Primary-side-control with active PFC offline LED controller

December 01, 2011 // By Naixing Kuang and Zhijun Ye
Naixing Kuang and Zhijun Ye of Monolithic Power Systems outlien how to achieve primary-side-control with an active PFC offline LED controller.

The rising demand for high-efficiency lighting has helped propel the demand for LEDs in a variety of general illumination markets, including industrial, commercial, and residential lighting. However, LED penetration in general illumination, especially residential lighting, has met with significant resistance. Part of the difficulty stems from the development of a cost-effective method of adapting a DC device to run using the AC mains signal—which typically requires more circuitry than incandescent bulbs. This circuitry can contribute significantly to the cost of LED luminaires while eating into the expected energy savings and luminous output that remains among the biggest issues to limit market penetration. Reducing up-front costs and improving overall energy efficiency will help LEDs penetrate the general illumination market.

Monolithic Power Systems MP4021 power solution uses several methods to both reduce component costs and to improve overall energy efficiency. In particular, the MP4021 uses primary-side control to eliminate the need for a separate optocoupler feedback circuit to control the power flowing through the device, thus significantly reducing complexity and component costs. It also limits primary-side turn-on switching loss using boundary conduction mode (BCM) and implements active power-factor correction (PFC).

The MP4021's primary-side control solution relies on a proprietary real-current control method to accurately estimate the current through the transformer's secondary side without additional circuitry. This method uses a sense resistor connected to the MOSFET to measure the current through the transformer's primary-side, as shown in Figure 1. The output current of the transformer, T1, can then be estimated as a function of the turn ratio of the transformer windings, the internal feedback reference voltage, and the value of the sense resistor. For a universal AC input application, the propagation delay of the gate-control circuit introduces error into the control system, which is vastly improved with the addition of a simple feed-forward circuit that limits the effects of this offset for good line regulation.

 Figure 1 : Current Sense with

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