Hi,
In your circuit.
- What is 1NA219-2 measuring, because it isn't load current?
- Why is your load between supply +V of 20V and the "output" of your "converter"?
- Should it not be between "output" and gnd?
Sorry but I think you need to read up on buck and boost converters, then decide on which pre-made DC-DC converter you are going to buy.
If it is for a CUBE Sat, is this DC-DC going to model space as well?
If so you need to model for PV cells, that is a CURRENT SOURCE rather than a voltage source.
Tom... 
