Digital Isolators: A Space-Saving Alternative to Gate-Drive Transformers in DC-DC Converters

Focus:

Pulse transformers have long been the most commonly used method to isolate gate-drive signals in isolated dc-dc converters. These devices provide excellent isolation but have limitations when operating at the high duty cycles encountered when driving synchronous rectifier MOSFETs in half-bridge and full-bridge topologies. A new alternative to pulse transformers, fully integrated digital isolators employ RF coupling techniques to transmit digital information across an isolation barrier. In this article, the principles of operation and benefits (small size, no duty cycle limitations) of digital isolator technology will be discussed along with their application issues. A half-bridge dc-dc converter reference design using Silicon Labs' Si8450 digital isolator will be presented. The article begins with a discussion of the isolation requirements of a half-bridge power converter with control-driven synchronous rectifiers, and describes the operation of basic transformer-isolated gate drive and transformer-isolated gate drive with dc restore.

What you’ll learn:

• How to reduce design size and eliminate duty-cycle limitations when driving synchronous rectifier MOSFETs in half-bridge and full-bridge topologies
• How to replace gate-drive transformers (pulse transformers) with digital isolators in half-bridge and full-bridge power converters
• How to understand operation of digital isolators employing RF coupling techniques

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Author & Publication:

Bob Bell, National Semiconductor, Phoenix, Ariz. and Don Alfano, Silicon Labs, Austin, Texas, How2Power Today, Mar 30 2010

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