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  • Triac Dimmable LED Driver Using PIC12HV752

  • Created: Oct 13, 2016

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The LEDs these days have been popularly being used in lighting industries. This is because of its improved performance and cost-effectiveness as compared to its competitors. Their efficacy (light out per watt in) is very competitive with fluorescent lamps, while their life cycle is significantly longer at 100,000 full-power hours at half brightness. Dimming LEDs can extend the life cycle further, and can reduce power drain. By far the most common dimming method is the TRIAC phase-control dimmer, which directly replaces the AC ON-OFF switch, but can be used only with incandescent lamps and specially designed fluorescent ballasts. What makes the incandescent lamp works well with a TRIAC dimmer because it is purely resistive. Thus, in order to design a LED driver compatible with TRIAC dimmer, the input characteristics of the LED driver should be resistive too. This is achieved through the use of high Power Factor (PF) flyback converter topology. This topology has an inherent Power Factor Correction (PFC). PFC can make the LED driver look like a pure resistor from the AC input side by making the input line current in-phase with the input line voltage.

This circuit then presents a LED driver solution that is compatible with a traditional TRIAC dimmer. Microchip’s PIC12HV752 microcontroller is used as the main controller of the whole circuit. The PIC12HV752 is a low-cost 8-pin chip with on-chip core independent peripherals that are suitable for power conversion applications. In this design, PIC12HV752 microcontroller controls the circuit at the primary side, using on-chip core independent peripherals. The Complementary Output Generator (COG) peripheral (pin RA2) provides a Pulse Width Modulated (PWM) signal, which drives the input of the MCP1416 MOSFET driver to turn ON/OFF the MOSFET (Q2). The main purpose of the COG in the circuit design is to convert two separate input events into a single PWM output. The COG uses two independently selectable event sources to generate the PWM. The COG output Q is set to high only when a rising edge triggers the rising source input. During this time, the COG turns on the MOSFET. The MOSFET turns off when a low-voltage level is detected on the falling source of the COG.

Also, this design includes additional circuits for improved reliability. Inductor L2 protects the primary side components by suppressing the large input current spikes. The snubber circuit, which is comprised of R26 and C13, is used to protect Q2 from a large voltage spike caused by the leakage inductance of T1. The bleeder circuit, which is comprised of C1 and R17, draws additional current in order to maintain the TRIAC holding current at low input line voltage.