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Old 7th August 2003, 12:05 PM   #1
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Question can output transistors blow when no load connected??

Hi all,

Do you wether the output transistors can blow up even without a load connected.
This is how it happened:

I was testing my (DIY) amplifier (push/pull class AB) and the sound was pretty nice, so I decided to look at the signal with a scope - without any load connected. the signal was neat and I increased the amplitude of the input signal (1 khz sine) until the amp started to clip due to the supply voltage ( +/- 60V). At this moment the fuses (3A) of my supply flashed up and my output transistors were short cicuited between collector and emittor. How can this happen without any load?
the output devices are rated 200W 16A 160V (BDV66D / BDV67D). first I was thinking about a too high quiescent current, but I'm not sure (the transistors were still cold). Is it possible that one of the transistors blowed anyhow, so that the other blew too due to the short circuit? It's also amazing that the transistors blew really fast, the fuses didn't blew first.

If someone recognize this problem please tell me!
BTW: the circuit looks like a standard class AB with two current sources.

Best regards,

HB.
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Old 7th August 2003, 12:21 PM   #2
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oh yes, other question:

what the hell is a (integrated) damper diode?? is that a kind of backwards diode to protect the transistors against currents of inductive loads??
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Old 7th August 2003, 12:22 PM   #3
subwo1 is offline subwo1  United States
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Doing what you did should not cause any problem. The power supply is 120 volts peak to peak, and the transistors are rated 160 volts. Could there be an anomoly in the design. Could you get a diagram uploaded?
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Old 7th August 2003, 12:27 PM   #4
subwo1 is offline subwo1  United States
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Sounds like that is what they mean. In TV's they keep the flyback voltage from going too high. They can lso do as you mentioned. If that is actually what they do in your transistors, they may have gotten overloaded with too much voltage and melted down. But I don't see why.
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Old 7th August 2003, 01:11 PM   #5
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take a look at the manufacturer's chart on Safe Operating Area (SOA) -- all is not (necessarily) as it appears. Your transistors could also have suffered from insufficient heat sinking.
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Old 7th August 2003, 05:55 PM   #6
dimitri is offline dimitri  United States
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>At this moment the fuses (3A) of my supply flashed up and my output transistors were short cicuited between collector and emittor. How can this happen without any load?

Hello HB,
From my experience, this can be happened when some oscillation/instability occure near (after) the clipping point,
and a huge current flows through both output transistors.
This is also usual when you test your
amp with subsonic signal.

Could you please enclose the circuit schematic?
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Old 7th August 2003, 06:12 PM   #7
dhaen is offline dhaen  Europe
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"Integrated damper diode" would generally mean an integral diode that is normally reverse-biased, but will protect the device against reverse voltages.

I agree with Dmitri's opinion that instability was the likely cause of failure.


Cheers,
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Old 7th August 2003, 06:12 PM   #8
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Bipolar transistors have a finite turn-off time, so if the test frequency you used was somewhat high, one transistor wont turn off before the other one turns on, resulting in a rail-to-rail short.
Another "reason" (more like pure speculation) is that the amp was clipping, causing large HF components, making the devices not turn off fast enough. In TTL gates, transistors are not allowed to saturate because they'd turn off too slowly. Clipping usually means saturation, so I'm guessing this is the reason.
<flame_guard>BTW, MOSFETS will not suffer from this problem.</flame_guard>
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Old 7th August 2003, 09:19 PM   #9
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Exclamation It can be oscillation

To me it sounds like an oscillation problem. Some amplifiers have a tendency to oscillate when they are unloaded. And since you had the gain turned to over clipping point, this is absolutely a possible reason
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Old 8th August 2003, 06:39 AM   #10
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Does the amp have a zobel network at its output?

You may have one already incorporated, but without seeing the schematic I'm just guessing.

With no zobel network and being unloaded, depending on closed loop gain, miller compenstion etc. etc.. your amplifier could easily become a high voltage oscillator, exceed its safe SOA of the transistors, and the smoke will come out!! Once you let the smoke out of the transistors, it's all over. The smoke must stay inside..

make sure you have a zobel network at the output (will help stop ringing if any on the output signal), good supply decoupling capacitors near the output BJT's (or FET's for that matter) for stability. Make sure your miller compensation around the VAS is adequate (phase angle should not reach -180 deg. before unity gain.) If you have paralleled output transistor make sure you use ballast resistors at the output of each transistor for proper current sharing.

This is where I'd start,

Good Luck!!

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