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:
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 😱
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:
An externally hosted image should be here but it was not working when we last tested it.
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 😱
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 !
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 !
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)
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)
that is odd, but kinda feels that something kills your irs
are all the voltages, temps and current with in specs?
are all the voltages, temps and current with in specs?
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 40000µF Decoupling. Also ±5V are stable long before the power stage is switched on.
Temps were 67°C 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.
VCC is stable at 12V referenced to V-. V+ and V- are ±36V with 40000µF Decoupling. Also ±5V are stable long before the power stage is switched on.
Temps were 67°C 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.
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
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
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.
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.
🙂
🙂
Thanks a lot the hint with the "pullup" on the Power stage Output seems to solve my problem 🙂
but i'm still wondering why IRF reference design has no need for this...
but i'm still wondering why IRF reference design has no need for this...
Nice to hear this! 🙂 Congratulations! 😉
The reference design has the pullup resistors directly on the bootstrap capacitors (between Vb pin and +V rail), which are intended to solve this problem. But somewhy it did not worked this time. Maybe this bootstrap pullup resistor is broken in this your sample? 😕
The reference design has the pullup resistors directly on the bootstrap capacitors (between Vb pin and +V rail), which are intended to solve this problem. But somewhy it did not worked this time. Maybe this bootstrap pullup resistor is broken in this your sample? 😕
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well i got this resistor also in my design. I built three of these amp pcbs all with the same issue. So i don't think the resistor is broken but i'm going to check tomorrow. Only got one for testing at the moment here but this one seems to work fine with a 68k pullup on vout.
So tomorrow there will be a lot of stress tests and hopefully my amp modules work fine 🙂
So tomorrow there will be a lot of stress tests and hopefully my amp modules work fine 🙂
Did you implemented the "startup sequencing" in your design also? The key point here, I think, is that the CSD pin should stay low for some seconds after power on, allowing the bootstrap capacitor to charge via pullup resistor. To test this possibility, you could try to short CSD pin to GND (VSS pin) and release it manually in some seconds after power on.
That resistor is in on the IRS2092 datasheet circuit.
I believe it is one of the conditions before the 2092 will start running.
IR seem to be a bit lax sometimes with datasheet circuits.
Between datasheet circuits and app notes circuits there can be differences.
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