LNK406EG 14W dimmable, high efficiency LED driver design

This article describes a high power factor, thyristor dimming LED driver that provides a 28 volt rating and 0.5 A current rating for LED strings over an input voltage range of 90 VAC to 265 VAC. The LED driver uses the LNK406EG device in the LinkSwitch-PH family of ICs.

LinkSwitch-PH ICs can be designed to be cost-effective and have a very small number of component LED drivers that not only meet power factor and harmonic limits, but also provide an end-user experience. Features include an ultra-wide dimming range, flicker-free operation (even with a low-cost AC-input TRIAC dimmer) and fast, smooth turn-on.

The topology used is an isolated flyback that operates in continuous conduction mode. The output current regulation is completely detected from the primary side, eliminating the need for a secondary feedback component. There is also no need to detect external current on the primary side, but inside the IC, further reducing components and losses. The internal controller adjusts the MOSFET duty cycle to keep the input current sinusoidal AC, ensuring high power factor and low harmonic current.

The LNK406EG also offers a variety of sophisticated protection features, including automatic restart in loop open loop or output short circuit conditions. Input overvoltage provides enhanced immunity to input faults and surges. Output overvoltage protects the supply when the load is disconnected. Accurate hysteresis thermal shutdown ensures that the average PCB temperature is within safe limits under all conditions.

In any LED lighting device, the performance of the drive directly determines the end customer (user) perception of the lighting, including start-up time, dimming, flicker and consistency between the drives. The focus of this design is to be as compatible as possible with dimmers and to be as compatible as possible with dimming ranges at 115 VAC and 230 VAC. Even so, design simplification is possible with two single-input voltage operating ranges, including applications that do not require dimming or dimmer (high quality) dimming range.

First, the circuit schematic

Figure 1 circuit schematic

Second, the circuit analysis

The LinkSwitch-PH is a device that integrates a controller and a 725 V MOSFET for LED driver applications. The LinkSwitch-PH uses a single-stage continuous conduction mode flyback topology that provides a primary-side regulated constant current output while maintaining a high power factor for the AC input.

1, input filtering

Fuse F1 provides protection in the event of a component failure, while RV1 is used to clamp the maximum voltage that can be generated during a differential mode surge test. The RV1 is rated at 275 VAC, slightly above the maximum specified operating voltage of 265 VAC. The diode bridge BR1 rectifies the AC line voltage, and capacitor C2 provides a low impedance path (decoupling) for the primary switch current. In order to keep the power factor above 0.9, it is necessary to ensure a lower capacitance (sum of C1, C2 and C11) values. The EMI filtering function is provided by inductors L1-L3, C1 and C7 with Y1 safety requirements. Resistors R16 and R17 across L1 and L2 suppress resonances typically found in conducted EMI measurements between input inductance, capacitance, and AC input impedance.

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