is it OK to stack several DC-DC converters with their inputs connected in series to support a larger input voltage?
Connecting the chargers in series

Connecting several such switchers in series will be a problem since they would have to share the same supply current (all would have to draw the same current all the time). All the chargers have some input resistance which vary according to their immediate needs. If one of the chargers draws less current then:


It's input resistance rises so it will get less current.
This causes the voltage on this charger to rise.
Other chargers get lower a little lower voltage and when they "notice" it they lower their input resistances to get the required current.
This effect is highly unstable. Eventually only one charger (unfortunately the one that requires the least power) will get all the input voltage and (most probably) be destroyed.


To verify this logic assume that one charger finished charging and does not need any current at all. The same thing happens.

The only way out is to dissipate excess (unneeded at the moment) power but that is probably not something you would like to do. :)

High input voltage SMPS

A single SMPS supply working from 1000V input is also quite difficult to do since most MOSFETs and IGBTs are only rated up to 1200V. A 600V supply should be doable.

Output current regulation in multiple-output SMPS

You could do a multi-output transformer based switched supply and regulate the output voltages and currents with a magnetic amplifier (this Ferroxcube flyer is a proof that this is used in real circuits)