Electronics > Beginners

Problems with stability of power supply circuit

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David Hess:

--- Quote from: exe on August 05, 2019, 07:06:37 pm ---
--- Quote from: David Hess on August 05, 2019, 12:33:59 pm ---The LM358 is slow enough that external compensation should not be required unless something else is wrong.  With an electrolytic output capacitor, the only remaining problem is driving the high capacitance of the power MOSFET.
--- End quote ---

I'm not sure how if it is really slow enough. Datasheet from TI mentions this:


--- Quote ---Capacitive loads which are applied directly to the output of the amplifier reduce the loop stability margin. Values
of 50 pF can be accommodated using the worst-case non-inverting unity gain connection. Large closed loop
gains or resistive isolation should be used if larger load capacitance must be driven by the amplifier.
--- End quote ---

I myself built this circuit and it oscillated at currents above 0.3A or so (pass element is irfz24n or irfz44n). I had to add compensation. It still caused oscillation when I connected it to my lt3080 power supply. So, I had to increase capacitor even further, to quite some big value. Not sure how big my gate resistor is.
--- End quote ---

That is why I mentioned the problem of driving a heavy capacitive load.  Either an external buffer to drive the gate of the power MOSFET is required or external compensation between the output and inverting input needs to be added.


--- Quote from: KE5FX on August 05, 2019, 07:20:10 pm ---
--- Quote from: David Hess on August 05, 2019, 12:33:59 pm ---The LM358 is slow enough that external compensation should not be required unless something else is wrong. 

--- End quote ---

Using a slow opamp is no guarantee of stability.  The opposite is more likely to be true IMO.
--- End quote ---

With a slower operational amplifier, each degree of phase is equal to a greater amount of time so a given amount of delay within the feedback loop is fewer degrees of phase lag.

Emitter or source follower power supplies using slow operational amplifiers like the 324/358, 741, 301A, or my favorite, the 308, almost never require extra compensation unless something is overlooked like driving the large capacitance of a power MOSFET directly which is the case here.  Note that higher gain relaxes the requirements further (attenuation within the feedback loop) and a gain of 1 (voltage follower) is the worse case.  Incidentally, these operational amplifiers are slow enough that the output transistor can be replaced with an integrated regulator although a drive transistor should be used to handle the regulator's quiescent current.

Faster parts like the OP27, LT1007, or TL071 are much more likely to require extra compensation and parts which are faster yet will always require extra compensation.  But they may be worth using despite this for lower noise.

Jim Williams discussed this at a practical level in appendix C of Linear Technology application note 47.

mtimmermans:

--- Quote from: exe on August 05, 2019, 07:11:05 pm ---
--- Quote from: mtimmermans on August 05, 2019, 06:59:03 pm ---Is this the correct way of doing it, or am I misunderstanding something?

--- End quote ---

Didn't read your message as I have to go, but check this thread, it provides quite a comprehensive guide how to to this analyzis. Also provides complete schematic of an electronic load: https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/

--- End quote ---

Thanks for the suggestion! However, also this method seems to depend on where you inject the disturbance. I added the disturbance after R3, and now the circuit looks like:


The however changed the loop gain to


Since the simulated loop gain changes when you add the disturbance at different points, how do you know the correct location to add the disturbance?

exe:

--- Quote from: mtimmermans on August 06, 2019, 11:11:53 am ---Since the simulated loop gain changes when you add the disturbance at different points, how do you know the correct location to add the disturbance?

--- End quote ---

You add disturbance to the input where feedback is taken (imho), and watch output changing. Disturbance should be small so it's "small signal analysis". So, I'd say before R3. There should also be a big inductor so you can measure open loop response. But I'm no expert.

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