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Does this CV load circuit actually work? (with ltspice schematic)
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cbc02009:
Hello all,

I have this circuit I designed in LTspice that seems to work really really well as a CV load, but before I go build it I'm trying to figure out if it really works in reality, or if it's just simulation quirk.

The first attached picture is the circuit. it uses a LM358 to drive the gate of a P-channel mosfet. (the op amp in the LTspice is just Linear's version of the 358). The inputs to the op amp are a reference voltage and a voltage divider from the load voltage.

My thinking is that if the Vin drops below Vref, it will increase the voltage at the gate, thereby bringing Vgs close to it's threshold and increasing it's resistance. If Vin rises to far above Vref, it will lower the gate voltage, thereby decreasing it's resistance, increasing current draw and decreasing the voltage across the mosfet.

This should, as long as my thinking as sound, keep the voltage across the mosfet pretty constant, which shows in this particular output (shown in the second picture).

I just want to double check with people more experienced than me that my thinking is right. Any input would be greatly appreciated.

I've also attached the LTspice schematic.
ocset:
Attached i have edited it for you.
This allows you to have a low voltage opamp (5V)  and drive many different vout values.
Also,  there is some frequency compensation added as your uncompensated opamp will likely have instability issues in a real circuit.
Ian.M:
As usual Treez is over-complicating things.   

You should only need a single RC negative feedback network to roll off the HF response of the OPAMP, if that's even needed, which you wont know until you examine the feedback loop's response in the frequency domain to see how much phase margin you have.  A quick initial check of its step response didn't show any instability, so you're probably O.K. *IF* the LTspice OPAMP model is realistic for the OPAMP you are actually using.
N.B I edited the sim to match the .PNG you posted as the .asc file appears to be an older version.

Also, if one has a 12V rail available, there's no need to mess around with a low voltage OPAMP + level shifting the gate drive,

Also, your choice of MOSFET isn't up to the job for the currents and voltages in your sim - see fig 7 in the FDS4435A datasheet, the SOA curves.   The DC one indicates that it is only good for a small fraction of the current and voltage your sim is applying.  In general MOSFET DC linear region ratings should be treated with great suspicion except for devices specifically marketed for linear region operation, so even if you derate it by by 50% and keep the ambient temperature under 25 deg C it may not be reliable at even 0.5W dissipation.

You'd probably do better with a BJT Sziklai pair (so you can use a NPN power transistor) as the shunt pass device, as power transistors from reputable manufacturers generally perform in accordance with their specifications.

Finally, unless you need to vary the load control voltage input (source V1) dynamically, you'd probably be better off with a fixed shunt regulator controlled by a potential divider, based on a TL431 or TLV431 + a Sziklai pair to boost its current capability.
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