Electronic Design

Controller Operation

The IR2085S controller was optimized for the bus converter application as it is applied in distributed power architectures that use an intermediate voltage bus. This International Rectifier device combines a pulse-width-modulation (PWM) controller and a half-bridge driver (see the figure).

Each of these functions typically requires a separate eight-pin SOIC, but the IR2085S integrates the same functionality into just one of these packages. At the same time, the chip omits certain protection features that are normally external to the isolated converter in networking-related distributed power applications. Yet it includes enable and current-limiting functions.

The controller provides a gate-drive signal to a primary-side half-bridge driver, which has 1 A of gate-drive capability. The gate-drive circuit is optimized for the company's primary-side low-charge MOSFET, the IRF7493. This MOSFET specifies a maximum on-resistance of 15 mΩ at VGS = 10 V and a typical gate charge of 31 nC. The controller chip drives the primary-side MOSFETs directly, requiring no additional drivers or buffers.

An on-chip bias circuit generates the controller's 10- to 15-V supply. A soft-start feature gradually raises duty cycle from zero to 50% to limit inrush current at startup. The inrush current typically lasts for a period of 5 ms.

International Rectifier
www.irf.com

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