A power supply for an LED lamp has a set of coupled coils, primary-side power circuitry including a converter power switch for conducting input power, and secondary-side power circuitry including a dimming power switch. Power control circuitry includes converter control circuitry which generates a converter control signal for the converter power switch to maintain a desired undimmed level of lamp current at a normal operating value of a lamp voltage. Dimming control circuitry generates a dimming control signal for the dimming power switch to pulse-width modulate the lamp current at a duty cycle corresponding to a desired dimming. Operation of the converter control circuitry is modified during dimming to prevent an automatic increase of the lamp voltage in response to a decrease in lamp current, avoiding undesirable overshooting of the lamp current and providing more accurate and effective control over dimming operation.