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| Is 550uF too big for a power supply that has CC limit? |
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| Doctorandus_P:
--- Quote from: Zero999 on January 29, 2019, 12:23:04 pm ---It's all but impossible to design a PSU with constant current and constant voltage and have good load regulation for both. A capacitor will improve the transient response of the CV mode, at the expense of CC. No capacitor an a large inductor will improve the CC mode, at the expense of CV mode. --- End quote --- Could it be a good idea to put a MOSfet in series with the ouput capacitor and disconnect it in CC mode? Another point of attention are the sense lines. If these are connected to the outside world to compensate voltage drop over external wiring they can get attached to external voltages and this may lead to excessive current through the 1n4148 diodes. If Vsense_Pos gets grounded somehow it looks like your power supply wil output the maximum voltage it's capable of. Owtch. |
| bloguetronica:
--- Quote from: David Hess on January 30, 2019, 04:19:33 am --- --- Quote from: bloguetronica on January 30, 2019, 03:09:05 am ---I think that this other power supply, that has the corresponding schematic attached, might have stability issues too. I ran into oscillations caused by a misplaced ground connection (www.eevblog.com/forum/projects/usb-controlled-precision-power-supply-(or-voltage-reference)/msg2007782/#msg2007782). Normally, this supply is stable, but the constant current load that that you see is being supplied via a SMPS and causes some oscillation, as long as it is not grounded to earth. --- End quote --- Maybe I am reading the schematic wrong but it looks to me like the remote sense to the voltage feedback loop is backwards, but it reminds me of a circuit I was studying a couple weeks ago from the LT1010 datasheet shown below. The OPA703 is slow enough that it should be able to control the output through a TIP31 emitter follower without problems. The first example shown below is a good example of what I was talking about. D2 clamps the current control operational amplifier to prevent integrator windup. C1 and to a lessor extend C2 maintain stability despite the current control loop being in series with the voltage control loop with two relatively fast operational amplifiers. This design requires no output capacitance but has what is effectively a fast full class-ab output stage in the LT1010. Note that the 2 ohm resistor in series with the output of the power stage aids stability when a capacitive load is present and this is an advantage of having the current shunt in series with the output. The second example is from National Semiconductor is the low output capacitance design I was thinking of. D2 takes advantage of the external compensation feature of the LM301A current control operational amplifier to clamp it preventing integrator windup preserving fast response. Q2 makes the output stage more like class-ab with the ability to pull the output down. The third example uses 100 microfarads of output capacitance for a 1/2 amp output current to control transient response because no clamping of the operational amplifiers is used. In practice I think that capacitance could be 22 microfarads without problems but they wanted extra stability. --- End quote --- The remote sense is not backwards, that much I can guarantee. The resistor networks that you see are a means to provide internal lead compensationfor both the positive and ground leads. The schematics you've shown seem to be completely different applications. --- Quote from: Doctorandus_P on January 30, 2019, 04:44:06 am ---... Could it be a good idea to put a MOSfet in series with the ouput capacitor and disconnect it in CC mode? ... --- End quote --- No, that would actually cause instability. You don't want a capacitor connecting and disconnesting when the supply is constantly jumping between CV and CC modes. --- Quote from: Doctorandus_P on January 30, 2019, 04:44:06 am ---... Another point of attention are the sense lines. If these are connected to the outside world to compensate voltage drop over external wiring they can get attached to external voltages and this may lead to excessive current through the 1n4148 diodes. If Vsense_Pos gets grounded somehow it looks like your power supply wil output the maximum voltage it's capable of. Owtch. --- End quote --- Any supply with remote sensing is capable to do that. Some even will output the maximum when the sense wires are not connected. This one will output 10% more of the intended voltage in this case. However, I can't guarantee cases of bad usage (the cases you mentioned). The diodes are there just for EDS protection. Kind regards, Samuel Lourenço |
| David Hess:
--- Quote from: bloguetronica on January 30, 2019, 01:17:39 pm --- --- Quote from: Doctorandus_P on January 30, 2019, 04:44:06 am ---... Another point of attention are the sense lines. If these are connected to the outside world to compensate voltage drop over external wiring they can get attached to external voltages and this may lead to excessive current through the 1n4148 diodes. If Vsense_Pos gets grounded somehow it looks like your power supply wil output the maximum voltage it's capable of. Owtch. --- End quote --- Any supply with remote sensing is capable to do that. Some even will output the maximum when the sense wires are not connected. This one will output 10% more of the intended voltage in this case. However, I can't guarantee cases of bad usage (the cases you mentioned). The diodes are there just for EDS protection. --- End quote --- If the sense inputs are relatively high impedance, then medium value resistors can be used between force and sense at the power supply to prevent failure if a sense line becomes disconnected. |
| bloguetronica:
--- Quote from: David Hess on January 30, 2019, 04:20:53 pm --- --- Quote from: bloguetronica on January 30, 2019, 01:17:39 pm --- --- Quote from: Doctorandus_P on January 30, 2019, 04:44:06 am ---... Another point of attention are the sense lines. If these are connected to the outside world to compensate voltage drop over external wiring they can get attached to external voltages and this may lead to excessive current through the 1n4148 diodes. If Vsense_Pos gets grounded somehow it looks like your power supply wil output the maximum voltage it's capable of. Owtch. --- End quote --- Any supply with remote sensing is capable to do that. Some even will output the maximum when the sense wires are not connected. This one will output 10% more of the intended voltage in this case. However, I can't guarantee cases of bad usage (the cases you mentioned). The diodes are there just for EDS protection. --- End quote --- If the sense inputs are relatively high impedance, then medium value resistors can be used between force and sense at the power supply to prevent failure if a sense line becomes disconnected. --- End quote --- Indeed! And that is the purpose of R18 and R19. They were chosen as a sort of compromise. The output voltage will not get higher than 10% when the sense is disconnected on both sides. However, they don't have such a low value that may affect precision due to lead resistance (think of a shorted R18 and R19 that would defeat the purpose). They also wont dissipate too much current if the sense leads are switched. However, if the sense leads are switched, or if one sense wire gets shorted to the opposite polarity, the load will probably be damaged despite the supply being not. Kind regards, Samuel Lourenço |
| bloguetronica:
I think that R5 might be causing some delay and, therefore, instability. I wonder if adding a capacitor in parallel will improve things. Notice that I can't simply short out R5 because I don't want to short the IC4B amplifier's output. I employed a similar solution today to one of my supplies. See this post: https://www.eevblog.com/forum/projects/improving-the-fau200-power-supply/ Of course, I'll probably have to add some capacitors elsewhere in the loop (near IC4B), as described in previous posts. Kind regards, Samuel Lourenço |
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