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| Is there any variable DC-DC converter suitable for pre-regulation? |
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| bloguetronica:
Hi Tim, In my case, I need to control the current more or less precisely to the milliamp. The current limit is set by a DAC, and it has to be set proportionally and precisely. That's whay I'm using a full op-amp as a comparator. I can't dispense the voltage control op-amp either for a similar reason (because I want a very precise voltage controll, even far more precise than the current control). So, I'll use the resistors around the op-amp as you suggested. Also, the voltage divider to control the base of the transistor is also a good idea to diminish the sensitivity (I could use a MOSFET, but unfortunatily it has a too high of a Vth gs). Kind regards, Samuel Lourenço |
| Wolfgang:
Hi Sam, are you sure you have understood how control loops work ? What you do here is running them uncompensated, which is a major design flaw. You should think about your gain and phase margins under several load and current/voltage mode conditions. A good idea is to simulate load step responses and tune compensation so that overshoot and settling time is minimized and no "ringing" occurs. The voltage/current steering solution is also not optimal. Normally you try to use switching diodes to priorize the active op amp. In order to speed up the switchover, some minimum amp loading is also beneficial. |
| bloguetronica:
Hi Wolfgang, For the voltage control op-amp, I make sure to use a unity stable op-amp. The op-amp used for the current control is the loose end here. The voltage control part works very well and also compensates the output lead inductance (the reason behing the odd resistor configuration around the op-amp). Kind regards, Samuel Lourenço |
| Wolfgang:
Hi Sam this is a basic misunderstanding. Even if your op amp is unity gain stable, the control loop as a whole can still oscillate. I am afraid you need to dig yourself a little bit more into the theory of loop stability. Why not try some Jim Williams Appnotes from LT ? Or even Wikipaedia is a good place to start. Why do you think that even a methusalem like the LM723 has a frequency compensation pin ? I am absolutely sure that lead inductance is of no importance here. The delays created inside the op amps and in the power stages including filtering are much more important for stability. Not all people like math, but here its needed to understand what is going on, sorry. Another very good reading is Horowitz and Hill Art of Electronic 3rd Edition Chapter 9 about power supplies and regulators. regards Wolfgang |
| bloguetronica:
Hi, I've been simulating ways to solve the oscillation issue. First, I've implemented the regeneration resistor plus the voltage divider. It improved the performance a bit, but not solved it. Then I proceeded to implement the op-amp compensation circuit, but saw no significant difference. The circuit is still oscillating. It is better to consider taking the whole current control circuitry to the garbage and implement a solution that I can understand - one that only implements the voltage control plus the well needed lead impedance compensation circuitry (aka, remote sensing). Tested this scheme without the current control and worked very well. I'm ready to trow the towel on this CC crap thing and just use a fixed SC protection as I did before. What is the point of implementing someone else's circuitry if it was done already? If I'm to reinvent the well, I'll not copy it, especially if I don't understand it. The simple solution, on the other hand, I understand it very well. I can call it a solution of my own. The only thing "new" in this circuit will be the DC-DC pre-regulator, but if that fails as well, I'll ditch it too. No problems with that. --- Quote from: Wolfgang on September 25, 2018, 09:17:08 pm ---... I am absolutely sure that lead inductance is of no importance here. ... --- End quote --- Lead impedance (not inductance) is important and has to be compensated. Remote sensing is the key word. Anyway, it is not the lead compensation circuitry that causes the oscillation. It is the damn current control circuit (unless the simulator is wrong). Kind regards, Samuel Lourenço |
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