You might want to try this circuit. It uses an N-channel power MosFet to drive the load.
Further it uses two timer ICs, one to generate the clock pulse and the other one being a monostable being triggered by the clock signal and changing its output pulse duration by applying a variable control voltage.
In the simulation the setting of the two potentiometers is shown for 100% duty cycle (very close to). Turning the lower pot (RV1=100K) in the opposite direction the duty cycle drops to 2%. Clock frequency is ~300Hz, varying slightly with duty cycle.
Boncuk