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opamp transistor follower oscillates to buggery when in CC mode

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nemail2:
Hi

so I did make this cheapass LabPSU circuit which is attached below and did build it up with a PCB.
(details and more files like board layout here: https://github.com/mamama1/LabPSU_CA1)

The github version is not the most recent one, i have changed R6 from 1k to 4k7 and changed T1 from TIP3055 single NPN to TIP142 Darlington. Everything else is unchanged.

Now, when the PSU goes to constant current mode, the whole thing oscillates to buggery. I had this issue before in a similar design and some nice fella told me how to compensate it.

Principle how CC mode works: IC3B goes to 0V on output pin 7 when the read current (from MAX4080 via shunt R3) on IC3B pin 6 is higher than the set current on IC3B pin 5. D6 starts conducting and pulls IC3A output pin 1 low and the whole thing should settle at the set current.

In this circuit it works perfectly, without any oscillation: https://github.com/mamama1/LabPSU_Darlington/blob/master/schematics.pdf

Attached is also a measurement of how the output of the MAX4080 looks like (after D2 and R5) when it oscillates.

I hope I might get away with just changing the values for R10, C11, R11, C13 and maybe R5. in that case I wouldn't have to respin the PCBs.

Any suggestions would be highly appreciated :)

Thanks!

iMo:
Try to connect C11 to pin 1 (IC3A) instead to the darlington's base..

David Hess:
Why is D2 at the output of the current sense amplifier there?  The working schematic has a very small bypass capacitor across R5 there.  IC3B may be unhappy if D2 is forward biased.

nemail2:

--- Quote from: imo on July 24, 2019, 09:02:55 pm ---Try to connect C11 to pin 1 (IC3A) instead to the darlington's base..

--- End quote ---

that changed the oscillation to this (attached). notice the smaller amplitude, but it is still there...


--- Quote from: David Hess on July 24, 2019, 09:13:09 pm ---Why is D2 at the output of the current sense amplifier there?  The working schematic has a very small bypass capacitor across R5 there.  IC3B may be unhappy if D2 is forward biased.

--- End quote ---

D2 was a suggestion of another dude to make the current control faster in case of big current spikes... what bypass capacitor across R5? I'm confused... so maybe try to remove D2?

nemail2:
removing D2 does this to the oscillation (additionally to connecting C11 to pin 1 of IC3A instead of the darlingtons base.

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