With legislation in place for the phase-out of incandescent bulbs and the steady advancement of LED technology, LEDs have become ever more important as a light source. While there are still many challenges facing the LED before it becomes the primary light source, there is no question that LEDs will continue to be used in ever more lighting applications. Two challenges standing in the way of LED adoption in low-power bulbs are the size of the driver and cost.
The most commonly used topology for offline LED drivers is the flyback topology. In order to have accurate, constant LED current, typically a secondary LED current feedback is needed. An opto-coupler is also required to transfer the signal past the isolation barrier. In the primary side, a controller and discrete power devices are necessary for power conversion. For low wattage bulbs, this can be a problem since there is little space for these components.
An example of how STMicroelectronics is solving this problem is the HVLED805. The device integrates a high-performance, low-voltage PWM controller chip and an 800 V, avalanche-rugged power MOSFET in the same package. This highly integrated device minimizes the number of external components, resulting in a compact and very cost effective solution.
The chip utilizes a proprietary primary-sensing technique for constant output current (CC) and constant output voltage (CV), eliminating the need for the opto-coupler, the secondary voltage reference and the current sensor, while still maintaining quite accurate regulation. The device is a high-voltage primary switcher intended to operate directly from the rectified mains. In addition, the HVLED805 offers protection features that considerably increase the end-product’s safety and reliability (auxiliary winding disconnection, shorted secondary rectifier or transformer’s saturation), which make the IC well-suited for constant current LED drivers.
The schematic in Figure 1 is a design for a LED driver less than 5 W. For Energy Star, there is no power factor requirement. The total component count is only 22--the lowest component count solution in the market. The accuracy of the LED current is within +/-5% of setting point.