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Use active PFC offline LED controller for primary-side control

Posted: 14 Dec 2011     Print Version  Bookmark and Share

Keywords:power-factor correction  boundary conduction mode 

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. 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 the figure. This signal controls the MOSFET gate to minimise 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, 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 IO to the LED string, and a power factor that exceeds 0.9.

The MP4021 also comes with several protective features including over-voltage protection, short-circuit protection, and over-temperature protection. It is housed in a SOIC8 package that make it suitable for commercial, industrial, and the emerging residential lighting markets.

About the authors
Naixing Kuang and Zhijun Ye are from Monolithic Power Systems Inc.

To download the PDF version of this article, click here.

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