Simplest way to create regulated 400VDC from 12VDC
Your current is low .This means that a diode pump could have cheap small caps .Boost converters can be operated into a diode pump .You could make say 50 volts with the boost converter and use a multistage diode pump off the drain to make your needed 400VDC .This does mean 8 pump stages but 4148 diodes and 100n caps are cheap.The advantage of this is that you can use readily available boost chips and a standard garden variety SMD coil because the expected peak to peak volts across the coil is only 50VDC.The boost ratio is not too high so old school hard switching peak current mode will work OK.What I have done also is use a soft switching boost converter made from discretes running a higher boost ratio using BAV21 pump stages at 160 volts per stage .