Cost Analysis Of An 80-W Transition-Mode PFC Circuit
In a typical 80-W transition-mode PFC circuit, 90% of system cost is due to the input capacitor (C1), boost inductor (T), boost diode (D1), MOSFET, bulk capacitor (C5), and the controller IC (Fig. 2). The optimal design choice is to have the converter's switching frequency at maximum input line voltage and maximum rated output power. A higher switching frequency means lower efficiency, but also lower cost of C1, C5, and T. A higher switching frequency also increases the cost of the MOSFET and diode (or their heatsink) because of higher switching losses.
One very important consideration in selecting a switching frequency is EMI. FCC limits start at 150 kHz, in a way dictating some sweetspots in switching frequency selection. For example, with a 40-kHz switching frequency, the third harmonic will be at 120 kHz. Thus, to save cost, the EMI filters must be designed to suppress only fifth- and higher-order harmonics.
The next highest levels of switching frequencies span 100 to 130 kHz, depending on efficiency and size requirements. Typically, at 200 kHz and above, the advantage of the smaller size of C1, C5, and T is eliminated by the heatsinking requirement for the MOSFET and diode. Numerous application notes are available on TM-PFC ICs that can give precise data on the switching losses in the above mentioned six parts.
Cost Analysis Of A 400-W CCM PFC Circuit
A CCM topology is used in high-power systems where the MOSFET usually is the most expensive component, followed by the diodes and then the controller (Fig. 3). Switching frequency is the first design choice. The same considerations hold true for a TM-PFC circuit. Based on the switching frequency, one or two types of diodes should be tested in the actual circuit, and the selection can be made based on cost versus performance requirements.
Increasing the MOSFET size reduces the conduction losses. But it also increases losses due to its output capacitance and gate charge. Newer MOSFET technologies with lower output capacitance and lower gate charge for a given on-resistance would provide the best efficiency, without much cost increase.
For applications operating at very high power and requiring the highest possible efficiency, you can look into more-complicated current-mode topologies. A tapped-inductor boost topology, for example, can relieve some stress on the boost diode. A magnetic snubber will work too. Some advanced PFC controllers have zero-current or zero-voltage switching topologies to reduce switching losses.
All of these advanced topologies improve efficiency at the cost of extra components and complexity. It's arguable how much value these advanced topologies bring to the table. Perhaps the old-fashioned way of using a better diode, better MOSFET, and lower switching frequency is still a less expensive way to go in many applications.