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Boost lighting efficiency with voltage-mode CRM PFC

Posted: 14 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:critical-conduction mode? power factor correction? total-harmonic distortion?

There are two control different modes for critical-conduction mode (CRM) power factor correction (PFC) controllers: current-mode and voltage-mode. While current-mode controllers have been used for quite some time, voltage-mode controllers offer significant advantages particularly for applications such as lighting, where high-efficiency, low total-harmonic distortion (THD) and high power factor are becoming more of a premium.

The new voltage-mode controllers offer a few other features and are specifically designed for lighting applications, such as the ability to maintain good power factor, low THD at light loads, zero overshoot at turn on (which reduces the stress on electrolytic capacitors and therefore increases reliability), and a PFC-ready pin for power sequencing of downstream converters. The new voltage-mode devices are designed to be backwards compatibility with older current-mode designs, to allow some of the benefits of this voltage-mode controller to be implemented in older designs without layout changes.

For both the voltage-mode and current-mode CRM PFC controllers, the boost switch is turned on when the boost inductor current reaches zero. This is measured with a zero-current-detect resistor connected to the auxiliary (aux) winding or VCC winding before rectification, which feeds that signal into the control IC. Proper optimization of this resistance can result in a level of quasi-resident switching for the power MOSFET in voltage-mode converters. The main difference between a voltage-mode and current-mode controller is how the controller decides when to turn off the power MOSFET.

For a current-mode controller, the MOSFET is turned off when the inductor current sensed through the current-sense resistor meets the desired current reference. In this case, a percentage of the rectified AC-line voltage (which is sensed through resistor-divider network) is used to generate the current reference which is input to the control IC via the multiplier pin. This means that there is some power loss in sensing the AC line voltage.

Figure 1: CRM PFC current mode (upper) vs. voltage mode (lower).

Additionally, there is also a more significant power loss in sensing the MOSFET drain current. The MOSFET drain current is the inductor current, and that current is being sensed and compared with the reference measured from the AC line input. The value of that signal needs to be high enough to be used for compensation and control. Therefore, the overcurrent protection voltage needs to be above that signal for a current-mode device. This voltage is approximately 1.2 to 1.4 V.

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