CCM Voltage mode flyback output capacitor size?

[treez]

On page 6 of AN57 by powerint.com, it states that the resonant frequency of the Power stage LC should be greater than 500Hz...
http://www.powerint.com/sites/default/files/product-docs/an57.pdf

Why is this?...I could not find out from the powerint.com forum.

 

[mrinalmani]

Consider the following:
ω = 1/√(LC)
X_c = 1/ωC = √(L/C)
Since the circuit is at resonance, or almost at resonance, the current in the tank is constant regardless of L and C.
Let the current be I. Then,
V_c = I×√(L/C)

Now for a given capacitance, if the resonance is lowered by using a larger inductor, it leads to larger voltage across the capacitor. Since larger voltage rating capacitors do not have large current capability(as mentioned in the text also), we do not want to lower the resonance unnecessarily. Lower resonance is associated with larger circulating energy in the tank.

 

[treez]

surely the resonance should be kept well away from the crossover frequency? Ray ridley, in his book "power supply design: volume 1 control" (page 47) states that the LC resonant frequency should be less than half of the crossover frequency.
This doesn't tally with what powerint.com are saying.
Only one of them can be right, not both of them.
They are saying pretty much opposite things.
One of them is wrong.

 

[mrinalmani]

Please give another reading. Both the texts say that high cross-over frequencies give better performance. However since higher frequencies are more susceptible to noise, a compromise is made. ( And this compromise seems inconsistently large in the 'Powerint' text). Another thing is that the cross-over frequency is tuned on the basis of the resonance frequency and not vice-versa. In no text does it say that the resonance frequency should be small. If the cross-over is to be large relative to the resonance, does it mean we trim down the resonance itself?

Perhaps, the lower limit of approx. 500Hz if crossed, may lead to difficulty in selecting standard sized capacitors.

 

[treez]

cross-over frequency is tuned on the basis of the resonance frequency and not vice-versa

As long as your crossover frequency is at least twice as much as the resonant frequency then you are ok..surely?

Alternatively, the crossover freq should be less than half of the LC resonant frequency.

The main thing, and surely you agree, is that the LC resonant frequency and the crossover frequency cannot be the same frequency, or even close in frequency...surely this is so......Ridley as good as says so on his page 47 , as above