I am trying to run a transient simulation of a step down dc-dc converter using ideal components. This includes a feedback loop consisting of an error amplifier. I am interested in transient response without including any compensation.
I have used a voltage controlled voltage source without any saturation limits for an error amplifier. If I use an ideal opamp, it would saturate even though there is a feedback loop. Why?
If I use an ideal comparator to compare the control voltage with a sawtooth signal of desired frequency fs (same as switching frequency), I would get a PWM output whose frequency will not be fs. Below is the schematic and corresponding transient simulation (Multisim):
Since there is not saturation limit set for voltage-controlled-voltage source, I can have arbitrarily large gain to reduce the error. But the ideal comparator will compare the control voltage with sawtooth signal even if the control voltage is very high initially(that is why I've not included it in plot above). But the output is a PWM signal with a different frequency.
So I tried using a PWM control block instead of a comparator.
But the problem is that unlike an ideal comprator this block will not output a PWM signal with such high values of control voltage. In order to achieve this I have to set the amplitude of sawtooth reference used by PWM block to very high values so that Vcontrol and sawtooth signal intersect each other.
But the result is a pefrect PWM signal with frequency same as fs.
I wanted to analyze the effects of changing amplitude of sawtooth signal, gain of VCVS on transient response. But for obtaining a proper output I have to simulate using impractical values. Is there any other way to obtain a proper PWM signal using an ideal comparator?
Input voltage = 12V
Required Output = 6V
Steady state duty cycle is 50%
Switching frequency fs = 1Mhz