High Efficiency 3.6W Isolated LED Driver Design

This article introduces a high-efficiency LED driver that can provide 12 V output voltage and 0.3 A output current in the input voltage range of 90 VAC to 265 VAC . The LED driver uses the LNK605DG device from the LinkSwitch-II series of ICs

from Power Integrations. The topology used in this device is an isolated flyback running in discontinuous conduction mode. The output current control is completely sensed from the primary side, so there is no need to use secondary feedback components . There is no need to sense the external current on the primary side, but it is performed inside the IC, which further reduces components and losses.

The LNK605DG also offers a variety of sophisticated protection features, including automatic restart in the event of an open loop or output short circuit. Accurate hysteresis thermal shutdown ensures that the average PCB board temperature is within a safe range under all conditions.

2. Circuit diagram

3. Circuit Analysis

The LinkSwitch-II device consists of an integrated controller and a 700V power MOSFET for LED driver or charger applications. The LinkSwitch-II is configured for a single-stage discontinuous conduction mode flyback topology, providing primary-side regulated constant voltage and constant current outputs.

3.1 Input Filtering

The AC input supply is rectified by the bridge rectifier BR1. The rectified DC is filtered by large capacitors C1 and C2. Inductors L1, C1, and C2 form a π-type filter to attenuate differential-mode conducted EMI noise. This configuration, combined with the E-shiELd™ technology of the PowerIntegrations transformer, enables this design to meet the EMI standard EN55015 Class B requirements without the use of Y capacitors and with a large margin. The transformer construction also enables good EMI repeatability. The fuse resistor RF 1 provides protection in the event of a power failure. This resistor usually needs to be selected as a wirewound resistor to withstand the instantaneous power dissipation when the input capacitor is charged when it is first connected to the AC.

3.2 LinkSwitch-II Primary

The LNK605DG device (U1) integrates the power switch , oscillator, CC/CV control engine, startup, and protection functions. The integrated 700 V power MOSFET provides ample voltage margin in universal input AC applications. The device is powered from the BYPASS pin through the decoupling capacitor C4 during startup. The rectified and filtered input voltage is applied to one end of the primary winding of T1. The 700 V power MOSFET integrated in U1 drives the other side of the transformer primary winding. D1, R3, R4, and C3 form an RCD-R clamp circuit to limit the drain voltage spike caused by the leakage inductance. Diode D2, C5, and R10 generate the primary bias supply. This voltage is generated from the transformer bias winding and supplies bias current to the BYPASS pin through D2 and R10. The bias supply for the LNK605DG is optional. If the circuit structure is powered from the bias supply (same as this design), the no-load power consumption will be reduced to less than 50 mW.

3.3 Output Rectification

The secondary winding of the transformer is rectified by D3; a Schottky barrier diode is selected for efficiency and filtered by C7. In this application, a ceramic capacitor is used for C7, which has a longer service life than electrolytic capacitors due to its higher operating ambient temperature. Resistors R1 and C6 are used to attenuate high frequency ringing and reduce diode voltage stress.