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Old 20th July 2004, 09:10 PM   #1
pflodin is offline pflodin  Sweden
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Question What makes a power amp fail...

As some of you allready have seen, I have encountered a massive breakdown on one of my power amplifiers. In order to avoid (or at least try to avoid..) future events like that, I would really want to hear your theories about what could have happend.
The stor was like this, I was running this amp on a disco as a subwoofer amplifier driving two paralelled 18" subwoofers. The amp was running in bridge mode and the total load impedance was 4ohm (2*8ohm). The amp was rated 1500w RMS in bridge mode @ 4ohm.
The sound level was moderate at this time so I didn't push the amp so hard. And than suddenly, HMMMMMMM and then blackout (mais fuse 16A blown). When I investigated the amp I found 10 out of 16 power transistors shorted.
I allso found one emitter resistor (0,33ohm) witch was infinate resistance. I'm pretty shure that the transistor witch was connected to that resistor was one of the few "survivers".

Can the faulty resistor caused all this ? I mean, can the breakdown of one device cause so much unbalance so it starts an an chain reaction of transistor breakdowns, or was this a case of spontaniouse combustion ?

Is bridge mode more stressfull for the output transistors ?

/Peter
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Old 21st July 2004, 09:10 AM   #2
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Hi Peter,
Sorry to hear of the demise of your amp. Sounds like your transistors got cooked: their safe operating areas were exceeded. The open-circuit resistor might be a cause - if it went open-circuit then presumably only 3 transistors would be sharing the load rather than 4. But IME resistors don't just go open-circuit - this is a typical symptom of severe current overload due to a transistor failure. Is the resistor scorched?

Consider the powers involved in bridge mode:
With +/-80V psu rails and a 4-ohm load, the maximum average load power, at full volume, would be about 3000W. But the worst case dissipation for the transistors will occur at much lower volume when the 4-ohm load is dissipating 1300W. At this point the power transistors, in total, will also be dissipating 1300W. In bridged mode, according to your schematic, there will be 16 power transistors sharing the work. So, on average, each will disspate 1300/16 = 83W, and each will have a peak dissipation of about 100W, and each a peak Ic of about 6.4A.

At maximum output power, the peak Ic per device will be about 10A.

This assumes the load is 4-ohms resistive.

BJTs generally fail for one of three reasons: the silicone die gets too hot - usually needs to be <<200C, secondary breakdown occurs due to excess current and voltage, or the bond wires melt due to excess current.

If you know the transistor type you can check these figures against the SOA (safe operating area) chart on their datasheet. You'll have to estimate the temperature of their cases as their performance degrades very quickly with temperature. This is tricky - you'll need to estimate the room temperature at the time and the thermal reistance between heatsink and air and so on. Does the amp have any temperature cut-off device or other temperature limiter?
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Old 21st July 2004, 09:23 AM   #3
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Is the schematic that you posted actually from the amp you are using ?
I am asking because in this case it might not be that good for sub-use because of this coupling x-former used for base-drive. OTOH it of course depends on the correct dimensioning of this x-former.

Have you got any frequency-response data ?

Regards

Charles
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Old 21st July 2004, 10:02 AM   #4
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This amp is old, old, old.

I got news for you, things die, they don't live forever.

When an amplifier is advertised as being "able to run 2 ohms" it really means "will sound good and run for a long time into 4 ohms" and every now and then run it at 2 ohms in an emergency.

Life-span halves with every 10*C difference. 8 ohms is probably Delta 40*C, 4 ohms 80*C, 4 ohms bridge 160*C. Say the life is 10,000 hours at 40*C, it will be 625 hours at 80*C, and a whopping 39 hours at 160*C (change in transistor junction temperature above ambient).

You were just asking for it.

Was it really 4 ohms?

I doubt it. Most 8 ohm woofers measure 5R6 or so, and it is nor a resistive load either.

"The sound level was moderate at this time so I didn't push the amp so hard"

Too bad, it might of held up longer.

Class AB amplifiers run their hottest at about 25% of rated power.

Replace ALL of the outputs at the same time, I would go new emitter resistors and insulators too.

Replace the driver IC too, its about shot, and it costs little or nothing.
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Old 21st July 2004, 10:35 AM   #5
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Specs...
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Old 21st July 2004, 10:37 AM   #6
pflodin is offline pflodin  Sweden
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block...
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Old 21st July 2004, 10:42 AM   #7
pflodin is offline pflodin  Sweden
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And schematics. courtesy of Renkus-Hinez corporation USA.

And Yes , this is for my AMP. The output devices are H001A B1 (I have now learned at this forum that It's probably an inhouse version of MJ15024).

/Peter
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Old 21st July 2004, 01:44 PM   #8
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Find attached a SOA graph for MJ15024.
I have sketched the operating curve for each of the 8 output devices, in bridge mode and driving a 4-ohm resistive load.

To draw the operating curve, I choose a Vce for a transistor, then work out the load voltage as 80 - Vce. Then the load current as (80-Vce)/2. It is 2-ohms not 4 that the amps "sees" in bridged operation. The Ic is the load current divided by 4 as there are 4 transistors per half. This is somewhat conservative because it doesn't account for the voltage drop across the emitter resistors.

The upshot is that, at 25C case temperature, your operating line is fine for a 4-ohm resistive load in bridged mode. The maximum power dissipation of MJ15024 is 250W and yours will be 200W peak. With thermal derating, you can still operate ok up to 60C case temperature.

The load must no go much below 4-ohms and must not be capacitive otherwise the SOA will be breached and devices will fail. Note that the worst case amplitude for the transistors is about 51V (not 80V) so they will work hardest when the output power is about 1300W (rather than full power of 3200W).

Now, having said it'll be ok, you need to keep those case temperatures below 60C at all times. The total power dissipated by each amp will be about 650W average and that is a hang of a lot of power to get rid of, especially in a hot sweaty disco. I think this is somewhat unrealistic and you'll have trouble.

As a precaution I'd check that there is a mains thermal cut-off switch located on the heatsink close to the transistors and rated at 50C or so to be on the safe side. This way if things get too toasty the amp will turn off before the transistors fail. Also test the amp to make sure the fans still work!
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Old 21st July 2004, 01:57 PM   #9
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Default Erratum

In post #2 I said the peak power of each transistor would be 100W. This should have said 200W.
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Old 21st July 2004, 02:27 PM   #10
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Quote:
The load must no go much below 4-ohms and must not be capacitive otherwise the SOA will be breached and devices will fail.
Not even one of these two requirements might be properly met when driving a speaker at sub frequencies.
DC resistance may very well drop to 3 Ohms. Around peaks the impedance looks either inductive or capacitive.

Regards

Charles
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