Please help MOSFET transistor problem.

[bicepius]

Hello, I just stumbled across this forum and need help with a problem I am having.

I am a computer and laptop repair tech and do the odd bit of soldering for dc jacks, caps etc but thats about the extent of my electronic knowledge.

The problem is with a cars instrument cluster which I am trying to repair for my Dad. the backlight has gone and needles have stopped working. Now it is a common thing with this particular car (merc s class) and there is qite a bit of info on what parts to change. Apparently these are the parts that fail on the pcb; 1 Transformer and 3 mosfet transistors, which I have de soldered and soldered on new replacements. Now the instrument cluster does work but all 3 of the transistors get hot, and 1 of them gets really hot and starts to smoke after 30 seconds of having the igntition on and the instrument cluster lights then start to flicker on and off. If I unplug and let it cool down then it goes through the same cycle. I have asked about this somewere else and somebody suggested replacing the 330 coil(which looks like a small capacitor next to the 3 transistors but I did test it and the resistance seems normal. I will show pictures below of the parts on the pcb.

I would really appreciate any advice. Thankyou.

Luke.

Pic of Transistors.

Pic of red Transformer on seperate pcb.

**broken link removed**

 

[MikeMl]

Sounds like a bad design, where the voltage on the gate of the BUZ102 mosfet is not sufficient to turn it on hard enough to prevent it from dissipating much power and thereby self-heating the (original design problem, mosfets have no heatsink except the PCB traces).

If you replaced the mosfets with ones that have a higher gate voltage threshold than those used by the OEM, you made the original screwup even worse.

You should buy replacement mosfets with the lowest available gate threshold voltage (i.e "logic-level mosfets")...

 

[bicepius]

Hi thanks for your help. I have replaced the mosfets with exact same model and rating. (two BUZ102S and one BUZ102SL all made by infineon) I dont know why it should not work correctly now? What could cause it to be too little voltage at the gate apart from bad design?

Thankyou .

Luke.

 

[ben7]

It shouldn't start to smoke, maybe another transistor blew when the power ones blew.

You should check the little ones on the bottom right hand corner of the pic.

Also, how much current do they draw, more than normal?
That may be a indication that something else is still wrong.

 

[indulis]

Start by looking at the gate signals with a scope to see if they are being driven properly. If they look OK, then look for what is drawing current (getting warm). Do you have a guess at what the MOSFET's function are? MOSFET's and a inductor together sound like a DC-DC of some kind.

 

[Dragon Tamer]

The reason one of your MOSFETs could be getting hot the cooling down is because it is a pulsating circuit, it gets hot when it's on, and cools when it's off. You should make sure that the load resistances don't force too much current to go through the MOSFETs. As a safety precaution, I would also try to add a heat sink of some sort for your MOSFETs, but you will need to make it yourself, and it will need to be really small.

 

[indulis]

You should make sure that the load resistances don't force too much current to go through the MOSFETs. As a safety precaution, I would also try to add a heat sink of some sort for your MOSFETs, but you will need to make it yourself, and it will need to be really small.

This is an instrument cluster out of an S class Mercedes Benz… Don't you think if a heat sink was needed they would have added one? There was probably a reason they used D²PAK FET's.

 

[Dragon Tamer]

You would think, but the you have to consider that if it's a common problem in S classes, then there must be something Mercedes is doing wrong. So much for their motto; "The best or nothing" yea right. Mercedes cars are always plagued with problems. I have friends that own them and they are always complaining that there is something broken. One of my friends owns one that stops itself if it "sees" an obstacle, and it will just suddenly slam on the breaks in the middle of the freeway at random.

 

[mujiber]

Hey Luke, my dad's got the same problem and I'm trying to fix it for him. Were you able to figure out what was wrong? The transistor in the top right of your pic was getting really hot and burning the board. I've removed it and am thinking to replace the three transistors pictured, but I'm worried that the transistors aren't the root of the problem, and your post seems to support that. I'd like to know if you've made any further progress.