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Normally an op amp is used in the feedback loop as a lead-lag amp which has more than sufficient open-loop gain for any normal requirements. The lead-lag compensation parameters are determined by the output LC filter values and the PWM gain transfer function.
Are you using something besides an op amp for the error amplifier?
I am just using a high gain amplifier as the error amplifier. But don't know what value of gain is required to a specified accuracy level. Suppose at the input I am having a source of 5V which may drift to 8V and I want the output to be at 3V with say "x%" of accuracy.
Then how do I select the gain of the error amplifier??Please help..
Apparently you are looking for the DC regulation quality. You need to derive the modulator gain or "PWM gain transfer function". In a usual buck converter in CCM it's Vin/Vramp.
Vramp is the peak-peak magnitude of the pwm ramp generator voltage.
Besides sufficient DC gain, you'll need to care for stable feedback operation, using a frequency compensation as mentioned by crutschow.
The 'magic box' is the DC/DC converter The schematic employs negative feedback to ensure Vout stays at Vref even for variations in Vin or other parameters such as the load current.
My question is how to calculate the required gain of the feedback amplifier??
Why do you need a feedback amplifier? :-? If the converter is designed to keep Vout at Vref independent of line and load variations, then there is already negative feedback so you don't need to add an amplifier for that same purpose.
If you are asking about an amplifier that's already in the circuit, then you need to show the rest of the circuit to get a correct answer to your question. Is that a problem to do?
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