Electronic Design

Active-Clamp PWM Controller Boosts Efficiency, Power Density

Crafted from National Semiconductor's analog bipolar-CMOS-DMOS process, the LM5025 is a 100-V pulse-width-modulation (PWM) controller for forward converters. Yet its integration of the active-clamp/reset technique, which is used in distributed power architectures, sets it apart from other PWM controllers.

Designed for high switching frequencies, it provides higher efficiencies and greater power densities than conventional forward regulators in communications, automotive, distributed, and industrial power systems, as well as multi-output power supplies.

The active clamp enables operation at 50% duty cycles and higher primary-to-secondary turns ratios in the power transformer. It executes over a wide input voltage range of 13 to 100 V. Also integrated are a 100-V startup bias regulator and a user-programmable oscillator operating at 1 MHz. Couple that with total propagation delays of less than 100 ns, and the clamp is qualified for a vast array of 48-V communication isolated power-supply applications.

With the active-clamp output, the LM5025 can be configured to provide overlap time (for p-channel switch applications) or dead-off time (for n-channel switch applications). NSC's 3-A peak compound gate-driver technology suits the LM5025 for direct drive of a high-power MOSFET, reducing board space and leading to increased power density compared to alternative PWM controllers that require a gate-driver chip.

The IC comes in a thermally enhanced 5- by 5-mm LLP-16 chip-scale package or a TSSOP-16 package. It costs $1.30 each in 1000-unit quantities.

National Semiconductor Corp.
(408) 721-5000

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