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Oscillation in PSU simulation

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xavier60:
In this example, Q5 monitors the voltage across the CS resistor. Its Collector can draw drive current away from the Darlington's Base.
https://www.eevblog.com/forum/projects/how-to-design-fast-bench-supply-with-cc-and-cv/msg3018424/#msg3018424

StillTrying:
I think the 2 improvements have created a problem where the +5V current setting reference voltage is no longer referenced to ground ?

ElectronSurf:

--- Quote from: xavier60 on April 17, 2020, 12:05:09 pm ---In this example, Q5 monitors the voltage across the CS resistor. Its Collector can draw drive current away from the Darlington's Base.
https://www.eevblog.com/forum/projects/how-to-design-fast-bench-supply-with-cc-and-cv/msg3018424/#msg3018424

--- End quote ---

Sorry for asking unrelated questions to the topic; does op amp slew rate plays a role in fast switching to CC mode too?

Zero999:

--- Quote from: ElectronSurf on April 17, 2020, 11:10:36 am ---The scope is DSO138, it's a toy. I didn't say ltspice isn't working, I meant the falstad isn't working and I have to learn ltspice.

For current sensing I ordered INA128 in-amp so I thought it's better to simulate it as a simple differential amplifier for now. and to be able to measure the voltage remotely I'm going to need another in-amp which I'm planning to home-brew it with three OP07. do you think having two in-amp and two op amp is going to oscillate because of the loop you mentioned? (the keyword is "hobbyist", this guy (me) don't know anything about electronics)

--- End quote ---
Why do you want to use an instrumentation amplifier? They're not designed to be used in a loop like that. There's certainly no point with the voltage amplifier, just use precision resistors and a low offset/bias current op-amp. The internal matched resistors inside an instrumentation amplifier make more sense, for the current amplifier, but precision matched pairs of resistors can be used with the op-amp circuit I posted if you want better performance.
https://www.mouser.co.uk/ProductDetail/Vishay-Thin-Film/ORNTA50-1T0?qs=sGAEpiMZZMvrmc6UYKmaNbqQAD9%252BUsfF3rw6ojp5gC0%3D
https://www.mouser.co.uk/ProductDetail/Vishay-Thin-Film/AORN100-1AT5?qs=sGAEpiMZZMvrmc6UYKmaNV6xYUiB84CiwIR2BTir2S8%3D



--- Quote from: ElectronSurf on April 17, 2020, 12:19:54 pm ---
--- Quote from: xavier60 on April 17, 2020, 12:05:09 pm ---In this example, Q5 monitors the voltage across the CS resistor. Its Collector can draw drive current away from the Darlington's Base.
https://www.eevblog.com/forum/projects/how-to-design-fast-bench-supply-with-cc-and-cv/msg3018424/#msg3018424

--- End quote ---

Sorry for asking unrelated questions to the topic; does op amp slew rate plays a role in fast switching to CC mode too?

--- End quote ---

Yes. When switching from one mode to another, the op-amp must swing from close to the positive rail, all the way down to just above whatever the output voltage is supposed to be. For example, suppose the output is set to 20V, the voltage amplifier's output will be somewhere around 21.5V and the current amplifier's output will be saturated, as close to its positive rail, as it can. When the output is short circuited, the current amplifier's output will have to rapidly drop down to 1.5V or so, whist the voltage amplifier's output shoots all the way up close to its positive rail. This takes time and a large current will flow. When the short circuit is removed, there will be some voltage overshoot.

Here's a link to a simulation of a circuit, showing the current and voltage overshoot. Note that it's not very stable and will oscillate with a capacitive load, because the current amplifier is within the voltage amplifier's loop: U1 pulls down U2's inverting input, via a diode.
https://www.eevblog.com/forum/projects/how-to-design-fast-bench-supply-with-cc-and-cv/msg3007168/#msg3007168

Diodes can be added to clamp the op-amp's output voltage to the inverting input, just limiting the output rate of change and is discussed in the thread linked to above.

ElectronSurf:
I wanted to use in-amp to measure the current accurately (0.1 mV/16 bit ADC) at the high side of circuit. unfortunately I couldn't find any seller who sells thin film resistors where I live, so the only option is to use a simple voltage divider or an in-amp/differential amplifier. is there a third option? can I compensate for the created loop instability somehow?

About the op amp slew rate; does JFET op amps speed makes them a good solution for lab power supply application?

This for example: https://www.analog.com/media/en/technical-documentation/data-sheets/OP249.pdf

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