When do I need separate MLCC (input) and AL-ELEC (bulk) capacitors on my SMPS design?
My preferred input capacitors for SMPS circuits are ceramic (X7R); generally speaking that is sufficient for most designs but it does depend on the load current to some extent (because that determines how large a 'gulp' of current is taken from the input during switching).

I always specify X7R devices with Vin * 2 as a rated voltage as that gives me some confidence they will remain within 20% of their nominal value over time and temperature.

I do not like using dry tantalums for a number of reasons; They are prone to failure simply due to reflow even when properly derated and with a low impedance source (which is what the power input to a switch mode supply should be) they can become spectacularly pyrotechnic. In addition, they have effectively zero capacitance above perhaps 400kHz (far below the transition rate of the switches in a SMPS supply).



When using multiple input capacitors, I make a small 'mini-plane' for the power and return sides to ensure low inductance paths as the ceramic devices are the ones that can respond at the rate at which the switch transitions are operating.

Aluminium devices can be used as a bulk source, but are there for slow transitions only.

I use Aluminium devices for hold up applications at present (although I am evaluating different techmologies for the future).

My answer to the question is:

When would I not use ceramic capacitors on the input to a switch mode power supply?