Problem in Simulation of Voltage Mode Control

[shomikc]

Hello all,

I am having trouble simulating a comparator. Please see the image of the circuit.



The circuit converges upto 40useconds only. Even then it does not work properly.



Please help me with the simulation of the comparator.

Thanks.

[FvM]

LM741 isn't suited for single supply operation and not specified for 5V supply at all. Redesign with reasonable parameters.

I also won't expect useful output from a PI controller without closing the feedback loop.

[shomikc]

Hello all,

This is my second post. I am new to all topics in Power Electronics. I have simulated a Voltage Mode controlled Buck Chopper.



Unfortunately the controller only seems able to handle changes in output current but not changes in input voltage. This is seen in the next image.



Could anyone please tell me what I am doing wrong.

Thankyou.
Shomik C

[FvM]

I don't understand how you arrived at the strange controller design. What's the reason behind dividing the process value and set point to a low mV voltage? How do you expect a stable output voltage without an integral term in the transfer function?

[shomikc]

Dear Sir,


Dear Sir, Thankyou for the reply.

Please forgive me on my ignorance. As I am completely new and working on my own I am facing problem with basic issues which experienced persons might assume to be just common sense.

I thought that dividing the reference value to a low mV input for error amplifier will lead to low mV output of the error amplifier which is then the input for the compensator amplifier which has the positive terminal grounded. And from books I read that the op amp to work in linear range with amplification the difference between the two inputs has to be very small.

Also I thought having a capacitance of 100pF will give me the integral term.

Thankyou.

[shomikc]

Sir,

Thank you.

As soon as I put the capacitance in series with the resistor the output of the op-amp of the controller goes into saturation.

And also input voltage perturbation with my circuit shown is not handled only load current variations seem to be handled properly.

Thank you.

[FvM]

A typical error amplifier + compensator is implemented with a single OP, look at popular PWM controller ICs or text book schematics.

The main disadvantage of voltage downscaling is the increasing error due to OP offset voltage. Apart form this point, it should work. Having a controller transfer function with integral term is however mandatory to avoid a stationary error. A popular PI transfer function is generated by putting the integrator capacitor in series with a resistor. Proportional and integral gain can be adjusted separately by changing both values.

Now you have a pure P controller with an additional low pass corner at 230 kHz, far beyond feasible control loop bandwidth.

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And also input voltage perturbation with my circuit shown is not handled only load current variations seem to be handled properly.

Doesn't make sense. What you probably see is the control amplifier saturated with constant pwm duty cycle and no control action at all.

As a first step, you may want to reduce the inverting amplifier gain from 150 to below 1 so that the integrator output range doesn't exceed the linear pwm range by a huge factor. Or more generally, design controller parameters by calculation instead of choosing it arbitrarily.

[shomikc]

Ok Sir,

Thank you. I will try that. Your input has been very helpful.