RT8458A Single-stage offline LED driver for 9W LED Bulb

Transcription

1 RT8458A Single-stage offline LED driver for 9W LED Bulb 1. Introduction The document describes a non-isolated, high efficiency LED driver designed to drive an LED string with a nominal voltage of 32 V at a current of 277 ma from an input voltage range of 90 VAC to 265 VAC. It is designed to fit inside a 9W LED Bulb fixture. The RT8458A is a PWM controller with an integrated high side floating gate driver. It is used for step down converters by well controlling the external MOSFET and regulating a constant output current. The output duty cycle of the RT8458A can be up to 100% for wider input voltage application, such as E27 and PAR30 off-line LED lighting products. The RT8458A also features a 46kHz fixed frequency oscillator, an internal 220mV precision reference, and a PWM comparator with latching logic. The accurate output LED current is achieved by an averaging current feedback loop and the LED current dimming can be easily controlled via the ACTL pin. The RT8458A also has multiple features to protect the controller from fault conditions, including Under Voltage Lockout (UVLO), Over Current Protection circuit (OCP) and Over Voltage Protection (OVP). Additionally, to ensure the system reliability, a RT8458A is built with the thermal protection function. In the following paragraphs contains the LED driver specification, schematic, PCB diagram, bill of materials, conducted EMI measurements, and typical performance characteristics. 2. System schematic The RT8458A is a high side floating gate driver is used to control the Buck converter via an external MOSFET and regulate the constant output current. The schematic of the proposed single-stage LED driver shows in figure 1. Figure 1. Schematic diagram of RT8458A single-stage LED driver 1

2 Reference Part Number Description Package Manufacture U 1 B6S Bridge Rectifier MDI PANJIT High Efficiency PWM U 2 RT8458A Buck LED Driver TSOT-23-6 RICHTEK Controller D1 ES1J Super fast, 600V 1A SMA PANJIT D3 ER206 Ultra-Fast Recover Rectifier, 2A 600V DO-15 PANJIT ZD1 Zener Zener Diode 18V DIP PANJIT R 1 RTT055113FTP 511KΩ R0805 RALEC R 2 RTT051004FTP 1MΩ R0805 RALEC R 3 0.8Ω R1206 RALEC R 6 WR06X0022F 22Ω R0603 WALSIN R D1 WR06X0000F 0Ω R0603 WALSIN R 12 WR06X3300F 3.3KΩ R0603 WALSIN R X2 910KΩ DIP WALSIN L 1 MCD K IND 0.85mH MCD-0406 MAG.LAYERS L 2 IND 1mH, Shield MCD-1012 MAG.LAYERS C X1 Film Cap, 47nF/ 250V TDK C 1 C2012Y5V1H225Z 2.2uF/50V C1206 TDK C B332K nF C0603 WALSIN EC1 KSH106M400S1A5H20K 10uF /400V EC-2P/13 TEAPO EC2 KSY226M050S1A5C11K 22uF /50V EC-2P/8 TEAPO Q 1 IPP60R385CP NMOS, 600V PG-TO220 Infineon Table 1. Bill of Materials 2

3 Front / Back Module EVB Dimensions Components Height: 16mm Figure 2. Photographs of (Top and Side views) PCB Assembly 3

4 3. Performance The testing result of 9W RT8458A single-stage LED driver describes in this paragraph. 3.1 Efficiency fac [Hz] Vac [V] Pin [watt] Vout [V] Iout [ma] Pout [Watt] Efficiency. [%] % % % % % % % % Table 1. In table 1, the output current regulation with line input voltage is 0.36%. 3.2 Output Voltage and Current in Normal Operation Figure 3. Waveform of LED output voltage and Output Current In Figure 3, the output LED averaging current is 277mA and the output voltage is 32.3V with 4

5 lower ripple. 3.3 Output Current and Inductor Current in Normal Operation Figure 4. Waveform of LED output current and inductor current The inductor peak current in figure 4 is about 600mA which RT8458 operated in continuous conduction mode. 3.4 Thermal Measurements The power supply assembly was installed into the lamp socket base and the temperature of the each component was measured, while the room ambient temperature was 25 C. The power supply was driving the three load LEDs (delivering 10 watts). Inductor Diode MOSFET Bridge Diode RT8458A Components L3 D1 Q1 U1 U2 Temp. [ C] Table 2. 5

6 3.4 Conducted EMI Test The measurements were taken with the power supply driving the 10 LEDs. The worst case input voltage was at 230 VAC, where the margin to the test limits was about 6 dbµv. Figure 5. Conducted EMI at 230 VAC. 3.5 Radiated EMI Test Figure 6. Radiated EMI at 230 VAC. 6

7 More Information For more information, please find the related datasheet or application notes from Richtek website: Richtek Technology Corporation Headquarter 5F, No. 20, Taiyuen Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863) Fax: (8863) Taipei Office (Marketing) 8F, No. 137, Lane 235, Paochiao Road, Hsintien City Taipei County, Taiwan, R.O.C. Tel: (8862) Fax: (8862) Richtek Technology Corporation 7