[Technology Report]
PFC And Efficiency Mandates Inspire New Power Discretes
Semi makers tweak power switches and boost diodes, while reports vary on OEM adoption of synchronous rectification.
A common element among new developments in discrete semiconductor power devices involves PFC, or power factor correction (see "Power Factor Basics," below). About 30% of the world's markets—including Europe, China, Japan, and several states in India—now require PFC in switching supplies for computing gear. Though PFC hasn't been mandated yet in the U.S., the IEEE is at work drafting standards.
Stephen Oliver, who manages International Rectifier's marketing to OEMs that make ac-dc front ends, believes that PFC has broader benefits than regulatory compliance. "If you put PFC on your front end, you automatically have a universal-input supply," he says.
"One design can run from mains voltages from 80 to 264 V," he explains. "Moreover, PFC up front means you have a very cleanly controlled bus voltage, usually at 380 or 400 V. Downstream from that, you can use lower-voltage MOSFETs or diodes, which means that with the same size chips, you can trade off performance versus cost. For example, instead of using maybe an 800-V MOSFET, you can use a 500- or 600-V device."
Without PFC, switching supplies feed harmonics back onto the powerlines. The problem arises when systems draw current from the energy storage capacitors in the ac-dc front-end supply in narrow, high-amplitude pulses.
These pulses contain harmonics that interfere with other equipment on the line and reduce the maximum power that can be drawn. In addition, the distorted line voltage causes capacitor overheating, dielectric stress, and overvoltages in insulation.
The problem with power factors that are significantly less than unity involves the rms current content of the harmonics created by the switching supply. The rms current is:
where n is the order of the respective harmonic currents. Eliminating the harmonics by making the power-supply input current approach a sinusoid decreases the rms input current and brings the circuit power factor closer to unity. Without PFC, a typical switched-mode power supply (SMPS) exhibits a power factor between 0.6 and 0.7.
DISCRETES' ROLE IN PFC Historically, switched-mode power supplies with PFC have used a boost converter topology that contains two semiconductor components: a power switch (MOSFET or IGBT) and a boost diode (Fig. 1). This is where semiconductor manufacturers are concentrating their product differentiation efforts.
Diodes used in boost converters are considered "soft" or "snappy" depending on their reverse-recovery characteristics. Consider what happens in a boost converter operated in continuous current mode (see "PFC Operational Modes," p. 60). The boost diode and the switching device operate in the hard-switched mode, which involves a short recovery period in which the diode conducts briefly in the reverse direction (see "Reverse Recovery," p. 67).
The diode's reverse-recovery characteristics increase the switching device's turn-on losses and generate EMI. If the reverse current characteristic is snappy—that is, if it shuts off too abruptly—there are voltage spikes and ringing. To deal with this source of EMI, designers have either slowed down the switch turn-on di/dt with a softer-switching diode and/or added snubber circuits. But a slower switch turn-on rate increases turn-on loss, and efficient snubbers are tough to design.
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