First, in order to use the 50W power LED you need also a proper heat sink, like this one:
A simple PWM controller can be made using a 555 circuit, some examples here:
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For the power LED you need a 36V power supply, for the 555 a lower voltage 12V - 15V , so you may use two separate power voltage supplies or a supplementary power voltage regulator (LM317) to be used to generate the 15V from the 36V power supply.