TL494 car amplifier PSU, cooking mosfets!

[AmmoJammo]

I'm meant to be repairing a car amplifier, that runs a TL494 switchmode controller in the PSU.

It had smoked all the power supply fets, and somehow also cooked one of the dual rectifier diodes (two diodes in a TO220 package) of which there is eight in total.

I replaced all the mosfets, and the one bad diode, and now it powers up, but one bank of 4 mosfets cook themselves.

The circuit is very similar to here:

http://sound.whsites.net/project89.htm



Except, rather than having two complementary pairs of transistors, there's 4 pairs total, driving 4 banks of mosfets, each bank has 4 mosfets.

None of the drive transistors appear to measure bad, but I am measuring in circuit... If the TL494 output had gone bad, I would have thought two banks of mosfets would be getting hot, as each output is driving two transistor pairs, I've since removed all the diodes, as I thought some may have gone bad (considering one was in 2 pieces) but they all appear to measure just fine...

So, I'm stumped!

[james_s]

Is the rest of the amplifier ok? Is it possible there are more shorted parts that are drawing the whole thing down? How does the drive waveform look on the mosfets?

[AmmoJammo]

Haven't had a chance to stick a scope on it yet, not sure I have a suitable one here at the moment.... 

None of the amplifier output devices appear to measure bad.

I also don't want to leave it on for more than a few seconds, as these devices appear to just be hard on.... I'll have to get a scope onto it sometime... I was trying to avoid removing the mosfets again 

[fable]

Drive transistors are bad and maybe tl494 too.You need to remove them and then check  to really  be sure

[AmmoJammo]

The transistors are:
"2X" - KST4401
"2T" - KST4403

(this is for my own reference more than anything else

[Seekonk]

If it is anything like the the schematic provided, the drivers are ok.  These are powered all the time so the high current would happen any time power is connected.  A bad chip would cause current when it was powered.  For your own reference, both transistors are the same.  Another fast copy and paste?

[T3sl4co1l]

It's a shit circuit in the first place -- made to cook the most transistors in the least cost, while still managing to work most of the time.  No current mode operation, not even an inrush protection circuit.  The only reason it doesn't explode on startup is the very slow soft-start time, which drops the voltage difference in switching loss and leakage inductance.

I'd recommend buying a new one entirely, but they all use the same shit circuit.

Tim

[SaabFAN]

Without a scope you won't be able to find the problem, unless it's something trivial, like a broken Feedback-Loop.

What I would suggest to check, from personal experience with TL494-Based SMPS, would be these points:

- Transformer OK? - Shorted Windings due to overheat might be a possibility and cannot transfer power, leading the chip to run the MOSFETs constantly at 50% DutyCycle (btw. my FullBridge SMPS-Design blew the MOSFETs after the Diode-Bridge burned out due to overheating during a max current-test)
- Feedback-Loop OK? - No feedback-voltage = Constant 50% DutyCycle
- Chip OK? - Car Supplies are NASTY (Voltage Spikes are expected to be as high as 80V with A LOT of Power behind them, according to some Application-Notes I read!). If the Chip isn't protected, it might have burned out and the outputs have been shorted to VCC - Other Failure-Modes: Switching-Frequency too high, TurnOn-Overlap, TurnOff-Comparator damaged, internal Feedback-Amps damaged, etc.


If you have the patience to build an SMPS yourself, I'd be happy to post the updated schematic of my FullBridge SMPS, or even send you a few boards.
It requires only 4 MOSFETs, should be adaptable to a transformer with center-tap and includes all the suggestions T3sl4co1l made (I hope I did implement them correctly^^), as well as overheat protection and a resettable fuse.
Components can be found on Digikey, or Ebay and are less than 30€.

[james_s]

I don't even see a feedback loop. The TL494 is run open loop and the output of the DC-DC converter is unregulated. It's not the best of designs, but it's adequate for a car stereo amplifier. Despite the unimpressive circuit design, these things are often remarkably reliable when not abused.