Posted on Monday, March 21, 2011 • Category: LED
There is a major advantage to using LEDs for lighting: various dimming techniques allow seamless control of the light output from the LED source. While LEDs are efficient light sources, the dimming feature also allows for considerable power savings. Control over the light output also helps to set the desired ambience.
PWM dimming is preferred to analog dimming for several reasons. For many applications PWM dimming maintains the color of the light output regardless of the dim level. For circuit design, PWM control is more immune to noise; the control signal need not be accurate in both voltage levels and dimming frequency; and the driver circuit design is less complex. PWM dimming usually requires a control line that carries the PWM dimming signal, in addition to the two power-supply lines. This standard configuration is, however, a drawback for applications that use common dimming for a large number of lights; the configuration also makes it difficult to replace the incandescent light installations with two-wire supply lines that depend on supply chopping for dimming control.
Traditional, crude LED driver circuits that work with power-supply dimming are problematic. Those drivers turn off power to the LEDs gradually as the input filter capacitors discharge to the minimum operating voltage of the driver. That process can cause the input and output filter capacitors to discharge to low levels. Then when the supply is turned back on, a huge surge of current flows to replenish the capacitor charge, thus causing EMI issues and premature dimmer damage. To prevent these various issues, those circuits use large inductor filters that increase cost.
The LED driver reference design described here addresses this basic design challenge with PWM dimming. This LED driver implements PWM dimming based on supply chopping; it does not cause any supply current overshoot. The design provides up to 90% efficiency while operating from a 24V supply. It allows a unidirectional supply input with an efficient semi-MOSFET bridge rectifier at the input. Figure 1 shows the top view of the design board.
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