i am using dc dc converter. i don't exactly know how it works internally. if anyone could suggest me some good reading material about it..
also i want to know if the input GND and output GND are isolated or not?

 

probably works like a switch mode power supply, just dosnt use the input rectifyer, uses the input dc instead. the grounds may or may not be connected, depends how it was built and wired.

 

There are thousands of different DC/DC converter products. Which one is yours?

ak

 

To determine if the grounds are isolated or not, just measure the resistance between the two.
If they are isolated the resistance should be at least a megohm.

 

i m using NMH0512 and Vicor vi-200 vi-j00

 

All three of those are fully isolated converters. The NMH0512 is a standard little flyback critter with bipolar outputs. The two outputs are not isolated from each other, and are hard-wired to be +12 and -12 with a common output ground.

The two Vicor parts are part of their first generation product line. Vicor pioneered the high density DC/DC converter, and their circuit topology is weird. In a general sense it is a forward converter, but it has a variable switching frequency that increases with load.

ak