How to limit current precisely?
Below is a high-side constant current circuit using a PNP transistor and op-amp: -



It needs to be modified a little to take one hundred volts though: -


Op-amp needs a top rail of V+ (as shown) and a lower power rail maybe at V+ - 10 volts
PNP transistor needs to be rated for at least 150 volts
Vref needs to be positive rail referenced by using a precision voltage reference such as a shunt reference - it should replace R1.


How it works - lets say Vref is 1 volt below V+. Due to op-amp negative feedback Vref is forced to be across R2. With a voltage of 1V across R2 and a resistance of 10 ohms for R2 it implies that current taken by the load is limited to 100mA.

Having said all of this, the volt drop of R2 (1V) may be too much and a smaller value of Vref adopted. It could be reduced to 100mV and R2 dropped to 1 ohm without much difficulty. An op-amp with output rail-to rail capability is needed as well as the op-amp being able to sense its inputs at the positive rail too. It also needs to be fast and possibly avoid saturation when the load taken is under 100mA - this could be done with extra circuitry.

Anyway, that's the general approach I would consider.