Electronic – Unexpected LTSPICE Output – Voltage Mode Buck Convertor

Your schematic is showing current-mode control.

In voltage-mode control the error amplifier compares the output voltage to a reference, then provides an output that is compensated and compared to a sawtooth wave that will vary the PWM duty cycle to drive the error lower.

At low frequencies (esp DC) we want the error to be as small as possible, so we want the error amp/compensation to look like an integrator. Any error will then build up and drive the PWM to force the error to zero. Only zero error will allow the output of the compensation to reach steady state at DC.

If we just used an integrator for the compensation we would have to close the loop at a very low frequency which would mean poor transient response (poor rejection to disturbances). That's because an integrator provides 90 degrees of phase shift and we would have to close the loop well below the L-C output filter resonance which provides another 180 degrees of phase shift. We also wouldn't be able to control transients causing the output filter to ring because our control bandwidth would be lower than the LC resonant frequency. Still, we want to put a pole at DC for good DC regulation.

The output filter L-C has 2 poles. So in order to cancel those two poles we can put 2 zeroes in the compensation. Now we still have that 90 degree phase shift until the switching frequency effects and error amp bandwidth start to come in.

With the two zeroes we form in the compensator we get a couple of poles as well (if we want them or not).

So we put one additional pole in the compensation somewhere around half the switching frequency to filter out noise and ripple, and a pole at the capacitor ESR zero frequency to cancel that. Now we can close the loop at a reasonable frequency with a reasonable phase margin and still get good DC accuracy.

Without researching your circuit i want to warmly recommend something. Prepare three stages on your feedback, and have their sum on another opamp. One stage is buffer, another one is integrator and the third one is derivator. Each should have a nice convenient potentiometer to set the gain. Then you will be able to tune your system for best performance.

Edit

Most importantly, simulate before you go to solder anything!!!

Edit

Based on assumption that your components are appropriate for voltage and current, and that you know the basics, I will explain what you should do to close the loop.

I will add a little bit more info. First of all, your PWM controller must have a saw-tooth waveform of constant frequency and "command signal" which is basically voltage, that you compare to this saw-tooth. If the command is smaller than the saw-tooth, your switch is on, otherwise- off. Make sure it works for a constant command, this is an open-loop PWM controller.

Now you have to close the loop. Closing the loop basically means making your error zero. So you have first of all to measure it by subtracting your output voltage from a reference voltage. Reference is generated by a zener diode with resistor. Or you can take an IC that will do it inside more accurately. The output voltage is usually higher than your reference, so use resistor divider, so error is 0 when the divided voltage equals to your reference.

Now when you have your error, feed it to the three opamp circuits I mentioned: one with just gain, another one- integrator, third- derivator. Add all three- the result should be used as the PWM command signal (remember?).

And you will have to tune the system, so use potentiometers around opamps.