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Implementing synchronous rectifiers with digital isolators

Posted: 17 Jun 2010     Print Version  Bookmark and Share

Keywords:digital isolators  synchronous rectifiers  controlling synchronous MOSFETs 

Many of today's switching power supplies implement synchronous rectifiers to improve efficiency. In isolated topologies, the power-supply controller typically is located on the primary side of the isolation barrier, while the synchronous rectifiers are located on the secondary side. The control signals must bridge the isolation boundary with minimal delay.

Gate drive transformers are the most popular method of accomplishing this. While less common, optocouplers are sometimes used. Some topologies lend themselves to implementing self-driven synchronous rectifiers, where the gate drive signals are derived directly from the power transformer. The emergence of capacitive-coupled digital isolators presents a new solid-state solution to driving synchronous rectifiers.

Digital isolators provide a solid state method of controlling synchronous MOSFETs in isolated switching power supplies. Additionally, the digital isolator circuitry consumes less than 100mW, minimising the impact on residual power loss. Although the example shown here is a simple forward converter, digital isolators have much potential in topologies where more complicated logic is required for synchronous rectifiers, like full-bridge and interleaved topologies.

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