Voltage-mode CRM PFC for lighting offers higher efficiency, improves reliability
Thursday, December 1, 2011 in Energy Efficiency
When it comes to lighting, efficiency and reliability are critical. Critical-conduction mode (CRM) power factor correction (PFC) controllers are available with two different control modes: current-mode and voltage-mode.
Though current mode controllers have been used for a while now, voltage mode controllers offer important advantages-particularly for applications-such as lighting, where high-efficiency low THD and high power factor are becoming more of a premium. The newest voltage-mode controllers designed for lighting applications are specifically designed to maintain good power factor, low THD at light loads, zero overshoot at turn on, and a PFC-ready pin for power sequencing of downstream converters. Also, the new voltage-mode devices are conveniently designed to have backwards compatibility with older current-mode designs. This allows some of the benefits of this voltage-mode controller to be implemented in older designs without layout changes.
The main difference between a voltage-mode and current-mode controller is how the controller decides when to turn off the power MOSFET. For the current-mode controller, the MOSFET is turned off when the inductor current sensed through the current-sense resistor meets the desired current reference. Another major difference between the current-mode and voltage-mode PFC controllers has to do with the placement of its compensation network.
The newest generation of voltage-mode PFC controllers, such as the FL7930C from Fairchild, has added a few important features, such as a soft-start feature with zero overshoot, internal THD optimizer, and a PFC-ready pin. The THD optimizer allows for improved total harmonic distortion, while reducing the number of external components required and frees up an external pin which is being used for the PFC ready function. The PFC-ready pin tells the downstream DC/DC converter that the output voltage is at 89% of the set-bus voltage, making power sequencing much easier.
However, there’s a lot more to it. Read the full article at EE Times Design to learn the differences between these two controllers, how voltage-mode improves reliability and efficiency, and more about the FL7930C.
This article, posted in Power Management DesignLine, was written by Alexander Craig, Principal Application Engineer, at Fairchild Semiconductor.