6.3 LED Driver Control

The LED driver functions either as a non-synchronous or, optionally, as a synchronous boost converter, taking the battery voltage and increasing it to drive the LED array. The LED array must inherently have a forward voltage higher than the battery to maintain proper control of the LED current.

A relay (K1) is used to route power to the high side of the power topology and isolate the LED array from the solar input. This relay should be sized to the maximum required forward current of the system, which is most likely the solar charge current.

A second relay (K2) is also implemented to allow the LED array to be powered either by the battery or by an external LED driver ballast. This would most commonly be used in hybrid applications where an AC-powered (existing) ballast would be used to back up the battery-based system when daytime solar input doesn’t fully charge the battery. This relay is optional and may also be replaced by diodes if the LED currents make the choice more cost-effective than a relay. Note that the current in this relay is likely less than the K1 relay.

If LED drive currents are low, Schottky diodes could be implemented instead of the relays if the power dissipation can be properly handled. This might reduce system costs compared to relays.