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Chip set for isolated and nonisolated dc-dc converter applications

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INTERNATIONAL Rectifier has introduced a new chip set solution for wide-range, universal telecommunications input (36V to 75V) and 48V fixed-input systems.

The chip set is designed for isolated or nonisolated dc-dc converter applications. It enhances system level power supply performance such as bulk 48V conversion for system boards up to 220W or for powering the radio amplifier in base station systems.

It consists of DirectFET MOSFETs and the IR2086S controller IC. The controller IC simplifies isolated dc-dc converter circuits using primary side full-bridge topologies. The IC is optimised for bus converter applications, using a self-oscillating 50% fixed duty cycle.

It has an integrated soft-start capacitor that gradually increases duty cycle to 50% over 2000 cycles to limit in-rush current during start up and maintains equal pulse widths for the high and low-side MOSFETs in the full bridge throughout the start-up sequence.

Other features include hiccup current limit protection, ±1.2A gate drive current, a programmable switching frequency up to 500kHz, and adjustable deadtime from 50ns to 200ns to protect against shoot-through current.

To complete the chip set a designer can choose from four DirectFET MOSFETs, the 100V IRF6644 and IRF6655 or 40V IRF6613 and IRF6614. The new MOSFETs use IR trench technology with DirectFET packaging technology, enabling reduction of specifications such as on-resistance and gate charge.

A typical telecom base station power amplifier stage requires a 28V regulated output from a wide-range (36V to 75V) input. International Rectifier’s DirectFET MOSFET and controller IC chip set can be used in a two-stage process to create a converter that is 92.5% efficient at full load, with 28V out at 250W. The first stage uses the IRF6655 and IRF6644 in a nonisolated synchronous buck configuration to produce a regulated intermediate voltage. This is followed by a simple full bridge fixed-ratio bus converter using the IR2086S and IRF6614 on the primary side and the IRF6644 on the secondary side for self-driven synchronous rectification.

This solution eliminates complex secondary side control schemes that are used in single stage regulated solutions. The 50% nature of the bus converter also allows the use of 100V MOSFETs on the secondary side, rather than 150V or 200V MOSFETs that can reduce efficiency by up to 2%.

The 100V IRF6644 can also be used to achieve up to 95.7% efficiency at a full load of 220W with 8V out at 27.5A in standalone fixed 48V input bus converter applications. These are suitable for most networking and high-end computing systems.

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