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Old 28th March 2011, 03:37 PM   #1
mstro is offline mstro  Germany
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Default IRSAUDAMP4 - IRS20957S Problem

Hello,

Since i read a lot in your forums i think this is the right place to put my question on

I have built a Class-D Amp based on the IRF Reference Design IRAUDAMP4(A). Worked pretty fine for a time but now i'm getting Problems with it.

The PWM modulator is similar to the refernce design (self oscillating, TLC081 as integrator, IRS20957 Gate Driver, and IRF6645 Mosfets).

Now to the problem:
On Startup one of the Channels isn't starting to oscillate. Measuring the PWM Input Pin of the IRS20957 shows a constant High.
In the folowing Picture the Output Voltage of the Power Stage is shown:
Click the image to open in full size.

When i short the Input Pin to Ground, the channel starts to oscillate properly at a frequency of about 380kHz.
It seems like the input is some kind of inverted after startup, so the oscillating won't start.

The Other Channel of the Board works fine and they have the same Layout.

Has Anyone had similar Problems with this modulator stage?
Any help on this would be great, I'm kind of stuck here

Thanks for your help and greetings,
Manu
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Old 28th March 2011, 06:47 PM   #2
savu is offline savu  Romania
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Hello Manu ...
try to change the opamp ...
can't think of anything for the time being

regards
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Old 28th March 2011, 09:29 PM   #3
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I believe soem class d chips need an input signal to start oscilating.

I can believe one IR chip working and the other not after my experiences with the IRS2092. I can get resetting with one chip on clipping and plug in another and there is no resetting at all !
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Old 29th March 2011, 07:01 AM   #4
mstro is offline mstro  Germany
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Thanks for your replys.

I changed the opamp to another tlc081, tlc071, opa827 everywhere the same error.

So i replaced the Gate Driver and the channel worked fine for about 10 Hours. Now i got the same problem again.

An Input Signal results in half sine wave (only negative part of the sine wave on the output)
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Old 29th March 2011, 08:19 AM   #5
luka is offline luka  Slovenia
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that is odd, but kinda feels that something kills your irs
are all the voltages, temps and current with in specs?
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Old 29th March 2011, 08:43 AM   #6
mstro is offline mstro  Germany
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the external components are equal to the reference design, the layout of the power stage is also equal to the reference design.

VCC is stable at 12V referenced to V-. V+ and V- are 36V with 40000F Decoupling. Also 5V are stable long before the power stage is switched on.

Temps were 67C at maximum. But when it really gets to hot OTP triggers.(testet with no heatsink on the power stage). I tested the Amp with about 90W Output on 4 Ohm Low Inductance Power Resistors mounted on a big Heatsink. So Current Ratings shouldn't exceed the specifications of the chip.
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Old 29th March 2011, 03:01 PM   #7
mstro is offline mstro  Germany
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So after a lot more testing i think i located the problem at the gate drivers input stage.
It seems like after startup the input is inverted, what results in a non switching oscillator.

Any guesses how this could happen?

Next step i try is some input protection for startup
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Old 30th March 2011, 01:09 PM   #8
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The first thing I think of when you mention that it's not oscillating at first is that the output transistor is set in a open mode to long letting the current rushing in which will stress the transistor.
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Old 31st March 2011, 11:47 AM   #9
81bas is offline 81bas  Germany
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No startup without input signal - it is the common problem in self oscillating class D amps with bootstrap supply for high side gate driver. Try to connect the amp output (before inductor) to positive power supply rail via 50K-100K resistor. This usually solves the problem with bootstrap supply at startup. This resistor creates some small initial offset voltage at output after power on and forces the oscillator to start with negative pulse, effectively charging the bootstrap capacitor of the high side gate driver. During normal work (after oscillation starts), this resistor does not affects the amp output because of the low amp output impedance.
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Old 31st March 2011, 11:54 AM   #10
luka is offline luka  Slovenia
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indeed, this is on irs2092 too
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