Power components

Primary-side-control with active PFC offline LED controller

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

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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 Feed-Forward Compensation

To improve overall power efficiency, the device uses BCM to improve power transfer from the AC source to the LED string while maintaining a near-constant output RMS current. BCM makes the transformer work on the boundary between the continuous and discontinuous mode, which is quite different from the well-known resonant converter and has several advantages in both energy efficiency and output noise. Unlike conventional fixed-frequency operation, the device uses a variable-frequency signal. This operation produces a constant turn-on time over each line half-cycle: The turn-off time is determined by the magnetizing energy discharged through the transformer to the load of the secondary-side, as shown at the bottom of Figure 2. This signal controls the MOSFET gate to minimize the turn-on switching loss and thus limit the level of switching noise that induces EMI noise and leaches power from the system.

The MP4021 also employs active PFC to help improve the power factor. As shown in Figure 2, the current envelopes of the transformer's primary and secondary sides follow the sinusoidal shape and phase of the rectified mains signal: The device achieves these envelopes though an internal multiplier that connects to the tap of a resistor divider connected to the rectified mains signal. The multiplier output acts as the sinusoid reference for the PWM generator, and allows the device to retain a steady RMS output current of I_O to the LED string, and a power factor that exceeds 0.9.

Figure 2 : Current from the Transformer's Primary- and Secondary-Sides, and the Corresponding MOSFET Gate Signal.

The MP4021 not only simplifies the application circuit, but also comes with several protective features—including over-voltage protection, short-circuit protection, and over-temperature protection—in a compact SOIC8 package that provides designers with additional flexibility to engineer a safe and attractive luminaire for commercial, industrial, and the emerging residential lighting markets.

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