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PWM advances active-clamp converter designs

Posted: 18 Oct 2005 ?? ?Print Version ?Bookmark and Share

Keywords:national semiconductor? lm5026? pulse-width-modulation controller? pwm controller?

National Semiconductor says its LM5026 pulse-width-modulation (PWM) controller is the industry's most advanced type for forward converter toplogies using an active-clamp/reset design. National said this 100V current-mode device, designed to simplify loop compensation and reduce total parts count, boosts overall efficiency compared to standard forward converters in AC/DC and DC/DC telecom and industrial supplies.

"The LM5026 is the most advanced, feature-rich controller for the forward active clamp reset topology available today," said Paul Greenland, director of strategic marketing for the Power Management group. "With this addition to National's active-clamp controller family, engineers now have the flexibility to design with the single-channel, current-mode LM5026, the single-channel, voltage-mode LM5025 or the dual-channel LM5034, which controls two interleaved power stages. Together, this versatile family of controllers meets all the control and drive requirements of a wide range of power supply applications, while enabling higher efficiencies and power densities." The active-clamp technique allows operation at duty cycles greater than 50 percent to deliver a well regulated output from supplies whose input voltage varies widely.

The LM5026 includes a wide-range input start-up regulator working from a 13V to 100V input, high-frequency oscillator, current-mode PWM control circuitry and two drivers: a main 3A MOSFET stage and a 1A clamp MOSFET stage. The LM5026's current-mode feedback topology increases the effective bandwidth of conventional opto-couplers to simplify compensation and improve overall loop bandwidth and transient response. User-programmable under-voltage lockout and soft-start circuitry ease power-on sequencing and minimize start-up surge currents, while a programmable restart circuit supports three modes of over-current fault protection. The LM5026 also provides for synchronizing the oscillator (to an external master clock) to minimize EMI.

Additional information on the controller and its evaluation board is available online. The LM5026, available in 16-lead LLP and TSSOP packages, is priced at $1.10 each in 1,000-unit quantities.

- Vince Biancomano


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