USERS MANUAL ISL6745EVAL2Z AN1411 Rev 3.00 Offline High Brightness White LED Driver with High Power Factor for Universal June 9, 2009 Input Introduction The ISL6745UEVAL2Z utilizes Intersils double-ended voltage mode PWM controller for a SEPIC converter application requiring a constant output current. It consists of separate assemblies for the driver and for the LED board, which supports as many as 21 high brightness LEDs. The LED driver board converts a universal (85V to 275VAC) AC input to a DC output rated at 300mA. The design can be further optimized for applications that do not require universal AC input. ISL6745LEVAL2Z is for 80V to 140V applications. Worldwide, on average, 19% of the electric power goes for lighting 11 . In 2000, about 567BKWH of electric energy (or 16% of the nations electric energy in the United States) is for residential, commercial or industrial lighting 22 , 3 33. The commonly used lighting technologies include incandescent and halogen bulbs, fluorescent and compact fluorescent lamps (CFL), and high intensity discharge (HID) lamps. Generally, the efficiency of the lighting sources is indexed by efficacy, or lumen per watt weighted to the sensitivity of the human eye. The efficacy of typical energy saving CFL lamps is around 7%~8%, compared with 2% for the conventional the incandescent bulbs 4 . With the advancement of new materials and manufacturing processes, the economics and performance of high brightness LEDs are now attracting the attention of academia and industry. For instance, in 2006, Cree Inc. demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W 5 . Compared with commonly used lighting sources, white LEDs provide equivalent or superior efficacy, and improvements are announced frequently. Unlike the CFLs, LEDs contains no mercury, and FIGURE 1. TOP VIEW OF DRIVE AND LED BOARDS are less susceptible to breakage. They have a very long Key Features expected life (100,000 hours, more than 10x as much as CFLs), and high on/off repetition capability. Of particular Universal AC Input interest, they are easy to power when compared to many Dimmable LED Brightness traditional sources. The practical application of LED lighting High Power Factor for commercial and residential applications has become a reality. The typical operating current of the high brightness Overvoltage Protection LED is about 300mA~1000mA. The typical drive voltage for Overcurrent Protection high brightness white LED is about 2.5V to 4V. Transformerless The ISL6745UEVAL2Z evaluation board is a reference design for powering high brightness white LEDs. It utilizes Specifications for ISL6745UEVAL2Z the ISL6745 voltage mode PWM controller in the Input AC Voltage 85V to 275VAC (50Hz~60Hz) single-ended primary inductance converter (SEPIC) topology to convert a universal ac input to a variable 300mA Outputs DC . Up to 300mA DC output. See Figure 1. Output DC Voltage Ripple 5% This application note describes the circuit operation design Typical Efficiency 75% constraints, description of the evaluation board, and its Typical Power Factor >0.95 performance. AN1411 Rev 3.00 Page 1 of 15 June 9, 2009ISL6745EVAL2Z Operation Mechanism and Design Constraints A simplified offline SEPIC converter for LED lighting applications is shown in Figure 3, where the voltage across C 1 is the rectified AC voltage. A current sense resistor is placed in series with the switch S . A second resistor is placed in series 1 GATE with the LED string to sense the load current. The control signals include the load current and switch current. The output voltage is sensed for the purpose of output overvoltage I L1 protection (OVP). Since the input capacitor C is very small, 1 the voltage waveform is the rectified input voltage, and the voltage applied to the SEPIC converter is approximately a I L2 rectified sinusoidal. To achieve high power factor, the envelope of the converter input current must track the rectified input voltage waveform. This is accomplished when the converter is IQ operated at a constant switching frequency, a constant duty ratio, and while operating in the discontinuous conduction mode (DCM). Assuming the impedance of the power source is negligible, the voltage across C is given in Equation 1, where 1 I D subscript I denotes input AC line, and V is the amplitude of the l input AC voltage with frequency . l (EQ. 1) v t = V sin t FIGURE 2. TYPICAL OPERATION WAVEFORMS C1 l l If the switching frequency of the converter is much higher than L1 C2 D5 the utility frequency, and C is properly sized, the voltage 2 across C will be equal to the voltage across C . A feedback 2 1 AC voltage, V , is created by the error amplifier. V is the m m C1 L2 C0 S1 amplified difference between the LED current, Io, and the reference voltage, Ir. Varying the reference voltage causes a Rcs proportional change in the LED brightness. The current signal Rs D1~D4 from the current sense resistor, Rcs, which is connected in series with the boost switch S1, is compared with the OCP overcurrent threshold for overcurrent protection. A sawtooth PWM Cs CFB carrier signal is compared with the feedback signal Vm and - PWM I ADJ CONTROLLER + generates the PWM signal to control the turn on/off of the V ERR switch. Typical waveforms of the inductor current, switch and vo - diode currents are shown in Figure 2. + OVP FIGURE 3. SEPIC CONVERTER FOR LED LIGHTING TABLE 1. SEPIC CONVERTER NAME DESCRIPTION AC 85V ~ 265VAC, 50Hz ~ 60Hz D1 ~ D4 Bridge Rectifier C Input Capacitor 1 L /L SEPIC Inductors 1 2 S High Voltage MOSFET 1 C Output Capacitor O C Intermediate Capacitor 2 R Current Sense Resistor S PWM ISL6745A PWM Controller AN1411 Rev 3.00 Page 2 of 15 June 9, 2009