Electronics > Beginners
Need tips on building a variable power supply
not1xor1:
OK
I realized that there was a sensible overshot in switching off from CC back to CV.
So I ran multiple simulations comparing the classical CV/CC switch by diode topology with that where the VC reference is decreased as needed.
I also compared LT1013 with LT1056 that with its 14V/µs of slew rate and 5.5MHz GBP is quite faster than LT1013.
The circuit topology where current control works on the voltage reference (like that ebay kit) does indeed completely avoid the overshot. Unfortunately it does that at the price of a much lower recovery time.
That is because both opamps are in the loop and must be overcompensated to avoid oscillations.
A countercheck of that is the fact that by lowering the speed of the CV feedback network of the CV/CC diode switch topology (by putting an RC in parallel with the DC feedback resistor) it is possible to completely avoid the overshot while still preserving a reasonable recovery time, even with the slow LT1013.
Another proof is that even a zener diode in the CV amp local feedback, while preventing saturation, does nothing on that overshot.
Of course if the same is true in the real world is a completely different matter ;D.
If anybody is interested I can clean the circuits to make them more readable and attach screenshots and or the asc files.
It would be better to check... just in case I made some coarse mistake... :)
wasyoungonce:
[quote author=not1xor1 link=topic=152870.msg19928
If anybody is interested I can clean the circuits to make them more readable and attach screenshots and or the asc files.
It would be better to check... just in case I made some coarse mistake... :)
[/quote]
Actually that would be great and helpful. Thanks for your efforts for others
Brendan
Sent from my iPhone using Tapatalk
David Hess:
--- Quote from: not1xor1 on November 26, 2018, 05:21:23 pm ---A countercheck of that is the fact that by lowering the speed of the CV feedback network of the CV/CC diode switch topology (by putting an RC in parallel with the DC feedback resistor) it is possible to completely avoid the overshot while still preserving a reasonable recovery time, even with the slow LT1013.
--- End quote ---
This is why it is seldom advantageous to use a faster operational amplifier in a power supply; extra frequency compensation is needed anyway to maintain stability slowing it down. The one reason I have done so in the past is for lower noise.
--- Quote ---Another proof is that even a zener diode in the CV amp local feedback, while preventing saturation, does nothing on that overshot.
--- End quote ---
Saturation should be handled separately. Ideally the transconductance node in the operational amplifier is clamped but only some amplifiers which support external compensation make this point available which is a good argument for using old LM301A or LM308 type operational amplifiers in high performance regulators. The 723 voltage regulator can be used this way also but its transconductance stage is pretty low performance although better than most applications require.
If enough output capacitance is used, then recovery from saturation is not a problem and most power supplies take this route. Saturation and overload recovery become a problem in a design with a minimum of output capacitance like that shown below where the current control loop is clamped like I described.
not1xor1:
--- Quote from: David Hess on November 28, 2018, 04:06:40 am ---
--- Quote from: not1xor1 on November 26, 2018, 05:21:23 pm ---A countercheck of that is the fact that by lowering the speed of the CV feedback network of the CV/CC diode switch topology (by putting an RC in parallel with the DC feedback resistor) it is possible to completely avoid the overshot while still preserving a reasonable recovery time, even with the slow LT1013.
--- End quote ---
This is why it is seldom advantageous to use a faster operational amplifier in a power supply; extra frequency compensation is needed anyway to maintain stability slowing it down. The one reason I have done so in the past is for lower noise.
--- Quote ---Another proof is that even a zener diode in the CV amp local feedback, while preventing saturation, does nothing on that overshot.
--- End quote ---
Saturation should be handled separately. Ideally the transconductance node in the operational amplifier is clamped but only some amplifiers which support external compensation make this point available which is a good argument for using old LM301A or LM308 type operational amplifiers in high performance regulators. The 723 voltage regulator can be used this way also but its transconductance stage is pretty low performance although better than most applications require.
If enough output capacitance is used, then recovery from saturation is not a problem and most power supplies take this route. Saturation and overload recovery become a problem in a design with a minimum of output capacitance like that shown below where the current control loop is clamped like I described.
--- End quote ---
In the simulations, the highest output voltage spikes appear when switching from minimum load (i.e. mA) to short circuit and get much lower when the switching is between an highest load and short.
In the real world they would be even lower due to the parasitic inductance of the PSU cables.
Besides that, we are in the order of the tenths of nJ (nano jouls on CV-CC switch) or µJ (micro jouls - CC-CV switch) of energy.
Even recovering from 100A to 1A yelds around 20µJ of extra energy. Nothing to be scared by. :phew:
OK... I've just realized the screenshots were relative to a 100mA/100A pulse rather than a 1/100A pulse which instead produces about 200µJ of extra energy.
Here is the circuit I used for the tests:
Here are the full simulation traces:
Here is the CV-CC mode switch zoomed:
Here is the CC-CV mode switch zoomed:
This screenshot shows the steady state energy (same time interval of the above one).
The extra energy is given by the difference of the two.
jah118:
--- Quote from: David Hess on November 23, 2018, 07:03:01 pm ---10 amps at 30 volts is beyond what a single power transistor can handle reliability without fold-back current limiting; 20 volts at 1.5 amps would be more realistic so I suggest building a lower power supply to gain more experience first.
The design as shown is certainly workable although the current limit is pretty basic.
--- End quote ---
hmm. i think i then will have to run 3 or 4 of them(2n3055) in Parallel,(and i have som massiv coolers from and old radio)
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