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| Lab Power Supply - The Lost Current |
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| C:
--- Quote from: xavier60 on February 26, 2018, 10:37:41 am ---The output of the CC op-amp starts slewing down when it senses that set current has been reached. --- End quote --- This is why I suggested you look at the anode & cathodes of D1 & D3 You have a limited rate of change on the outputs of the op amps. In addition more time is usually required for op amp's output to come off it's rails. You need to test both CC and CV mode before making a change. What happens to one should happen to the other. CV mode to current op amp. CC mode to voltage op amp. When you find a problem for one make same change for other. With your current connections, C6 is hiding current changes from R5. In addition R5 voltage drop changes is effecting Q2 directly. moving C6 to Q2 source would let R5 see more of the load current change & let CV control see need to change. Think of what circuit is doing. A negative change of Q2 gate is output increase. A negative change on R20 is output increase. Swapping current sense makes a negative change of R5 and output increase. With all working in same direction simpler control. You have D1 & D3 setting min and controlling Q2 Think of another analog OR that sets ready to take control max. Set voltage or D1 anode Set current or D3 anode Remember to keep testing with lower values of C6 as this will let you see problem areas better. |
| xavier60:
actually it is stable with 47uF for C6, I neglected to update the schematic. There is a delay of the op-amp's output initially dropping from sitting at full voltage. It is responsible for the remaining current overshoot. I have never seen any designs with the shunt exposed directly to the load. Ill see what happens. |
| xavier60:
Moving C6 to the other side of the shunt allows the shunt to see the capacitor's discharge current but nothing can be done about it anyway. I mainly set out to solve the problem that many PSUs must have, where the short circuit current is unlimited for some time. My Agilent U8002A will supply the full set voltage into a sub ohm load, tens of amps, for 100us before the CC loop begins to respond. |
| C:
--- Quote from: xavier60 on February 26, 2018, 01:58:17 pm ---Moving C6 to the other side of the shunt allows the shunt to see the capacitor's discharge current but nothing can be done about it anyway. I mainly set out to solve the problem that many PSUs must have, where the short circuit current is unlimited for some time. My Agilent U8002A will supply the full set voltage into a sub ohm load, tens of amps, for 100us before the CC loop begins to respond. --- End quote --- Something can be done, decrease size of C6 to decrease current dump. Increase control speed to reduce 100us. The more you reduce R1 the faster you can change. The more current you can pump in/out Q2 gate the faster you can change. If you have control then you can add functions. The faster you can make circuit respond the less time output is uncontrolled. Faster also reduces overshoot. Think about it. you can drive a hot rod on the street with proper control. A bad driver will end up in a tree, while a good driver has power to get out of way. The difference is how good is the control. One of the big problems for a power supply is the unknown load. I have seen power supplies that add a scaled internal voltage load & a scaled internal current load not effected by output load to get better control. Need to keep in mind total result & how each part effects local area & total. for example C3 has two effects, removes noise & slows changes. A cap across R20 does same for output. A cap across R21 adds output noise & speeds changes. There will be matching places for CC side. Using both an AC divider. You can use separate dividers and mix two at input to op amp. Remember that when you add a cap that the effect is different at DC then at high frequency. Try to have few caps and really think about it before adding one. Right now you are running op amps open loop with total circuit controlling gain. You have option of some local resistor feedback which would slow all changes. Note that you can use many feed back paths. Op amps are commonly used to combine two audio sources for example. Might think of three inputs to op amp, scaled output, reference input and local feedback that gets canceled or over ridden to keep output off rails. Old timers often used positive & negative feedback. If you have 1v positive feedback and 1v negative feedback. A set of matching resistors in series = 0 feedback. Remember that mode change is by conduction curve of diodes. Diodes have capacitance. Diodes only pull one way. Watch for difference in over shoot vs under shoot. Your LED's are adding error to output that is different between low and high. I would think of using a quad comparator for leds. When thinking of a change, really look at the many ways to accomplish same thing. |
| C:
You are using NE5532p http://www.ti.com/lit/ds/symlink/ne5532.pdf Q2 = PSMN020-150W https://media.digikey.com/pdf/Data%20Sheets/NXP%20PDFs/PSMN020-150W_2.pdf D1 & D3 =_____ Here is my thinking Q2 gate is a voltage control device, current is rate of change. The Q2 gate-source slows rate of change. And with out proper control can lead to ringing. D1 & D3 have non linear conduction curves but only effect positive output change. NE5532p is a voltage output device. From data sheet output short circuit current is 10-60 ma. The current limit of NE5532p limits positive output change A part of this current goes to over coming diode pull up resistor. diode pull up is source of negative output change output & Q2 gate. To get fast you need to change fast but must remember time delays of Q2 gate-source cap & NE5532p delay. Now look at CC op amp in CV mode or CV op amp in CC mode. Both are swinging full range which takes time. If you can build a window around these, then you could shorten the swing from a little above current output needs to control of output. You can connect two more diodes below D1 & D3 and build a max analog OR D1 & D3 are a min analog OR of CV/CC to control output. New Max analog OR can be used to limit the swings of the op amp's Really try to keep both CV and CC op amps the same circuit with only difference being difference of output & reference. Also note that you can regulate down to 0. With R15 connected as shown in last the rate of change will get slower the closer to 0 you get. Think of dropping the output and catching it with the op amps. You have option of using a constant current or a better third op amp to do this function. You could add a resistor between op amp output and diode to be able to sense op amp output current. You can window a control signal and slide the window. You can also window the window and gain even more control. |
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