Flyback Topswitch.
hello,
when i set the zero gain crossover frequency of a flyback with Topswitch,
do i set it.....
1. At a frequecny of one fifth of the switching frequency ?...or
2. At a frequency which is just less than the corner frequency of the output RC filter?
Most grateful for any reply
Comments
Setting the poles and zeros is widely described in various papers on loop compensation. I will try and summarize the process for getting a crossover around 1kHz...
You need 1 pole at the origin. You naturally get this by the integrator circuit action of the error amplifier. Additionally you will need a zero at approximately 100 Hz. Finally for the third pole you can use the 7 kHz internal pole of the TOPSwitch itself for loop compensation.
Better still would be to use PI Expert software which makes all these calculations for you and even synthesizes the feedback circuit and suggests component values and plots the expected Bode plot as well.
Hello,
normally at low line gain cross over is set between 1-2 kHz, providing enough low frequency gain (at 100 Hz) to effectively reject line ripple, sufficient phase margin for stability and good transient response. I've attached the results from RDR-142 as an example - note the loop bandwidth is lower at high line due to operation in discontinuous conduction mode.
Cheers
PI-Chekov