TOPswitch jx feedback loop
Hi,
AN-57 is , as you know, a Power Integration App Note.
Top RHS of page 6 of AN57 says the resonant frequency between c(out) of the flyback, and the "effective L(sec)" should be <500Hz.
This stipuation has never before occurred for any flyback converter, in any text or book.
Do you know what this is about?....And why it should be so?
We are using a Rubycon YXF capacitor of 35v, 1mF. Our L(sec) is 54.5uH...And D = 0.55 (at 90vac) and NS/NP = 19/84.....so our resonant frequency is 306Hz...therefore we violate the rule.....
Is it something to do with the voltage mode control.....?...Topswitch is voltage mode controlled from 100% of load to 55% of load.
Comments
Thanks and as is known...
Page 47 of "power supply design, volume 1:control" by Dr Ridley states that for a voltage mode flyback.."The crossover frequency should be at least twice the resonant frequency".......by "resonant frequency" , Dr Ridley means resonance of Cout and L(sec). [or rather "LE", the effective secondary inductance since its flyback and not full or half bridge)
As such, there are tight constraints on voltage mode flybacks in CCM as regards to feedback loop.....thats why i believe we need the full transfer function of the error amp and modulator is given by which ever TOPswitch.....Voltage_mode_in_CCM needs that bit more care than current mode.
As mentioned in AN-57, it is recommended to the LC tank formed by the output capacitor and the effective primary inductance should have the resonant frequency after about 500 Hz in continuous mode operation. It is better to use smaller capacitance with larger voltage rating capacitor. Figure 17 and 18 in Appendix A show the magnitude and phase responses of power stage, which include the transfer function of LC tank circuit for the reference.