Switching power supply

Self-oscillating, current controlled switching power supplies, used for example, as energy storage device charging circuits, and related methods are provided herein. In one implementation, a switching power supply comprises a power transformer having a primary and a secondary, a switching element that switches a voltage source to the primary, and a control circuit that controls the operation of the switching element in response to the measured primary current and secondary current. The switching element disconnects the voltage source from the primary when the primary current reaches a first threshold, causing the secondary current to conduct. The switching element switches back to the primary when the secondary current drops to second threshold. The switching power supply oscillates between charging and discharging the power transformer. In one embodiment, a high value energy storage capacitor is coupled to the secondary and is charged with the secondary current.