Frequency controlled dummy load to stabilize PFC operation at light load conditions

A method for maintaining a power factor controller in a continuous operation mode in an LED driver circuit wherein the power factor controller receives a rectified voltage from a first rectifier and generates a conditioned DC voltage to a DC-to-DC converter, the DC-to-DC converter including a DC-to-AC inverter operating at a variable switching frequency to provide an AC voltage on an inverter output, the inverter output AC-coupled to a second rectifier, the second rectifier generating a load current, the variable switching frequency varying from a minimum switching frequency at a maximum load current to a maximum switching frequency at a minimum load current, the power factor controller operating in a discontinuous operation mode at the minimum load current, the method comprising:

• coupling a frequency-dependent load impedance to the inverter output;
• and
• varying the frequency-dependent load impedance from a first load impedance at the minimum switching frequency of the inverter to a second load impedance at the maximum switching frequency, the first load impedance greater than the second load impedance, the second load impedance sufficiently small to cause an additional current to flow from the inverter output, the additional current sufficient to cause the power factor controller to operate in the continuous operation mode when the DC-to-AC inverter is operating at the maximum switching frequency.

A system for controlling the current through a DC load, the system comprising:

• a power factor controller that provides a DC voltage to a first voltage rail, the first voltage rail referenced to a second voltage rail;
• a DC-to-AC inverter coupled between the first voltage rail and the second voltage rail, the DC-to-AC inverter having an inverter output, the DC-to-AC inverter switching the inverter output between the first voltage rail and the second voltage rail at a variable operating frequency;
• an isolation transformer having a primary winding AC-coupled to the inverter output and having a secondary winding connected to an AC-to-DC rectifier, the AC-to-DC rectifier providing a secondary current to a DC load, the DC load current responsive to the operating frequency of the DC-to-AC inverter;
• and
• a frequency-dependent load impedance coupled to the inverter output, the frequency-dependent load impedance configured to have a first impedance when the variable operating frequency is at a minimum operating frequency, the frequency-dependent load impedance configured to have a second impedance when operating frequency is at a maximum operating frequency.