Electrical – Find v1 and v2 of a summing amplifier when it is in series with another Op-Amp

Fundamentally, you have way too much gain in your feedback loop, along with enough phase shift to create a very nice ~50 kHz oscillator.

First, I would simplify the circuit by eliminating the MOSFET Q1; instead I would consider swapping the inputs of the LM393 and using its open-collector output to drive the Vref node directly. Secondly, I would add a significant amount of negative feedback around the LM393, along with a capacitor to roll off the frequency response. You really do not need a lot of bandwidth in your control loop for a battery charger — a battery is not a highly dynamic load.

Edit #1, incorporating comments:

I understand about limiting the voltage swing on Vsense; that's what R1 and R2 are for. I'm saying eliminate Q1 and R3 and connect the LM393 to the junction of R1 and R2. Then, you need to swap the inputs to the LM393 in order to preserve the correct polarity of the feedback.

For negative feedback, just connect a capacitor between pins 1 and 2 of the LM393. Since pin 2 is now connected to your reference source, you'll also need a resistor between C7 and pin 2. Together, these components will roll off the frequency response of the comparator. I would start with values like 10K and 100 nF, giving a corner frequency of about 160 Hz. I don't know if this will be enough to make the system stable, but at least it gets you started in the right direction.

Edit #1, additional thoughts:

Let's take a step back for a moment. If we ignore PWM_Vset for the moment, what's really needed is to take the 200 mV that appears across the sense resistor and translate this to the 1.221 V that the regulator expects on its Vsense pin. This requires a simple noninverting amplifier with a gain of a little more than 6.

Based on the new circuit, it would be an interesting experiment to short out C7 and reduce R3 to 51K (gain = 6.1) and see if the regulator is now stable. If it is, we can then think about ways to make the setpoint adjustable.

Your first tick of time "t" produces an end-stopped output voltage at plus or minus 10V. It doesn't matter which because your next iteration will also produce an end-stopped output voltage of opposite polarity then you are back to the start again and you'll just produce end-stopped voltages each tick of time "t".

If you don't get alternate end-stops of plus then minus 10V then your code is wrong.

But, of course alternate end stops of 10V isn't how an op-amp works so you then have to look at what has gone wrong and build a small integrator into the code so that it more closely represents an op-amp. You have no mechanism for producing a value that can converge on 0V and that is your basic problem.