New PFC controller achieves 96% efficiency
By Paul Rako, Technical Editor -- EDN, February 19, 2009
Fairchild Semiconductor has introduced the FAN9612 two-phase BCM (boundary-conduction-mode) PFC (power-factor-correction) controller for 100 to 1000W power systems. Driving external, high-voltage FETs, the device can realize efficiencies of 96% and targets use in two-phase, interleaved PFC control. Dual-phase operation improves efficiency and also allows you to reduce the input filter size by 10%. BCM represents the precise transition point between CCM (continuous-conduction mode) and DCM (discontinuous-conduction mode). In CCM, the PFC inductors continuously carrying current; in DCM, the inductors “run dry”—that is, go to zero current—for part of the switching cycle. With BCD, each inductor goes to zero current, and the device then switches back on. The controller can automatically disable one of the two phases when driving light loads to improve efficiency.
The device features overcurrent protection and power-limit sensing for each channel, input brownout detection, and input overvoltage protection. You can design in soft start, and the restart operating frequency is above the audio range. The inductor size you select and the input voltage determine a switching speed of 18 to 600 kHz. The device also provides for input voltage feedforward to improve line regulation.
The FAN9612 provides PFC for digital TVs, desktop and entry-level server computers, front-end-telecom systems, and industrial-power systems. It comes in a 16-pin SOIC package, operates in the −40 to +125°C temperature range, and sells for $1.30 (1000).
