Revamped Planar MOSFETs Are Ready for PFC

May 1, 2008 12:00 PM
By Sungmo Young, Application Engineer, and Wonsuk Choi, Application Engineer, Fairchild Semiconducto

Switch-mode power supplies are increasingly being designed with an active power-factor correction (PFC) at the input stage to meet international regulations for harmonics. The boost topology in discontinuous current mode (DCM) is considered by many to be the most suitable PFC method for converters rated at less than 300 W of output power.

In this topology, the losses associated with turn-on of the boost switch are negligible, while turn-off and conduction losses account for the majority of the converter's power losses. Traditionally, planar MOSFETs have been widely used as the boost switch because of their low cost. In cases where higher efficiency was desired, some designers have used superjunction MOSFETs as the boost switch because of the lower R_DSON associated with these devices. But the tradeoff for that higher performance has been higher cost.

However, simulation and bench testing conducted at the device and system levels demonstrates that the latest planar power MOSFETs can provide excellent performance in DCM PFC applications where up to several hundred watts are required. In these applications, the new planar MOSFETs offer a more-efficient alternative to the superjunction devices currently being used.

MOSFET Technologies

Planar power MOSFETs have high ruggedness under unclamped inductive switching (UIS) conditions. This can occur when power supplies power up during an ac line transient. The latest generation of planar MOSFETs such as Fairchild Semiconductor's UniFET family can offer even greater UIS ruggedness than previous planar devices.

For example, a 265-mΩ, 500-V UniFET shows more than 80 A of avalanche current under a low-coil UIS test. Moreover, it does not fail at all in the test. On the contrary, a conventional planar MOSFET with the same on-resistance failed at around 40 A. The improved ruggedness will ensure enhanced reliability in a system.

In terms of switching performance, gate charge is one of the benchmarks to compare different devices. The UniFET has a smaller gate charge, faster switching characteristics and lower switching power losses than conventional planar MOSFETs such as Fairchild's QFET, a previous-generation MOSFET family. Some typical electric-characteristics benchmarks are shown in the table below.

The Benefit of DCM

Generally, PFC circuits have used a boost topology because it is simple and inexpensive. There are two modes of PFC boost circuit operation. One is continuous current mode (CCM) in which there is continuous inductor current. This mode has many benefits such as lower core loss, lower ripple current and a smaller input filter. But, it requires a very fast reverse-recovery diode for the boost diode, because the boost switch is being switched on while the inductor current is not zero.

In contrast, in DCM the controller switches on the boost switch when the inductor current is zero. This allows the use of slower, less-expensive diodes. The turn-on loss of the boost switch is also negligible. However, the DCM is usually used for small power supplies — 300 W or less where inductor current is relatively low and where low cost is an important design constraint.

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