Car amplifier blows - help

[viki]

hello i have a car amp, a sony xplode xm 1002 hx when i used it in normal sound it plays well, but when i pull it loud it blows the mosfets (IRF 540 and 9540) of one channel, i replace then, and it plays nice intil i pull it loud and blows agains the mos-fet.

What shoud it be? I was thinking about bad quality MOS-FET because it blows only when I put it in bridge, that mens that a 4ohm sub, its like a 2ohm for each pair of mos-fets

Tanks for any help

[Netlist]

Hi viki and welcome
This post is so funny, the name of your amp says it all.

I would go for low impedance that can not be taken by the amp.
Posting your query in the Car Audio forum might give you more results.

/Hugo

[ElectronicsTech]

mmmmm i would never buy anything sony because the price of the amps might tell you something

[cunningham]

First of all you need to make sure that your amp will handle 2Ohms. If it is one channel only that keeps xploding, then chances are something is wrong with that channel. Better check the bias current of the outputs. May be too high which could cause If you have a 12V power supply to test it would be nice. BTW check DC offset.

[ElectronicsTech]

im soo happy i bought my 15 volt power supply. it comes in handy

[viki]

How can I check bias current of the outputs? I see one variable resistor closer to each pair of the two MOS-FET, but I dont know how to ajust it?
Can I compare the values from the bias current of the channel that is nice and ajust to that value the other channel that blows?

Tanks

[DigitalJunkie]

Everything I've heard about the Xplode amps suggests that the name says it all.They have a habit of blowing up.Alot of people seem to have troubles with them frying outputs the day after they are replaced/repaired.

[Eva]

IRF540/IRF9540 devices are prone to thermal runaway in linear class AB applications. Put them in a poorly designed circuit like the one found in Sony car-amps and you'll probably get periodic failures

Reducing bias may be useful in order to reduce the likelyhood of output devices blowing, but it's a compromise since too little bias produces huge distortion on MOSFET amplifiers

Anyway, adjusting bias is mandatory after replacing MOSFET output devices since Vgs thresold varies a lot from device to device and from manufacturer to manufacturer in comparison with bipolar Vbe 'threshold'

The circuit should include small/medium power resistors in series with the source pin of the MOSFETs and bias current is easily calculated applying ohm's law to the voltage drop across these resistors with no load connected to the output. Be careful since bias current may skyrocket when the amplifier gets hot, so adjusting in cold is useless

I've found also useful to replace IRF540/IRF9540 by IRF640/IRF9640 since the latter devices show higher Rds-on and lower transconductance ['MOSFET gain'], making it easier to avoid thermal runaway and enhancing current sharing between paralelled devices

Another approach may be to discard the amplifier circuit keeping the heatsink/case and the SMPS [maybe tweaking its output voltages] in order to build your own car amp using some circuit design known to be reliable

Sometimes cases/heatsinks with transistor mounting hardware, toroids/ferrites and binding posts/connectors are much more worth than the circuit itself or the rest of the components, since standard cases and heatsinks are expensive and not suitable to fit inside a car, and ferrites are hard to get in small quantities

[viki]

I found two pins that are test points ,they are connected to the 0.03Ohm resistor that is in serie with the drain.
So I have to ajust the variable resistor until i get the right bias current?
What is the normal value for bias current in IRF540/9540 ?
Or should I compare with the value of the amp channel that is fine and ajust the one that blows to that value?

Tanks

[ElectronicsTech]

my company uses IRF540/IRF9540 but not in a class ab amp there are configured for a class D amp.