As a rugged, energy-efficient and high-performance alternative to halogen lamps, LEDs are increasingly used in 24VAC and 12VAC lighting systems. The power converter responsible for driving the LEDs should have a high power factor (above 90% to meet the industry-recognized green standards), should have high efficiency, use very few components, and should maintain very low temperatures during operation. Rise. They do not need to be isolated.
A solution that meets these requirements integrates a rectifier bridge and a current-controlled synchronous boost/buck converter. Specifically, a synchronous four-switch buck-boost converter can be paired with a four-switch ideal diode rectifier bridge to drive high-power LEDs; a lower power solution can use a standard diode bridge. Both solutions are shown here.
The LT3791 60V four-switch synchronous buck-boost controller IC can drive a constant current (DC or ripple) into a high power LED string. The device features an output current feedback loop that drives a constant current through an LED string and a CTRL dimming input that connects to a 120Hz half sine output of a rectifier bridge to produce a High power factor ripple LED current output.
The LT4320 is an ideal diode rectifier bridge that can replace four typical rectifier diodes to drive four MOSFETs for extremely high efficiency conversion from 60Hz 24VAC input to 24VRMS 120Hz ripple output. When the current reaches 5A and higher, the diodes in the standard rectifier bridge will dissipate a lot of power and heat up. The LT4320 helps low-power AC applications operate with high efficiency and low temperature rise by driving low-resistance external N-channel FETs.
98.1% power factor
Figure 1 shows an LED driver operating directly at 24VAC with a power factor of 98.1%. It is capable of driving up to 25V LEDs with 120Hz pulsating power with a peak LED current of 4.4A. At a frequency of 120 Hz, the human eye does not perceive the pulsation of light and considers the brightness of the light to be constant. Figure 2 shows the high power factor 24VAC input voltage and current waveforms. The 120 Hz pulsating LED current waveform is shown in Figure 3.
Figure 1: 24VAC to 60W LED driver (equivalent to a 600W halogen lamp) with high power factor and high efficiency
Figure 2: 60Hz 24VAC input waveform
Figure 3: 120Hz Pulsating LED Driver Waveform
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