Sophisticated Reference Designs Illuminate The Art Of Power Engineering

Power-supply design is an art with too few practitioners. Two recent reference designs illustrate this point.

One relates to power factor correction (PFC), which limits conducted noise. Switched supplies draw input power in short, high-current pulses that play havoc with utility systems designed for light bulbs and motors.

Designers who need to boost 85- to 265-V rms input voltages up to 300 V rms, with 500- to 3000-W capacity, can look to a boost-converter PFC evaluation board from BI Technologies. Based on the company's 7720 series PFC module, the board consolidates the required input bridge rectifier, fast/low-R_DS(ON) FET, ultra-high-speed diode, and an overtemperature detector into a single component.

The reference design contains the 7720 plus the necessary external circuitry, including the inductor, on a compact board. Only a powerline EMI filter need be added, if required. Able to handle up to 3 kW, the ready-to-run board accepts input voltages at 50 to 60 Hz. It achieves near-unity power factor and better than 95% efficiency.

For computer applications, International Rectifier's reference design for 30-W flyback ac-dc supplies delivers one-tenth the power of BI's design. The IRISMPS4 reference design is based on IR's IRIS4009 ac-dc integrated switcher. It can accommodate any common ac line input while delivering regulated 12 to 15 V dc.

A closer look illustrates the art and sophistication that must go into creating an Energystar/Blue Angel-compliant power supply.

IR's IRIS4009 switcher IC, the key component, combines a control chip and a power MOSFET along with other components in a single five-pin package. Compared to a discrete solution, the switcher cuts part count and board space by 25%. Yet the reference design's BOM lists 18 additional external resistors, 17 capacitors, a full-wave bridge, two discrete transistors, an optocoupler, 10 diodes, a transformer, and three inductors (counting one ferrite bead), plus the IRIS4009. This represents some serious engineering.

BI's PFC evaluation board costs $199 and IR's IRISMPS4 flyback reference design is $245, both in 100,000-piece quantities.

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