Abstract: A high voltage flyback power supply provides slope-based primary feedback to control the off-time of the power supply. When a negative slope is detected at a low side of a primary winding of the power supply, an off-time cycle is terminated and a next on-time cycle is initiated. The slope-based off-time primary feedback helps to keep the off-time of the power supply in sync with ringing of the primary leakage inductance and the primary-side capacitance of the power supply. Such a power supply is particularly useful for battery-powered systems with low voltage rails. The power supply can also include on-time feedback to shorten the on-time of the power supply after the secondary-side voltage rises beyond a certain relatively high voltage level. In this way, the transformer of the power supply is charged less when the power supply is driving relatively light loads.