No, the circuit I drew is just a controller; it would be used to control the power section.
You stated that the current should be constant; do you want this to be adjustable as well, e.g. using another 'knob'?
As your ON time can be quite long and potentially the load will only dropping 10-30V (correct me if I'm wrong), if a linear current source is used, the pass transistor will be dissipating a lot of heat and will need a large heatsink. The alternative is to use a SMPS (or at least a chopper), which will not suffer from the same issue, but will have a some ripple current. Which is acceptable to you? Either is quite simple.
Also, can you confirm that the circuit has to be powered from the 50V supply, or is there a lower supply available?
One last thing... does it matter to you whether the anode or cathode is switched?