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Old 13th March 2014, 12:07 AM   #1
Trileru is offline Trileru  Romania
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Default regulated supply oscillation or whatever that is

Hello,
I designed a pcb for a regulated supply using LM317/LM338/LT108x.
So I made the boards and populated them and then made some measurements using my (also) newly made TDA7297 amp. This chip has a current limit of 2A. When I run the chip past 1A I get some oscillations on the regulator board. Looks like a doubling of the amp's test frequency. The lower the frequency the larger the noise. I made all these screenshots at the same volume. You can deduct the test frequency from the frequency of the oscillation. I made the measurement on the output of the PS board. I also get something similar from another PS board used with a ta2020 amp, but the overall effect is smaller. The tda PS uses a LT1083 (or 84, can't remember now) regulator and the ta2020 amp uses a lm338 regulator.
I attached the schematic used for the power supply.
Also I had to shut off all cfl lights, removed all smps from sockets and shut down computer. Basically it was only the amp and oscilloscope running in the whole house
There are about 20cm of cable between PS and TDA board. And basically nothing is grounded, only the transformer. And there is no case at the moment
So is this normal for this type of power supply? Forgot to mention that I'm running the output of the PS at 16.5VDC. As I go past 1A I start to see the test sine wave (doubled in freqency) on the output of the PS.
The regulating chip is well cooled by a decent heatsink, it just stays warm. I modified the schematic to reflect the exact values of the components used on the PS board.

PS.
Yes, I'm using two paralleled resistors for R2 so I can set a more precise output voltage. It's staying there, yes, I like it that way Unless that's the culprit, which I don't think so.
pps.
The output capacitor is made of 2x 10uF tantalum capacitors soldered together. I only had a 68uF R2 bypass capacitor and as per application I adapted the output capacitor.
ppppppppppps
the ta2020 PS board uses a LM338 regulator with different R2 bypass capacitor / output capacitor combo, and still shows something similar. I know LT108x are prone to oscillation if the output capacitor is not in spec with the datasheet, or low esr. Also the TDA board has diodes on power input as I'm splitting into two power rails for the amplifier chip. On the ta2020 board power goes directly to some low esr FM caps.
I noticed that if I add a whatever value capacitor at about half the distance between the PS and the amp this effect decreases.
Attached Images
File Type: png reg.png (13.6 KB, 200 views)
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File Type: png 20140304_323620.png (26.6 KB, 196 views)
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File Type: png 20140304_323856.png (23.6 KB, 36 views)

Last edited by Trileru; 13th March 2014 at 12:17 AM.
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Old 13th March 2014, 07:42 PM   #2
Trileru is offline Trileru  Romania
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So, anyone has any ideas?
This is my first power supply that I made so I don't know if this thing is normal or if there's a problem somewhere. I don't have the experience to judge this. I just noticed this while looking with the oscilloscope on the output.
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Old 13th March 2014, 10:30 PM   #3
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I think the issue stems from the regulator output impedance.

I made some pretty tough rail PSs (4-5A) based on the LM723 and high gain high current pass devices (darlington), and with a high capacitance bank AFTER the reg/pass device (22000uf per rail). Voltage value is rock steady no matter what. To take care of the pass devices I have to turn on the PS through a soft start circuit. It's been working like a charm for a few months and I turn it on daily.

My advice here is try installing high gain darlington pass devices even if the required current is within the regulator capabilities. A really big output capacitor (10000uf or more per rail) will turn your PS super steady but you will need a soft start to turn it on.

Which is the capacitance value of the input bank?
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Old 13th March 2014, 11:10 PM   #4
Trileru is offline Trileru  Romania
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Input is 6700uF total. Output is only 20uF at the moment.
The thing is that I need to put some normal ESR capacitors there. I will do some testing with the output capacitor value.
Thank you!
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Old 13th March 2014, 11:29 PM   #5
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Try some resistance in series with tantals (1R or less).
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Old 14th March 2014, 12:27 AM   #6
Trileru is offline Trileru  Romania
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Quote:
Originally Posted by aparatusonitus View Post
Try some resistance in series with tantals (1R or less).
There is already, 47uF in series with 0.47R.
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Old 14th March 2014, 01:37 AM   #7
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I suggest (but don't take me too seriously ) removing the output snubber and R.load and doing this test again.

Best regards.
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Old 14th March 2014, 01:47 AM   #8
infinia is offline infinia  United States
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try testing the regulator alone with dummy loads 1st.
check DC stuff 1st , ensure Vin to Vout under max load is at least 3 VDC
make sure it works stable and heat sinking is good enough.


testing 2 new things at once by analyzing AC waveforms , is like trying to run before you can walk
back to basics gents
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Last edited by infinia; 14th March 2014 at 01:55 AM.
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Old 14th March 2014, 11:28 AM   #9
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A board layout would be very helpful.
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Old 14th March 2014, 11:42 AM   #10
esgigt is offline esgigt  Netherlands
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Trileru, from your description I get the impression that beyond 1A the stability margins of the regulator are exceeded.
You can solve this in several ways:
Quote:
Originally Posted by aparatusonitus View Post
Try some resistance in series with tantals (1R or less).
Or you can apply a small series resistor ( .2 a .3 Ohms) and some 10 to 20 mF on the output line. This will give less load regulation, but better ripple and noise performance.
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