Electronics > Projects, Designs, and Technical Stuff
Bench CC/CV PSU Based on Daves uSupply (Not Anymore)
iMo:
It simulates ok with an LM358. The current limit is not precise with I_limit<100mA, though. Bigger shunt (ie 1ohm) makes it better, for I_limit>=50mA it works precise then.
PS: a small negative V- for the opamps would help as well, imho.
With a bigger shunt you get higher voltage on it, therefore you have to decrease the dividers ratio in the current sense diff amplif to be still within the DAC range.
Also add some resistance in series with the output capacitors (ie 100-200mOhm) in the simulation to get more realistic ILoad currents when switching cv->cc.
jaycee:
--- Quote from: imo on March 12, 2019, 10:24:53 am ---PS: a small negative V- for the opamps would help as well, imho.
--- End quote ---
A small inverting regulator could do that. It also opens the choice of opamps up greatly. -5V would probably be enough. My own supply based on this topology uses an auxilliary transformer winding to get +5/-5V (around the output) to run the opamps, and uses TL082's.
--- Quote from: imo on March 12, 2019, 10:24:53 am ---Also add some resistance in series with the output capacitors (ie 100-200mOhm) in the simulation to get more realistic ILoad currents when switching cv->cc.
--- End quote ---
The capacitors on the output already include ESR - LTSPICE allows you to specify this. In my latest version of the files, you will see I specified a Panasonic FM series capacitor - the values for ripple current and ESR come from the datasheet.
Kleinstein:
Even the LM358 might like a little negative supply. No need for -5 V, but something like -0.5 V or -1 V would already be enough.
Another part that would like a slight negative supply would be a minimum load, as a constant current load usually needs at least some -100 mV. A minimum load of some 10 mA could avoid the rather nonlinear and slow part of the power stage.
The OPs initial idea was to start with an (old) laptop supply, so no easy choice for a 2 nd winding.
PeterZ:
There is even a dedicated chip to generate a small negative voltage (-0.232V) and allow the OAs outputs go down to zero:
LM7705 Low Noise Negative Bias Generator
https://www.ti.com/product/LM7705
iMo:
Ok, I see the capacitors models.. So no additional resistors needed in the simulation.
Btw, why not to wire the constant current load at the output (after the shunt)? It creates an offset on the shunt, that helps to go a little bit lower with the lowest I_Limits, and you can compensate for the current offset in software.
PS: the simulation shows -10mA at -3V for the 4x LM358.
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