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Old 13th October 2005, 05:27 PM   #1
Pierre is offline Pierre  France
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Default Turn on/off transients

Hello all.

There is something that most builders of Class-D amplifiers may have suffered: turn on/off ticks/pops.

I am currently using a IR2113 for gate driving, using the SD pin for turn on/off. There is always an annoying click at the speaker, specially at turn on. First I used a relay to turn on/off several modules at the same time, shorting that pin to GND or driver's VCC, and I thought that the transient was due to the relay bouncing. But now I have done it with a transistor that has no bounce and it still remains the same.

I have measured it and consist of a dirty and damped oscillation that reachs around 20Vpp for 1 cycle or two, and its shape seems to depend on the load nature.

I think that the main cause is the LC filter transient response, and perhaps bootstrap circuit estabilisation. I am sure that some designs don't suffer from that, but can anyone point to a way of avoiding that? (apart from the obvious solution of using relays)

Thanks!
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Old 13th October 2005, 08:02 PM   #2
alfsch is offline alfsch  Germany
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could you show the circuit?
i assume a prob with the loop.
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Old 14th October 2005, 05:38 AM   #3
Pierre is offline Pierre  France
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Yes, of course!
Please give me some time in order to sort it and draw a figure.
Meanwhile, a little description:

- Clock (triangle) based design, oscillating at 250KHz.
- Output filter: 20uH and 440nF, with 18 ohm + 120nF zobel in parallel with the output.
- Error amplifier: inverting configuration: 2k input resistor, 180k local feedback resistor. 6k8+330pF in parallel with that resistor. Feedback is taken AFTER output filter, through a 100k || 47pF network.
- Error amplifier is a NE5532 fed at +/-6V
- Output stage: IR2113, 10uF tantalum bootstrap capacitor.
- Gate resistors: 6.8 ohm in antiparallel with schottky.

That should give an idea of the circuit.

Anyway, I will do some test in open-loop (increasing the 2k resistor to something like 47K to reduce gain), to see if the click still happens.

Best regards,
Pierre.
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Old 14th October 2005, 07:54 AM   #4
IVX is offline IVX  Russian Federation
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Try to keep bootstrap precharged
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E1.
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Old 14th October 2005, 08:32 AM   #5
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What happens with a smaller bootstrap cap (e.g. 0.1 uF) ?

Regards

Charles
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Old 14th October 2005, 09:22 AM   #6
Pierre is offline Pierre  France
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The thing is that I haven't tried with a smaller cap.

My fear is that, when the amp clips (i.e. one of the mosfets is always on while the other is always off), the behaviour won't be as good as now as the bootstrap cap will eventually discharge.
But take for granted that I will test as there is margin for a reduction.

IVX: How can I get it pre-charged? That won't be easy to achieve practically, is it?
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Old 14th October 2005, 09:42 AM   #7
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I am not suggesting to leave it like that but as a trial. If it helps with the startup problem then I'd have another ace in the pocket for the clipping problem.

BTW: How does your amp perform otherwise (reliability, sound ... ) ?

Regards

Charles
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Old 14th October 2005, 10:10 AM   #8
Pierre is offline Pierre  France
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Thanks. I am all ears...
The amp is performing quite well, sound is very nice and seems also quite reliable (playing for some months at several parties with volume at full power during 7-10 hours). It is now using IRF640N mosfets. They did make the difference in my opinion.
But this "click" is quite annoying... let's see if we can fix it together ;-)

I will post an sch as soon as I have cleaned and updated it!

Best regards,
Pierre
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Old 14th October 2005, 10:30 AM   #9
IVX is offline IVX  Russian Federation
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Pierre,
I saw in the UcD400 33kOhm to +Rail, and 12v zener to overvoltage preventing.
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Old 14th October 2005, 11:12 AM   #10
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Quote:
I saw in the UcD400 33kOhm to +Rail, and 12v zener to overvoltage preventing.
IMO it is indeed a good idea to pre-charge the bootstrap cap. But I think you will also need a resistor to the negative rail - otherwise no charge-current will flow into the bootstrap cap.

Regards

Charles
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