The power supply acts as a power generation and conditioning system.  The generator, which is mounted on the engine and driven by a toothed belt from the engine output shaft, produces electrical power. The power supply then takes the electrical input and converts this to the appropriate outputs as required. The generator is mounted alongside the engine rather than concentrically on the crankshaft to allow for a higher power density motor to minimise the weight of the aircraft, insulate the generator from the heat of the engine and provide much easier air cooling of the generator.