Reduction of harmonic distortion for LED loads
Apparatus and associated methods reduce harmonic distortion of a excitation current by diverting the excitation current substantially away from a number of LEDs arranged in a series circuit until the current or its associated periodic excitation voltage reaches a predetermined threshold level, and ceasing the current diversion while the excitation current or voltage is substantially above the predetermined threshold level. In an illustrative embodiment, a rectifier may receive an AC (e.g., sinusoidal) voltage and deliver unidirectional current to a string of series-connected LEDs. An effective turn-on threshold voltage of the diode string may be reduced by diverting current around at least one of the diodes in the string while the AC voltage is below a predetermined level. In various examples, selective current diversion within the LED string may extend the input current conduction angle and thereby substantially reduce harmonic distortion for AC LED lighting systems.
1. A method of conditioning current in a light engine, the method comprising:
providing a pair of input terminals adapted to receive a periodic excitation voltage;
receiving a current of equal magnitude and opposite polarity into each one of the pair of terminals, said current flowing in response to the excitation voltage;
providing a plurality of light emitting diodes (LEDs) arranged in a first network, said first network arranged to conduct said current in response to the excitation voltage exceeding at least a forward threshold voltage associated with the first network;
providing a plurality of LEDs arranged in a second network in series relationship with said first network;
providing a bypass path in parallel with said second network and in series relationship with said first network; dynamically increasing an impedance of the bypass path as a substantially smooth and continuous function of said current amplitude in response to said current amplitude increasing in a range above a threshold current value;
substantially smoothly and continuously reducing current flow through the bypass path in response to a substantially smooth and continuous increase in a voltage drop across the bypass path in a range above a forward threshold voltage associated with the second network;
rectifying the excitation voltage to form a substantially unipolar excitation voltage to drive said current;
modulating the excitation voltage;
wherein modulating the excitation voltage comprises receiving a control signal and, in response to information contained in the control signal, applying the excitation voltage waveform that corresponds to the information in the control signal;
wherein applying the excitation voltage to the input terminals only during a portion of the period of the excitation voltage waveform that corresponds to the information contained in the control signal comprises delaying application of the excitation voltage to the input terminals during at least one of the periods, wherein a length of the delay is responsive to the information contained in the control signal;
wherein the light engine operates with a power factor above 90%.
2. The method of claim 1 , further comprising modulating the impedance of the bypass path at two times a fundamental frequency of the excitation voltage waveform.
3. The method of claim 1 , further comprising modulating the impedance of the bypass path at a fundamental frequency of a unipolar excitation voltage waveform.
4. The method of claim 1 , further comprising arranging said first network, said second network, said substantially smooth and continuous function of said current, and said threshold current value so that said current exhibits less than 30% total harmonic distortion in response to the excitation voltage having a substantially sinusoidal waveform.