Most boost converters with PFC may operate in either discontinuous mode or continuous mode. In discontinuous mode (see the figure, a), the switch is turned on when the inductor current reaches zero and turned off when the inductor current meets the desired input reference voltage. The input current stays more or less in phase with the input voltage, and the power factor is close to 1.
Higher-capacity supplies run in continuous mode, in which the switch does not turn on when the boost inductor is at zero current and the current in the energy-transfer inductor never reaches zero during the switching cycle (see the figure, b). The voltage swing is less than it is in discontinuous mode, resulting in lower conduction losses. Also, the lower ripple current results in lower inductor-core losses. The most important thing about continuous-conduction mode from a semiconductor standpoint is that it needs a diode with a very fast reverse-recovery characteristic.
The reasons for using different modes comes down to a tradeoff between cost and efficiency. Obviously, if every application could run in discontinuous mode, life would be simple. But conduction losses make discontinuous mode undesirable for converters that have to supply more than around 200 W.
"The kind of PFC you use depends on the power level," says Stephen Oliver of International Rectifier. "From 75 to around 200 W (some people say 150, others say 250), people use discontinuous mode because it's a cheap and simple solution. But because current is discontinuous, you have a very high peak current, which requires a large EMI filter and some oversized components." Typical applications for discontinuous mode, Oliver says, include smaller laptops, LCD-TV adapters, and silver box PC supplies.
At above about 200 W, designers don't use discontinuous mode because the peak current gets so large that the necessary EMI filter is rather hefty, and huge heatsinks are required. Designers use continuous-conduction mode here.
Oliver also notes that for different types of PFC, selecting the right diode is important. Discontinuous mode requires a fast diode with a low forward-voltage drop (VF). There's a tradeoff of around 1.5 to 2 V, where a low-VF diode is best for a discontinuous PFC. For continuous conduction mode, the tendency is to use a very fast diode at the cost of some sacrifice in VF.
