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Checking PWM signal on Class-D amps using IRS21844S problems
Checking PWM signal on Class-D amps using IRS21844S problems
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Old 22nd August 2017, 03:56 PM   #1
Troy Dionne is offline Troy Dionne  United States
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Default Checking PWM signal on Class-D amps using IRS21844S problems

I am trying to test functionality on a few amps that use the IRS21844s without risking damaging the output Transistors and I am not having much luck. Hopefully somebody can clear up how to do this because I can't seem to get it to work as described.

On amps that use the IRS21844s without any driver transistors between it and the output FETs I have tried removing the rectifiers, left the output fets in the circuit. Powered up pins 11(negative) and 13 (positive) with a 9v battery. Played a 100hz sine wave at high volume (not clipped) with the gain a good ways up, but I see nothing useable at the output FETs. I have tried this on amplifiers I know are working as well and still dont see anything that resembles a square wave or 100hz at the gate legs of the outputs.

On driver cards that use drivers (A1715/C2814) between the 21844 and the outputs, I have connected a 10K resistor across pins 6+7 (pins 1+2 are not used) of the TL072, and again played a 100hz signal into the amplifier at high volume. I see nothing that resembles a clean square wave or 100hz at the gate legs of the output transistors.

What am I missing? Id really like to find a way to check the circuit up until the outputs without risking them before I power it up fully with new outputs installed and risk them exploding.
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Old 22nd August 2017, 04:56 PM   #2
Perry Babin is offline Perry Babin  United States
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Do you have a square wave at the input of the 21844?

Do you have a square wave at the output of the LM211?
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Old 22nd August 2017, 05:03 PM   #3
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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I believe you are misunderstanding class D theory. Once the signal has been compared with the ramp/triangle, no more audible signal until following the output filter. In fact, the signal itself disappears, it is "mixed" inside the square one. Take a reading at this pdf.
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File Type: pdf Class D Audio 03.pdf (281.8 KB, 57 views)
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Old 22nd August 2017, 05:06 PM   #4
Perry Babin is offline Perry Babin  United States
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This is a self-oscillating design. It has no clock (triangle waveform).
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Old 22nd August 2017, 05:35 PM   #5
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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OK, but a basic guidance I believe it can be read.
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Old 22nd August 2017, 05:58 PM   #6
Troy Dionne is offline Troy Dionne  United States
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Quote:
Originally Posted by Osvaldo de Banfield View Post
I believe you are misunderstanding class D theory. Once the signal has been compared with the ramp/triangle, no more audible signal until following the output filter. In fact, the signal itself disappears, it is "mixed" inside the square one. Take a reading at this pdf.
It's my understandinng that with these amps, if you drive a strong 50-100hz signal into the amplifier with no rail voltage present you should see a square wave of that frequency at the gates of the FETs.
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Old 22nd August 2017, 06:00 PM   #7
Troy Dionne is offline Troy Dionne  United States
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Quote:
Originally Posted by Perry Babin View Post
Do you have a square wave at the input of the 21844?

Do you have a square wave at the output of the LM211?
With no signal coming into the amplifier I get a square wave at the input of the 21844, when I try to drive a signal into the amplifier there is no more distinct square wave at the input of the 21844 anymore.
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Old 22nd August 2017, 06:05 PM   #8
Perry Babin is offline Perry Babin  United States
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What about at the points before the 21844?
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Old 22nd August 2017, 09:50 PM   #9
Troy Dionne is offline Troy Dionne  United States
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On the amp I'm working on right now (4x 21844's, no driver transistors between the driver card and outputs) there is a 4.6v 2Khz square wave from the LM211. The same 2Khz wave is seen at the input to the 21844's, but at a much lower 740mV.

When a 100HZ signal is played I see square waves of the same voltages, but they have varying duty cycles so it appears as a bunch of overlapping square waves on my scope. This seems normal though correct?
.
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Old 22nd August 2017, 10:49 PM   #10
Perry Babin is offline Perry Babin  United States
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If you are not driving too much signal into the amp, you should see audio on the output of the TL072 with the 10k resistor installed.

That feeds the LM211 which will produce a square wave of the same frequency (100Hz) that swings 5v.

The output of the LM211 drives an emitter follower pair. You should have the same square wave on the emitters of that pair.

There should be no modulation.

You can shut down the driver ICs (may be interfering) by shorting Q10(?) collector to emitter). The emitter will be connected to the negative rail. The collector will drive the control pins of the 21844s.
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