Offline 8w LED Flyback Power Supply with PFC using NCP1014

The offline 8W LED driver circuit using NCP1014 as the controller is shown in Figure 1. The AC input voltage of this LED driver circuit ranges from 90v AC to 265v AC, and the output is 8W, driving a xLAMP MC-E from Gree. It can be used as a portable table lamp, cabinet light or corridor light, etc. MC-E contains 4 surface mount LEDs in one package. The rated current of each LED is 700mA. The actual current through the LED string is 630mA.

In Figure 1, R1 is a fusible fuse resistor, C1, L1 and C2 form an EMI filter, D1 to D4 are bridge rectifiers, U1 (NCP1O14), transformer T1, rectifier diode D7, capacitor C8/C9, etc. form a DC-DC flyback converter. The current sensing resistor Rsense (R6, R7 and R8), transistor Q1, optocoupler U2, capacitor C6, etc. form a feedback loop, and Q2, R13, R14, R15, R16, and D9 form a dimming circuit.

NCP1014 is an IC that integrates a PWM controller and a 700v power MOS-FET in the same 4-pin SOT-223 package, with a maximum drain current of 450mA. The switching frequency of U1 in Figure 1 is 100kHz (there are also 65kHz and 130kHz versions). NCP1014 contains a dynamic self-powered circuit. The DC high voltage can be directly applied to the pin DRAIN for startup. To protect the MOSFET in U1, an RCD clamp circuit composed of R2, C3 and D5 is set at the primary of T1. When the system enters normal operation, U1 is powered by the bias power supply composed of T1c, D6, C4, R3 and C5.

No smoothing capacitor is connected to the output of the bridge rectifier, so the DC voltage input to the flyback converter is a half-sine wave voltage with a frequency twice the AC line frequency. In the circuit of Figure 1, in order to obtain a high power factor, a large 47μF capacitor C6 is connected to the feedback input terminal FB of U1, which makes the feedback response slower during the half cycle of the input power supply and keeps the feedback input level basically unchanged. With this simple method, the power factor is >0.7 (see Figure 2) in the same input voltage range of 90v~200v AC, meeting the requirements of the US Department of Energy (DOE) Energy Star standard (version 1.1-12/19/08) for solid-state lighting (SSL).

The harmonic content of the AC input current meets the limits of IEC61000-3-2 for Class C (lighting) products. The third and fifth harmonic currents and their comparison with the Class C limits are shown in Table 1.

As long as the voltage drop caused by the current through the output current sensing resistor Rsense is about 0.6v, the PNP transistor Q1 will be turned on, and its collector current will pass through the LED in U2 to form a feedback loop. The output current is 630mA, the peak-to-peak ripple current is 25%, and the peak current is 12.5% ​​higher than the average current, so the current sensing resistor Rsense=0.6v/[630mA×(1+12.5%)]≈0.85Ω. Select R6=R7=1.8Ω/0.25W. R8=10Ω, 0.25w. The total resistance after parallel connection is about 0.83Ω. D8 limits the maximum output voltage (22v), and R9 and C10 provide a small discharge path and filtering. Q2, R12~R13 and D9 form a dimming circuit. By adjusting R13, the LED current can be changed, thereby achieving dimming.