p on June 04, 2013, 10:45:12 amSure have+57V and -56.8V under no load what so ever. And there is no overshoot when turned on.The confusing thing is that i Have had both boards working for atleast an hour each before failure.

author: wraper
author_rank: Unknown
timestamp: June 04, 2013, 10:59:38 am
content: Voltage shouldn't be the problem. IC is rated ±100 V and MOSFETs 150V.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 04, 2013, 11:06:37 am
content: Quote from: wraper on June 04, 2013, 10:59:38 amWell apparently it is... Desoldered both Q1 and Q2. powered it up and the IC went up in smoke?!?

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 04, 2013, 11:15:49 am
content: I don't have specific experience with this kit, but two common problems with MOSFET-based amplifiers (Class D or linear) is oscillation and feedback loop stabilization, both because of the wide bandwidth of the MOSFETs (compared to equivalent-power bipolars).A very good - and old - trick to use when troubleshooting something like this is to insert a 1-10 ohm power resistor in series with the supply lines (but not the common/ground return) and keep a scope probe clipped across it to trigger on any current spikes. The series resistance should allow you to run the amplifier long enough to probe the gate/drain for any oscillation (NB - this oscillation will often be up in the MHz range). There must be a resistor (and, sometimes, a lossy-ferrite bead) in series with each MOSFET gate.Class D amps are difficult to get working correctly, especially if you try for the entire audio range in bandwidth. Hence why they are most often confined to subwoofer applications.

author: wraper
author_rank: Unknown
timestamp: June 04, 2013, 11:21:41 am
content: I think it was already dead. MOSFETs blew up fuses before IC was able to emit any smoke. You desoldered mosfets, IC blow up itself. Simple as that.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 04, 2013, 11:35:46 am
content: Quote from: MagicSmoker on June 04, 2013, 11:15:49 amYup, I was planning to use it for an LFE channel.Now a new question, does anyone know of an amplifier module already built, or in a kit formthat will operate from a +57V rail and can deliver 200-250WRMS into a 4ohm load?

author: wraper
author_rank: Unknown
timestamp: June 04, 2013, 11:48:02 am
content: Desolder IC, hook up the power and measure voltage on pin 12 (Vcc) (referring to -50V rail). Also measure Vaa, Vss referring to ground. I think it may be something wrong there.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 04, 2013, 12:10:25 pm
content: Quote from: wraper on June 04, 2013, 11:48:02 amVaa = +5.6VVss = -0.04VVCC = -57Vall with respect to 0V from the power supply.There is 0V drop across ZD1. Q3 (Tip31) is not shorted and had the correct forward voltages (i.e. not blown)

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 04, 2013, 12:14:20 pm
content: Here's some images

author: wraper
author_rank: Unknown
timestamp: June 04, 2013, 12:33:15 pm
content: Seems that ZD4 or/and UC2 are shorted to the ground. There sould be -5.6V on Vss. Vcc is also totally wrong. There sould be +15V referring to -50V rail ( or -42V referring to GND). Check ZD1, Q3, R9, R10. Also check two 100uF and 1uf capacitors around them. Also check electrolytic caps for reverse polarity and also ensure that polarity on PCB is the same as on schematic. Misprint is possible.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 08, 2013, 01:22:30 am
content: ZD4 was shorted across it (same on both failed modules)D6 is also directly shorted.Pulled both components out to test.Both IC's are also dead. hole burnt into the top of both.

author: wraper
author_rank: Unknown
timestamp: June 08, 2013, 03:09:34 pm
content: Strange about D6, are you sure that you wrote correct components number? If D6 is shorted then -Vss would be connected directly to -50V rail.

author: _Steven_
author_rank: Contributor
timestamp: June 08, 2013, 09:22:40 pm
content: I had the same problem with a class A amplifier kit...a given golden horse and it is working now.De-soldier everyting.....even tested the opamps with a simple circuit with leds.test every part! Only soldier it again if you are sure it is a working component.One thing not to forget is that if you put more capacitance after the rectifier output...it can pull a lot of current. This can also blow the surges. But I think there is anther problem in this story. (even with 40.000µF no surge will blow here....a beautifull damping factor on the output....something I never experienced before)...and that's just one channel....nothing to do with this topic....I really prefer a clas A or AB above a class D amplifier. take a look at the UCD kits.....I think you will be using it for driving a subwoofer?

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 16, 2013, 10:22:25 am
content: Quote from: _Steven_ on June 08, 2013, 09:22:40 pmYup, it's intended for a LFE channel. I'm tempted to buy a pre-made class A+B amp. But not sure as I'm worried about heat

author: peter.mitchell
author_rank: Super Contributor
timestamp: June 16, 2013, 10:54:19 am
content: If you want to throw shit at a wall and see what sticks, you can buy something random from here;Aliexpressand is probably cheap enough that if it doesn't work it doesn't matter?I'll probably get destroyed by people saying "blah blah reliability, blah blah sound quality, blah blah chinese junk" but hey, you could just use them for ideas,hereis one that uses the same chip as yours (supposedly).

author: envisionelec
author_rank: Frequent Contributor
timestamp: June 22, 2013, 02:55:35 am
content: You're blowing FETs because of your crazy paralleled front panel LED arrangement. You can't do this because the RUN LED is off the gate of the lower FET. With double the current flowing through the one resistor, the effects on the circuit will be deleterious at best. It also looks like the legs of that LED are shorted together.Just run the kit as designed.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 22, 2013, 11:18:13 am
content: Quote from: envisionelec on June 22, 2013, 02:55:35 amThe LED's weren't being paralleled. I cut one of the legs to the board's LED, then wired up to the front panel one. Nope they weren't shorted.

author: Hobgoblin612
author_rank: Contributor
timestamp: June 24, 2013, 06:30:44 pm
content: Just a random thought here but in the picture you have LK2 in the "invert" position. This may be causing some weird transients if it is un-plugged while powered up or being fed by what looks like a low pass filter or pre-amp you have made there. In theory the output of IC2 could be at +5.6V in this situation.It is also possible that when you power it down one part of the circuit is dropping faster than another and destroying something. In this case it is also possible that a zener might become forward biased and cause ill effects.I haven't examined the circuit too closely so I'm probably completely wrong but its worth a try.

author: envisionelec
author_rank: Frequent Contributor
timestamp: June 25, 2013, 02:07:22 am
content: Quote from: GabYoung92 on June 22, 2013, 11:18:13 amWell, it's good to know I'm still wrong sometimes.

author: GabYoung92
author_rank: Regular Contributor
timestamp: June 25, 2013, 10:33:26 pm
content: Quote from: Hobgoblin612 on June 24, 2013, 06:30:44 pmYup, I made a LPF which is inline. It is set on invert as the input level from my reciever's subwoofer output wasn't loud enough. So i set the link to invert, modified the feedback resistor to amplify the signal to the amplifier and inverted the output connections to 'invert' it back to normal.The LPF should be at 0V when no input is connected as I AC coupled the input and have the input set to 0V with a voltage divider.Quote from: envisionelec on June 25, 2013, 02:07:22 amHaha, sorry about that!I did have a think about that though =]