Offline LED Lighting Simplified: High Power Factor, Isolated LED Driver Needs No Opto-Isolators and is TRIAC Dimmer Compatible – Design Note 490 Wei Gu Introduction As environmental concerns over traditional lighting increase and the price of LEDs decreases, high power LEDs are fast becoming a popular lighting solution for offline applications. In order to meet the requirements of offline lighting—such as high power factor, high efficiency, isolation and TRIAC dimmer compatibility—prior LED drivers used many external discrete components, resulting in cumbersome solutions. The LT®3799 solves complexity, space and performance problems by integrating all the required functions for offline LED lighting. opto-coupler. Its unique bleeder circuit makes the LED driver compatible with TRIAC dimmers without additional components. Open- and shorted-LED protection ensures long term reliability. No-Opto Operation Figure 1 shows a complete LED driver solution. The LT3799 senses the output current from the primary side switch current waveform. For a flyback converter operating in boundary mode, the equation for the output current is: IOUT = 0.5 • IPK • N • (1 – D) IPK is the peak switch current, N is the primary to secondary turns ratio and D is the duty cycle. The IC regulates The LT3799 controls an isolated flyback converter in critical conduction (boundary) mode, suitable for LED applications requiring 4W to over 100W of LED power. Its novel current sensing scheme delivers a well-regulated output current to the secondary side without using an L, LT, LTC, LTM, Linear Technology, the Linear logo and μModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. L2 750μH 90V TO 270V AC L1 33mH BR1 C3 0.22μF C1 0.068μF C2 0.1μF R1 200Ω R3 499k R4 499k R7 100k R6 D2 20Ω R8 100k C4 C5 4.7pF 10μF 4:1:1 R13 2k D3 VIN DCM VIN_SENSE R5 3.48k FB R16 32.4k 100k NTC FAULT BR1: DIODES, INC. HD06 D1: CENTRAL SEMICONDUCTOR CMR1U-06M D2, D3: DIODES INC. BAV20W D4: CENTRAL SEMICONDUCTOR CMR1U-02M Z1: FAIRCHILD SMBJ170A Z2: CENTRAL SEMICONDUCTOR CMZ5937B T1: COILCRAFT JA4429-AL M1: FAIRCHILD FDPF15N65 R9 40.2k Z1 C10 560μF ×2 D1 R16 20Ω CTRL3 GATE CTRL2 SENSE CTRL1 VINTVCC R10 15.4k GND FAULT CT COMP+ C6 0.1μF 20W LED POWER Z2 M1 C9 4.7μF RS 0.05Ω C8 2.2nF COMP– C7, 0.1μF Figure 1. TRIAC Dimmable 20W Offline LED Driver Using the LT3799 05/11/490 1A D4 R15 4.99k LT3799 VREF R18 100k R4 100k DN490 F01 the output current by adjusting the peak switch current and the duty cycle through a novel feedback control. Unlike other primary side sensing methods that need to know input power and output voltage information, this new scheme provides much better output current regulation since the accuracy is barely affected by transformer winding resistance, switch RDS(ON), output diode forward voltage drop and LED cable voltage drop. High Power Factor, Low Harmonics By forcing the line current to follow the applied sine-wave voltage, the LT3799 achieves high power factor and complies with IEC61000-3-2, Class C lighting equipment Harmonics Requirement. A power factor of one is achieved if the current drawn is proportional to the input voltage. The LT3799 modulates the peak switch current with a scaled version of the input voltage. This technique provides power factors of 0.97 or greater. A low bandwidth feedback loop keeps the output current regulated without distorting the input current. TRIAC Dimmer Compatible When the TRIAC dimmer is in the off state, it’s not completely off. There is considerable leakage current flowing through its internal filter to the LED driver. This current charges up the input capacitor of the LED driver, causing random switching and LED flicker. Prior solutions added a bleeder circuit, including a large, expensive high voltage MOSFET. The LT3799 eliminates the need for this MOSFET or any other extra components by utilizing the transformer primary winding and the main switch as the bleeder circuit. As shown in Figure 2, the MOSFET gate signal is high and the MOSFET is on when the TRIAC is off, bleeding off the leakage current and keeping the input voltage at 0V. As soon as the TRIAC turns on, the MOSFET seamlessly changes back into a normal power delivery device. Open- and Shorted-LED Protection The LED voltage is constantly monitored through the transformer third winding. The third winding voltage is proportional to the output voltage when the main switch is off and the output diode is conducting current. In the event of overvoltage or open-LED, the main switch turns off and the capacitor at the CT pin discharges. The circuit enters hiccup mode as shown in Figure 3. In a shorted LED event, the IC runs at minimum frequency before the VIN pin voltage drops below the UVLO threshold Data Sheet Download www.linear.com as the third winding can’t provide enough power to the IC. The IC then enters its start-up sequence as shown in Figure 4. CTRL Pins and Analog Dimming The LT3799’s output can be adjusted through multiple CTRL pins. For example, the output current would follow a DC control voltage applied to any CTRL pin for analog dimming. Overtemperature protection and line brownout protection can also be easily implemented using these CTRL pins. Conclusion The LT3799 is a complete offline LED driver solution featuring standard TRIAC dimming, active PFC and wellregulated LED current with no opto-coupler. This high performance and feature-rich IC greatly simplifies and shrinks offline LED driver solutions. VIN 100V/DIV VGATE 5V/DIV 2ms/DIV DN490 F02 Figure 2. MOSFET Gate Signal and VIN VOUT 20V/DIV VSW 100V/DIV 100ms/DIV DN490 F03 Figure 3. Output Open-Circuit Event IOUT 2A/DIV VSW 100V/DIV 100ms/DIV DN490 F04 Figure 4. Output Short-Circuit Event For applications help, call (408) 432-1900, Ext. 3565 Linear Technology Corporation dn490f LT/AP 0511 226K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2011 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●