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NSC dc/dc chipset improves 48V to 1V conversion

Posted: 03 Feb 2003 ?? ?Print Version ?Bookmark and Share

Keywords:national semiconductor? lm5041? lm5101? dc/dc converter chipset? dc controller chipset?

National Semiconductor Corp.'s LM5041/LM5101 controller/driver chipset takes a giant step toward achieving very high efficiency in 48V to sub-1V dc/dc power conversion.

The devices borrow on traditional current-fed topologies used in high-reliability, high-efficiency dc/dc converters for military applications, applying the same principles to very low-voltage and high-current, multi-output designs. A typical 50A, 2.5V buck-fed push-pull isolated power supply using the new chipset will deliver an overall efficiency in excess of 90 percent, said Don Ashley, strategic-marketing manager for power management.

"The cascaded buck-fed topology is optimized for low-voltage outputs using a cost-effective, transformer-isolated power conversion technique," said Ashley. "Existing controllers cannot efficiently step down a 48V input to a sub-1V output because the minimum on-time becomes too narrow." The solution, he noted, "is based on a controller-driver chipset tailored to the application."

The LM5041 is a current-mode pulse-width modulation controller that has a synchronous buck controller, push-pull controller with user-programmable dead time and integrated 2A drivers. The controller is applied in cascade fashion with the LM5101. It ties to the input bus directly with high-voltage start circuitry. The LM5041 also has a user-programmable, 1MHz oscillator with external clock synchronization. The second chip, the LM5101, is a 100V-rated half-bridge power MOSFET driver that drives the synchronous buck MOSFETs.

Use of the current-fed topology allows the designer to tolerate overlap (no dead time) in the push-pull switching stage, a phenomenon that results in cross-conduction and fatal transformer saturation in voltage-fed topologies. Furthermore, the absence of dead time, coupled with the fact that the buck filter inductance is reflected through the transformer, allows the use of minimal output filter inductance.

The filter capacitor at the output of the typical voltage-fed buck stage is removed, establishing the buck stage as a current source feeding the push-pull section of the converter. As a result of the current-fed operation, the switches in the push-pull stage need to be rated for only 40V or so, not the 100V usually required. In turn, the FETs' on-resistance is substantially lower for a lower-voltage-rated device, as will be conduction losses.

Meanwhile, the push-pull outputs of the LM5041 operate continuously at slightly over 50 percent duty cycle. This overlap ensures that the MOSFETs in the push-pull stage will be on at the same time for some fraction of the switching cycle; the overlap time is about 20ns to 40ns.

The output current from the push-pull stage will be a dc waveform without underlap, giving rise to a regulated output, which requires minimal energy storage and no output inductor. Current-fed switching also ensures that the synchronous rectifiers at the output are self-driven (no external driver required), with optimal current form factor improving efficiency up to 5 percent.

The favorable current waveform mitigates the effect of intrasecondary leakage inductance, improving cross-regulation in converters that have two or more outputs with appreciable power.

The LM5041, packaged in an LLP-16, is priced at $2.55 and the LM5101, in an LLP-10, at $1.75, both in 10,000-piece quantities. The chips will be available for sampling in April, with volume production in June and September.

- Vincent Biancomano

EE Times

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