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| Lab Power Supply - The Lost Current |
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| xavier60:
It did become unstable when I reduced C6 to 1uF Without changing the speed of the MOSFET, I was able to make CV mode stable by tweaking the CV op-amp's feedback components. So I hope this means that I can now more trust the stability when I make C6 larger again. I gave it some Proportional gain also which has helped the CV op-amp regain control much sooner from voltage overshoot after a load dump. This period after an overshoot can be a problem because the CV op-amp drives the Gate to a lower than usual voltage. If load is applied during this period, it catches the CV op-amp off guard and not able to turn the MOSFET on in time causing a very deep dip in output voltage. I really must make the PCB now. |
| C:
I could be wrong but just looking at it, it's still not very good. Guess I am not explaining very good. If the two op amp circuits so not look the same you have a problem. The difference between CV & CC is nothing but gain. Really think it through. R5 uses current to create a Voltage. R20 & R21 a voltage divider output is a voltage. Looking like you just want to hack a poor CC hack on a voltage regulator.. Adding cap's is very last step. Should have very good stability with out cap's. An osculation is trying to tell you it's bad. You have to find the reason it's bad and correct the real problem source not hack a patch. To see the true problem requires looking, testing and some times adding or modifying to find the actual problem. Remove feedback from CV amp and just use a resistor to gain some control. What will help is some non linear gain in path. Remember that this is one big mess of interactions and need to get all correct. If is osculates work to make it osculate at a higher frequency. First get you connections to Q2 area correct and good. With your 12V common connected to + output you are adding control noise into what you are trying to sense. This can cause problems. Moving 12V common to drain of Q2 is proper connection to prevent control currents effecting both sense. Start testing CC mode, just set CV to not harm load. Start with a load of the biggest cap you have as a load. The big cap will give you more time to use scope to look at circuit. With cap discharged you should see a quick ramp up to set current. Now think about CC op amp. It's acting like a comparator more then an an amp. This is a cause of osculation Remove C1 and replace it with a resistor. This resistor value is like current gain. As you are not changing to CV mode until load cap is charged, CV circuit should not be effecting circuit. Replace R1 with two resistors in series. Parallel one with back to back diodes. This should be the higher value resistor of the two. What this does is change your rate of change to be non-linear. You are using the conduction curve of the two diodes. With cap as load. when turned on, you will transition from diodes not conducting to diodes conducting hard. As current gets closer to set current, The rate of change will decrease due to diode curve. If this osculates decrease feedback resistor on CC op amp. Change values of two R!'s Resistor paralleled by diodes is center control. Other resistor is high rate of change control. Feedback resistor is how hard to go at high rate. All interact. Once you have best action. lesser value load cap should just have voltage across cap changing at a faster rate. If you have osculation, look for cause. bypass caps should be large between op amps V+ & V- A cap to V common is a source of noise. A huge cap on output has no high frequency, as caps are for High frequency No caps, Find the problem. When you have this make CV op amp match. only feedback resistor is different. Test it with out going into CC mode. When you have this, next step is working on mode change. Note when testing in CC mode with large cap you can switch in/out a resistor to discharge the cap. Q2 will be changing from open with resistor doing allmost all the discharge back to some on resistance when resistor is switched out. If you have problems, think of a good change, think you will save time posting the change first. Think along the lines of non-linear with smooth changes. To be clear an osculation keep going. Ringing that decreases is not osculation. For example op amps have no output down control, Only veering up control. A resistor with one bypass diode changes feedback of up vs down. |
| xavier60:
--- Quote from: C on February 27, 2018, 12:49:20 pm ---I could be wrong but just looking at it, it's still not very good. Looking like you just want to hack a poor CC hack on a voltage regulator.. --- End quote --- No, not at all. In my previous post I explained that I had eliminated instability when C6 is 1uF. I can even get it to be stable with no output capacitor. But I will be putting a 47uF on the output when I finish it. What part of the circuit looks like a hack? Because the CV and CC are working very well. Extra: Ill post an updated schematic after I have done the PCB and done more testing. |
| C:
If you still have 12v common connected to positive output. It is not as good as it could be and that connection adds control noise to what you are trying to sense. Proper connection for 12V common is between Q2 and current sense resistor. |
| xavier60:
--- Quote from: C on February 27, 2018, 02:10:04 pm --- If you still have 12v common connected to positive output. It is not as good as it could be and that connection adds control noise to what you are trying to sense. Proper connection for 12V common is between Q2 and current sense resistor. --- End quote --- That would reduce noise but makes voltage sensing a bit more complicated, needing a balanced input amplifier. This is the schematic for the Agilent U8002A. They seem to have tied the 12V common to the + output. Difficult to be certain with the way they use ground symbols. And I can't find the output capacitor to see what size it is. It appears to be about 100uF when I do an external measurement at the output terminals of my U8002A while it's totally powered down. http://d1.amobbs.com/bbs_upload782111/files_39/ourdev_639010D487UM.pdf |
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