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| Is 550uF too big for a power supply that has CC limit? |
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| bloguetronica:
--- Quote from: Kleinstein on January 29, 2019, 01:44:25 pm ---C12 is not just making the current regulation slow, it also adds phase shift and thus makes stability difficult. So C12 is definitely a bad idea. If one really wants it slower it would be a larger cap for C11. Usually with this kind of circuit current regulation is already relatively slow - if one makes it too slow the simulated capacitance may be more than the real 500 µF. One problem it the output of IC7 swinging rather high, well above the active zone that is at some 1-3 V. In the circuit the OP has only 5 V supply and this already helps, but there is still some delay before the current limit sets in. A resistor in series with C11 adds a little more higher frequency gain and phase boost. If not too much it can help with stability. One may not need this if C12 is left out. --- End quote --- In the previous circuit, there was no C12, and in CC mode the circuit oscillated and the current read incorrectly, with spikes of 1.5A when the limit was set to 500mA! Already tried a much bigger C11 (1uF!) and a resistor in series. The capacitor improved, but the resistor didn't helped. Anyways, I see conflicting opinions regarding the need of C11 without C12. Taking C11 out solves the current spikes in the totally opposite direction (current is set to low, still oscillating). Kind regards, Samuel Lourenço |
| bloguetronica:
--- Quote from: blackdog on January 29, 2019, 02:12:44 pm ---Hi, C12 is there to slow down the response and make the system stable as a whole (the CC mode is now constant with any load). Wrong! As others have indicated, remove this capacitor. The very common mistake when designing power supply is to make it very slow to get stability. You just need Speed........... All extra RC times, all extra active components like opamp and transistors within a control loop make this loop inherrent unstable (slower). These components delay this loop and cause the phase spmargin to get smaller and smaller, when you reach 180-degrees you have built a transmitter. :-DD And the next component that will cause you a headache is Q1 in your schematic. This transistor gives you extra loop gain that is hard to compensate. It is clear that you miss a piece of knowledge regarding loopgain, phase margin, phase margin, etc. Without this knowledge you can't design a good power supply. Good information about this is available on the TI website: https://training.ti.com/ti-precision-labs-op-amps-stability-1 I hope this helps, Kind regarts, Bram --- End quote --- The fact that Q1 ruins the phase margin is a hint, but I can't get rid of it. Hence, that's why C12 is needed, to dampen the response (I don't see any oscillation at the amp's output with it). Makes sense, and that's why any combinations of C11 plus/minus a 1K resistor in series wouldn't solve this. Now, C11 doesn't need do be huge, although without it the circuit will oscillate, due to the fact that the op-amp is unstable (a different issue here!). I can only make the necessary changes if I have hard evidence that they will work. As far as I'm concerned, the CC mode was working fine, after the modifications (one of them was the inclusion of C12), until the 5V regulator blew up. I don't like C12 either, but theory doesn't make practice (also it is not as bad as if C12 was there right at the op-amp's output). Kind regards, Samuel Lourenço |
| bloguetronica:
Ok, I think there was an issue with my question, and I got misunderstood. It is not that my circuit was unstable. It worked fine with a 270uF output cap. I don't have stability issues. I just wanted to know of 550uF was too much. Kind regards, Samuel Lourenço |
| Kleinstein:
The IC7 with C11 still has unity gain at high frequency. So C11 can not provide the full compensation for the CC loop. So C12 may indeed be needed. So there is a chance to keep C12 (though maybe a little smaller) as the main compensation for the CC loop and make C11 small enough or provide enough series resistance so that at the higher frequencies it is C12 doing the job and C11 has still finished it's job. |
| bloguetronica:
--- Quote from: Kleinstein on January 29, 2019, 02:52:27 pm ---The IC7 with C11 still has unity gain at high frequency. So C11 can not provide the full compensation for the CC loop. So C12 may indeed be needed. So there is a chance to keep C12 (though maybe a little smaller) as the main compensation for the CC loop and make C11 small enough or provide enough series resistance so that at the higher frequencies it is C12 doing the job and C11 has still finished it's job. --- End quote --- The OPA705 is unity gain stable. I could try using a 10K resistor in series with C12, which would effectively double the gain at high frequencies, but I don't see this as a substitution for C12. The issue here is that, I was initially going to use two 47uF caps. But now I found a 270uF cap that has the same footprint, and at some point I've decided to use two of them, as a replacement (the more, the merrier, right?). However, I was alerted by Dave's video that a too big output capacitance might impair the CC limiting of the power supply. Nevertheless, I saw that a single 270uF cap would be more than enough. And perhaps, I should replace it with a 150uF cap (different footprint). Kind regards, Samuel Lourenço |
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