Converting a ground-referenced positive/negative dc voltage to positive voltage
You can use the circuit from a "universal input" switchmode AC adapter with 5V output. The input is isolated from the output so the internal bridge rectifier will work fine. 

200VDC is about equivalent to about 140VAC RMS. A typical adapter will work down to about 90 or 100VAC so well under 150VDC. 

For example, this circuit using a chip from Power Integrations:



You didn't mention how much current you need- this circuit is good for 4A (20W). Where it says 90-295 VAC, you can apply about 130VDC to 400VDC with either polarity. 

The ratings of all the relevant parts should be checked to make sure they are valid for continuous DC.