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Class D Switching Power Amplifiers and Power D/A conversion

Fixing [and preventing] a fried Zobel network
Fixing [and preventing] a fried Zobel network
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Old 19th November 2018, 06:25 PM   #1
jsiddall is offline jsiddall  Canada
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Default Fixing [and preventing] a fried Zobel network

I am attempting to use some one of these 500W IRS2092 amplifier modules. I had some success with resistor loads (see the thread Ebay amp - is 500W believable?) but when I hooked up a real speaker I got sparks and smoke.

Some digging revealed that the fried component was a 10 ohm chip resistor R36, seen in this stock photo adjacent to the center screw terminal:

Click the image to open in full size.

Strangely this seemed to only connect to another resistor, labeled R21, which was connected to ground. Checking a known good module confirmed R36 is indeed a 10 ohm resistor. However, after removing the fried R36 I found R21 was open. It didn't make sense to me to build two serial resistors across the load so I checked it with a capacitance meter and sure enough, 0.1 uF. So I believe these two components form the Zobel network from the IRAUDAMP7S
reference design.

For some reason I am getting a bunch of high frequency noise that those tiny components weren't designed to handle. Indeed, the reference design says:

Quote:
"The Zobel network is not thermally rated to handle continuous supersonic frequencies above 20kHz.
These supersonic input frequencies can be filtered out by adding R2 and C2 as shown on main schematic Fig 33 and Fig 34. This RC filter works also as an input RF filter to prevent potential radio frequency interferences."
However, I confirmed there is a 330 ohm resistor connected to some small capacitor after the input capacitor. Presumably this is the 1 nF called for in the reference design so it is still unclear to me is what is causing the oscillation in the first place.

Any ideas?


Edit: I forgot to mention that I had adjusted the carrier frequency down to 330 kHz to reduce load on output drivers. However, having now read through the reference design notes I found this:

Quote:
"A single stage output filter can be used with switching frequencies of 400 kHz and greater; a design with a lower switching frequency may require an additional stage of LPF."
Hmmm... maybe messing with the frequency wasn't a great idea. However, I also found this:

Quote:
"Normally, when adjusting the self-oscillating frequency of the different channels, it is suggested to either match the frequencies accurately, or have them separated by at least 25kHz. Under the normal operating condition with no audio input signal, the switching-frequency is set around 400kHz in the IRAUDAMP7S."
So since I want to drive two of these in a bridge configuration, should I set them both as close as possible to the same, or make one higher one lower? Would 385/415 kHz be a reasonable combination?

Last edited by jsiddall; 19th November 2018 at 06:45 PM. Reason: Added image, info about carrier frequency
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Old 20th November 2018, 06:22 AM   #2
Eva is offline Eva  Spain
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Mutual interference will tend to synchronize the channels, so if the channels are wired for BTL adjust frequency to same value. R36 SMD 10 ohms 1206 is a big no-no, normal 1206 SMD resistors have very limited overload capability, even special SMD resistors (very low value) have inferior overload capability compared to a simple 3W wirewound. Use a 3W wirewound.
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Last edited by Eva; 20th November 2018 at 06:24 AM.
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Old 20th November 2018, 06:32 PM   #3
jsiddall is offline jsiddall  Canada
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Quote:
Originally Posted by Eva View Post
Mutual interference will tend to synchronize the channels, so if the channels are wired for BTL adjust frequency to same value. R36 SMD 10 ohms 1206 is a big no-no, normal 1206 SMD resistors have very limited overload capability, even special SMD resistors (very low value) have inferior overload capability compared to a simple 3W wirewound. Use a 3W wirewound.
Good advice, thanks. I have replaced the R36 SMD with a 1 W resistor since I could get those readily. Admittedly this isn't as good as a 3 W but if I watch it carefully for oscillation I can keep them from burning up before cut the power. I still need to be quick though because it takes literally a second for even the 1 W resistors to get hot enough to start to smoke once oscillation starts. I fear for the little chip capacitor "R21" also, as running a few amps through it would presumably cause it to fry quickly.

Unfortunately that is all the "effect" but I still haven't been able to eliminate the "cause": oscillation when these amplifiers are connected to an inductive load.

I also tried to be a bit more diligent with grounds: keeping the input and the load isolated (no ground loops) but still the amplifiers want to oscillate. I captured this in the moment before I killed the power and it looks like it is full power, 970 W @ 48kHz:


Click the image to open in full size.

This was captured with the inputs grounded.

I am thinking perhaps increasing C2 to lower the frequency of that low-pass circuit but that seems like a bandaid to what is another issue.

Any other ideas what to try next?
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Old 21st November 2018, 08:31 AM   #4
Eva is offline Eva  Spain
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A sustained oscillation requires feedback. There are two types of feedback paths to investigate.

One is direct path through IRS2092 self-oscillation network, like wrong parts. I recommend measuring these parts and posting a schematic. Improper part choice can lead the circuit to oscillate wrongly with anything but a resistive load at HF. The RC damper provides the resistive load at HF. The 100nF on the output damper RC seems a too low value, more usual value is 470nF, as much as LC output capacitor. A usual problem with ceramics is runout of capacitance as voltage approaches maximum, it must be a X7R 50V type at least, capacitance at ~50V can be measured with the help of a power supply, but requires extreme care to not damage capacitance meter, pre-charging the capacitor with a resistor then connecting the meter.

Another is indirect path, as ground loops. 50Khz is too low to be very effectively filtered by a simple input LPF network with low attenuation at 20Khz, -3dB @ 50Khz at best. However, 50Khz happens to be the usual resonant frequency of the LC filter. But there should not be 50Khz current flowing through ground terminals.
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Old 21st November 2018, 07:25 PM   #5
jsiddall is offline jsiddall  Canada
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I was thinking the same thing about feedback, so last night I started going through the reference design schematic and mapping the component labels, concentrating on the feedback loop. I replaced the actual component numbers in green but when I got to R13 (R7 in the reference design) I found what could be causing my issues:


Click the image to open in full size.


R13 should have been about 3.1K but was 100 ohm! Since everything else I found lined up with the reference design I would think that was a manufacturing error.


Anyway, I found some 1/4 W axial 3.1K resistors and will try soldering in a couple of those and see what happens.


Regarding the Zobel capacitance, the IR design does use 100 nF so I am guessing they just copied that.
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Old 22nd November 2018, 03:24 AM   #6
jsiddall is offline jsiddall  Canada
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I successfully replaced the incorrect 100 ohm resistor R13 with a 3.1K ohm (mine actually measured 2.95K) 1/4 W axial resistor and everything is working great. The amplifiers no longer oscillate whether connected to a resistive load, inductive load (real speaker) or no load at all.

Hopefully anyone else who purchased these incorrectly built amplifiers will benefit from this:

Click the image to open in full size.
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Old 22nd November 2018, 09:32 AM   #7
dzwer is offline dzwer  Bulgaria
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Great work!
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Old 2nd December 2018, 01:59 PM   #8
Dr1v3n is offline Dr1v3n  United States
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jssidall, thanks alot for posting your finds. I'm in the middle of a sub project where I use 4 of these little boards. 1 per each 150W 8" subwoofers. I will be checking the R36 and R13 values today and report back.

1 question tho: is the mod to R36 still required after fixing R13 to 3K?
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Old 2nd December 2018, 07:42 PM   #9
dzwer is offline dzwer  Bulgaria
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Mine was with 100ohm too.
Replaced with 3.01kohm and will use it for subwoofer too
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Old 2nd December 2018, 08:31 PM   #10
Dr1v3n is offline Dr1v3n  United States
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Visibly checking the boards, I do see R36 a large SMD package with 100 marking on it and R13 with a 1000 marking. I'll remove these 2 from the boards and measure them with a multimeter.
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