This engineering report describes a 45 W isolated flyback power supply with a single stage power factor correction circuit for LED lighting application. The power supply is designed to provide an 80V constant voltage across 0A to 580mA output current load. It is also capable of providing 580mA constant current output for LED Lighting applications. The board is optimized to operate from an input voltage range of 90 VAC to 265 VAC.
The design is a universal input flyback converter design with an added switched valley-fill PFC circuit. Through the PFC circuit, the unit meets the high power factor requirement in LED lighting application while reducing loss by directly transferring energy to the output.
The key design goals were excellent regulation, high efficiency, and high power factor across the input voltage range.
The LYTSwitch-6 device (LYT6068C) combines a 650 V power MOSFET, sense elements, a safety-rated feedback mechanism, along with both primary-side and secondary-side controllers in one device. Since LYTSwitch-6 uses an integrated communication link, FluxLink™, accurate control of the primary-side switch by the secondary controller is possible and close component proximity is utilized. The LYTSwitch-6 IC is designed to deliver a 45 W flyback power supply with a switched valley-fill PFC providing a high power factor with 80 V constant voltage supply throughout the input range of 90 VAC to 265 VAC.
Input Circuit Description
Fuse F1 isolates the circuit and provides protection from component failures. Varistor RV1 acts as a voltage clamp at the input in case of voltage spikes from transient line surge. BR1 rectifies the AC line voltage and provide a full wave rectified DC across the input capacitors C2 and C3. Capacitor C1, L2, C2, L3, and C3 forms a 2-stage LC EMI filter to suppress differential and common mode noise caused by the PFC and flyback switching action.
One end of transformer (T2) primary is connected to the positive output terminal of the bulk capacitor (C4) while the other side is connected to the drain of the integrated 650 V power MOSFET inside the LYTSwitch-6 IC (U4). A low cost RCD snubber clamp formed by D8, R17, R46, and C9 limits the peak Drain voltage spike across U4 at the instant turn-off of the MOSFET. The clamp helps dissipate the energy stored in the leakage reactance of transformer T2.
LYTSwitch-6 Secondary Side Control
The secondary side control of the LYTSwitch-6 IC provides output voltage, output current sensing and drive a non-sync FET providing synchronous rectification. The secondary of the transformer is rectified by D10 and filtered by the output capacitors C16 and C18. Adding an RC snubber (R48 and C14) across the output diode reduces voltage stress across it. The secondary-side of the IC is powered from an auxiliary winding FL3 and FL4.
PFC Circuit Operation
Without the added PFC circuit, the power factor of the flyback power supply is normally around 0.5 to 0.6 at full load condition. Input from the bridge rectifier (BR1) will just directly feed the bulk capacitor (C4) that charges and recharges till the next voltage peak fed to it. The input charging pulse current must be high enough to sustain the load until the next peak. This means that the charging pulse current is around 5-10 times higher than the average current with a high phase angle difference from the voltage waveform; hence, the expected PF from this standard configuration is low and THD is high.