Electronics > Power/Renewable Energy/EV's

Voltage mode CCM flyback has limitations on output capacitance

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Faringdon:
Thanks, it was indeed a recomendation at the bottom left  of page 4 of AN57. However, if such a recomendation is to be made, it must be backed up with some kind of reasoning...after all, this is the feedback loop we are talking about here...if it goes wrong, there can be potentially  100'000s of returned products.

As you ae cognisant, in a VM Flyback.....you have constraints already...eg, to stay away from RHPZ and the resonant point....so giving out willy nilly recomendations, i am sure you would agree, is a bad idea...as it may bring the cust into conflict with the other feedback loop requirements.



And what about the transfer function for the error amplifier and modulator for topswitch designs........is the one in AN57 to be trusted?
...If you may, take a look at equn 23 (AN57)...this is refuted by other versions of this equation in other documents , (ie docs not from Power Integrations.)

As such.....Why does AN57 include the open loop gain of the TL431 in the feedback equation (23), whereas AND8334 from onsemi does not include it? (AND8334 page 5)...Equn 23 of AND8334  does not contain the "KTL431" factor.

mtwieg:

--- Quote from: Faringdon on July 23, 2023, 06:01:54 pm ---However, if such a recomendation is to be made, it must be backed up with some kind of reasoning...
--- End quote ---
The authors aren't obligated to do any such thing, especially if they make it clear that they are making a recommendation, rather than a requirement. And you are not obligated to apply their recommendations either.


--- Quote ---And what about the transfer function for the error amplifier and modulator for topswitch designs........is the one in AN57 to be trusted?
...If you may, take a look at equn 23 (AN57)...this is refuted by other versions of this equation in other documents , (ie docs not from Power Integrations.)

As such.....Why does AN57 include the open loop gain of the TL431 in the feedback equation (23), whereas AND8334 from onsemi does not include it? (AND8334 page 5)...Equn 23 of AND8334  does not contain the "KTL431" factor.

--- End quote ---
It's unclear which equation in AND8334  you are comparing with. At a glance, the two circuits are different, and therefore the transfer functions should be different. I don't notice any errors in either.

The authors of AN57 perhaps thought that the open loop gain of the TL431 might be of significance to the reader, while the authors of AND8334 did not. So what?

langwadt:
is this like betteridge's law, default answer is no

temperance:
Think about what will happen if the output cap is not charged to the required Vout before the soft start period has been completed.

Faringdon:

--- Quote ---The authors aren't obligated to do any such thing, especially if they make it clear that they are making a recommendation, rather than a requirement.
--- End quote ---
Thanks, but as you know, having a limitation on the output capacitance, would  tend to mean that the output LC resonant frequency would be higher. This is a problem, since as Basso says on page 532 of his Book (Designing control loops for linear and switching power suppplies), the crossover frequency in voltage mode should be some 3x the output LC resonant frequency. Also, ayk, the crossover frequency should be some 3x lower than the RHPZ (if the SMPS  has RHPZ). If you limit Cout, then you can end up with a extremely small range in which you can cross over with stability.

They can say what they like, (ie they gave a recomended limitation on the Cout value) of course, i agree, but its unwise in this case, and unhelpful to the customer.

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