SMPS input filter cut off frequency

[Faringdon]

Hi...regarding SMPS input filters........

Page 66-77 of Basso Book "switch mode power supplies" covers Input filters for SMPS.
snva538 by ti.com also covers Input filters of SMPS.

snva538
https://www.ti.com/lit/an/snva538/snva538.pdf?ts=1713952246374&ref_url=https%253A%252F%252Fwww.google.com%252F

The "DCM design guide" (A "DCM" is a type of DCDC SMPS module) by VicorPower also covers Input filters for SMPS on pages 11 to 33

DCM design guide
https://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf

....Near bottom of page 23 of "DCM design guide" it states, (regarding an input LC filter for an SMPS)....
QUOTE>>>> Select the cut-off frequency below the crossover frequency (20kHz) of the DCM. <<<<UNQUOTE

That is, its saying the the cut-off frequency of the SMPS LC input filter should be below the SMPS's crossover frequency.

Neither Basso, nor ti.com, nor any other reference that i can find make this stipulation. Why do
VicorPower stipulate it?

[jpanhalt]

Neither Basso, nor ti.com, nor any other reference that i can find make this stipulation. Why do
VicorPower stipulate it?

Capt. Obvious: Have you tried asking here: https://www.vicorpower.com/technical-support

[Faringdon]

Thanks, yes i just logged on and filled in the contact page with the question. I dont have big orders for them so dont know how they will respond.
The point has been in this app note for many years.
___----_____
So...
The attached (LTspice and PNG's) shows a Buck converter with a feedback loop bandwidth of 10kHz
being fed by an input filter with cut
-off frequency of 15.9kHz.
The Buck is perfectly stable, with no problems even after full load to noLoad transient
and vice versa.

This demo's that the VicorPower App Note stipulation that the Input Filter fco should
be below the SMPS's Crossover frequency is inaccurate.

As such, why have they stated this?

After all, if the Buck's bandwidth is 10kHz, then it wont even be able to "see" any resonance at 15.9kHz...
...so why would this be important? The Buck's  input impedance will no longer be negative above 10kHz.

Surely as long as the Buck's input impedance is greater than the input filter's output impedance (as
"seen" from the Buck"), -and this over all frequencies equal to or less than the Buck's
crossover frequency....then all will be well?

[temperance]

Explanation and real world measurements included.

Cheers

[coppercone2]

I looked at snvA 801 and its written so poorly it makes me not want to read it at all 

i dunno if its auto translated from another language or what

I would definitely not try to learn from those Ti app notes, that is a shady reference document for people that know most of it is

just do what vicor tells you to do and don't do what Ti says unless you think it can improve something? god that app note is fucking irritating. that is something you pay someone for reading so you don't need to read it

[mtwieg]

The reasoning is likely as follows:
The power supply's control loop will attenuate line disturbances below the controller's crossover frequency.
The LC filter will attenuate disturbances above its cutoff frequency.
In order to attenuate disturbances at all frequencies, then these two bands should overlap, thus the LC filter fc should be lower than the controller's crossover fc.

Of course, whether this is actually worthwhile is very application-specific.
For example, if your primary concern is just passing EMC tests, then there's not much reason to attenuate below 150kHz (or whatever the lower test frequency your device requires).