An apparatus includes an active bridge section with input terminals that receive power from a constant current source where the active bridge section operates at a fixed switching frequency. The apparatus includes a resonant section with a resonant inductor, a transformer and a resonant capacitor. The inductor is connected in series with a primary winding and the capacitor is connected in parallel with a secondary winding of the transformer. The resonant section is connected to an output of the active bridge section. The apparatus includes an output rectifier section that receives power from the resonant section and includes output terminals for connection to a load, and includes a controller that regulates output voltage to the load where the controller regulates output voltage to the load by controlling switching of the active bridge section. The fixed switching frequency of the active bridge section matches a resonant frequency of the resonant section.