PI Expert automatically designs the best feedback circuit to help you meet your specifications. If you want to change the feedback circuit used or any of the component values you may do so by using the Feedback Bias or Components form.
When PWM control based devices like the TOPSwitch or the DPA-Switch family of devices are used, you have the ability to use secondary side TL431 feedback. For designs using the TOPSwitch family, it is recommended that the target crossover frequency should be between 500 Hz and 3 kHz. For the TOPSwitch family the LC filter should have a corner frequency higher than 10 kHz while for DPA-Switch the corner frequency should be higher than 100 kHz.
The estimated gain and phase plots allow you to see the bandwidth of your loop as you change component values and other pieces of valuable information. The Estimated Crossover Frequency and Estimated Phase Margin give you an estimation of the stability of the power supply system. This is critical for peak power designs where large load transients are expected and the potential for output voltage oscillation is greater.
For designs using TOPSwitch family it is recommended that the target crossover frequency should be between 500 Hz and 3 kHz. For designs using DPA-Switch the target crossover frequency should be between 3 kHz and at most 10 kHz. Higher crossover frequencies are not recommended as this may increase the noise-sensitivity of the control loop. The software designs the control loop for a minimum phase margin of 45 degrees. In some cases it maybe necessary to add a phase boost network if the estimated phase margin is not enough. The phase boost network is calculated so that the crossover frequency remains as close to the target crossover frequency as possible but that the phase margin improves by as much as 30 degrees.
PI Expert allows you not only to manipulate individual component values, but also to enter the desired Error Amplifier Gain and Crossover Target frequency. Given these specifications, PI Expert calculates the necessary component values to best meet your specifications.
Many designs use a post filter that is used to attenuate switching ripple on the output. This post filter should be designed so that its corner frequency is far from the crossover frequency and it does not affect the control loop in a detrimental manner and cause instability. For TOPSwitch family the LC filter should have a corner frequency higher than 10 kHz while for DPA-Switch the corner frequency should be higher than 100 kHz.
It is important to remember that the calculations and charts presented are only an estimate of power supply performance. Actual measurements on a prototype should be made to verify the design.