Spectral shift control for dimmable AC LED lighting

An apparatus for a solid state light engine, the apparatus comprising:

• a pair of terminals to receive an applied electrical excitation for a load;
• a rectifier receiving the applied electrical excitation and supplying a unidirectional current to the load;
• a first network in the load, wherein the first network includes a first plurality of light emitting diodes (LEDs) arranged in series connection to form a first current path for the unidirectional current supplied by the rectifier, the first plurality of LEDs having a first color characteristic;
• a second network in the load, wherein the second network includes a second plurality of LEDs arranged in series connection to form a second current path for the unidirectional current supplied by the rectifier, the second plurality of LEDs having a second color characteristic that is substantially different from the first color characteristic;
• a bypass path in parallel with said second network and in series relationship with said first network;
• a controllable impedance element in the bypass path;
• and
• a dynamic impedance control module coupled to the controllable impedance element, said dynamic impedance control module adapted to dynamically operate the controllable impedance element to increase an impedance of the bypass path as a substantially smooth and continuous function of said current amplitude in response to said current amplitude increasing above a threshold current value, and to permit said current to flow through said first network and to divert substantially all of said current away from said second network while a voltage drop across the bypass path is less than a forward threshold voltage associated with the second network, and smoothly and continuously transitioning substantially all of said current from said bypass path to said second network as said current increases in a substantially smooth and continuous manner while the voltage drop across the bypass path exceeds a forward threshold voltage associated with the second network,
• wherein a color characteristic of the combined light output of the first and second networks varies between the first color characteristic and the second color characteristic as a function of the applied electrical excitation.