Hi Please may i add the following, on the subject of big filters being needed even for high frequency SMPS...
The pointlessness of high switching frequency DCDC SMPS:
We are doing a DCDC Buck SMPS, 24Vin to 12Vout at 300W.
We could do it at say 500kHz switching frequency and so we only need a few 10’s of uF at the input filter, (with a 4x electro as a damper) and even less at the output. -So a very small solution…or is it?....
…Well, yes, if, that is, you are supplying it from a typical lab power supply with a huge output capacitor, and the load is just a resistor.
However, if you are selling it to the general PSU market, and your customers could supply it from a multitude of different sources, and may connect loads to it which may have up to several mF of input capacitance, or more…then you certainly cannot get away with such small filter capacitor banks, and , in fact, you would have been better off just choosing a lower switching frequency like 100kHz or so. -Your solution would likely be no bigger despite the lower switching frequency.
This is because in order to be able to handle the connection of large capacitance loads to the output, you need to have the output capacitor bank on the actual SMPS being relatively large. Also, at the input, maybe the source will have a big output inductor, and small capacitor downstream of that, and so your PSU will need an input filter comprising a relatively large input capacitor bank, so as to avoid input filter instability when the source gets connected.
As such, why do we always see encouragement of high switching frequency solutions.? They have more switching losses, and often , cannot be made much smaller than a lower frequency solution.