Electronic Design

Offline PWM Switcher Improves Design Flexibility, Efficiency

The TOPSwitch-FX is the latest member of Power Integrations' family of integrated offline PWM switchers based on the company's proven TOPSwitch power-conversion topology. In addition to integrating many new features to improve design flexibility and energy efficiency, the TOPSwitch-FX incorporates energy-saving EcoSmart technology to meet popular energy-efficiency standards established around the world.

Consequently, unlike previous 3-pin members, this model comes with two additional pins. One is a programmable multifunction pin M for simultaneously setting line undervoltage/overvoltage and line feed-forward situations using a single resistor. The other is a frequency pin F. The line undervoltage lets TOPSwitch-FX perform glitch-free shutdowns, while built-in line overvoltage protects the power supply from line surges. This resistor also permits a reduction in the maximum duty-cycle with line feed-forward to minimize output-line ripple. Alternatively, the same multifunction pin can be used to implement adjustable current limiting that enables higher efficiency with smaller and less expensive transformers.

The frequency pin F sets the switching frequency to 130 kHz when it's connected to the source pin. A half-frequency option can be selected by connecting it to the control pin. However, the F pin is offered only in the 5-pin TO-220 package. The additional pins can be disabled in applications that don't require them by shorting them to the source pin. This allows the device to operate in a three-terminal TOPSwitch mode.

Other new features include a fully integrated soft-start function, frequency jittering, a wider duty-cycle range, hysteretic thermal shutdown, and synchronization. While its 10-ms soft-start capability reduces peak currents and voltages during startup, the large hysteresis prevents pc-board overheating.

"Aside from meeting the stringent demands of communications products, the TOPSwitch has been tailored to deliver a new level of design flexibility and energy-saving features to environmentally friendly power-conversion solutions," says Balu Balakrishnan, vice president of engineering and marketing at Power Integrations.

The TOPSwitch-FX family has three members: the TOP232, the TOP233, and the TOP234. They're available in 5-pin TO-220s, 8-pin DIPs, and 8-pin surface-mounted packages. With a universal input in an open-frame design and adequate heat sinking, the TOP232, 233, and 234 can handle 15, 30, and 45 W of continuous power, respectively. Under similar conditions in a nonventilated adapter, the power capability is halved. TOPSwitch-FX pricing starts at $1.06 in 10,000s.

Power Integrations Inc., 477 North Mathilda Ave., Sunnyvale, CA 94086; (408) 523-9200; www.powerint.com.

See associated figure.

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