Power Electronics / Power Management


PFC/PWM controller combo for power supply design

24 October 2001 Power Electronics / Power Management

Power supply designers need reliable, cost-competitive solutions to meet new harmonic distortion standards. Fairchild thus designed the ML4800 series of combo controller chips which integrate leading-edge power factor correction with trailing-edge pulsewidth modulation into a single chip. With less external components and the ML4800, switchmode power supplies can be designed even more efficiently. Capacitors can be smaller and lower-cost, power line loading and stress on the FETs are dramatically reduced and 1 A gate drivers minimise the need for external driver circuits, according to the manufuacturer. The ML4800 also offers a feature called Trifault Detect.

Main features

The ML4800 consists of an average current-controlled, continuous-boost power factor corrector (PFC) front end and a synchronised pulsewidth modulator (PWM) back end. The PWM can be used in either current or voltage mode. In voltage mode, feedforward from the PFC output bus can be used to improve the PWM's line regulation. In either mode, the PWM stage uses conventional trailing-edge duty cycle modulation, while the PFC uses leading-edge modulation. This patented-leading/trailing-edge modulation technique results in a higher usable PFC error amplifier bandwidth, says Fairchild, and this can significantly reduce the size of the PFC DC bus capacitor. The synchronisation of the PWM with the PFC simplifies the PWM compensation due to the controlled ripple on the PFC output capacitor (the PWM input capacitor). The PWM section of the ML4800 runs at the same frequency as the PFC.



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