Two new 20V HEXFET power MOSFET families from International Rectifier are designed to increase efficiency by up to 4% in 12V-input DC/DC convertors

Two new 20V HEXFET power mosfet families from International Rectifier are designed to increase efficiency by up to 4% in 12V-input DC/DC convertors. The new IRF3711 and IRF3704 series devices are designed for multiphase buck convertors that power next-generation, GHz-class microprocessors such as the Intel PentiumT 4 and AMD Athlon in high-end desktop computers and servers.

This article was originally published on Electronicstalk on 15 March 2001 at 8.00am (UK)
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The 20V devices are also suitable for 12V-input point-of-load (POL) buck converters for distributed power architectures in telecom and datacom applications.

Advanced multiphase buck converters typically require two sets of mosfets per phase, the synchronous FET and the control FET.

Parameters beyond RDS(on) such as gate charge (Qg) and output capacitance (Coss) are optimised for each device for each application.

The 20V IRF3711 series is specifically designed for best performance in the synchronous FET application, while the 20V IRF3704 series is designed for best performance in the control FET application.

The 20V maximum rating of IR's devices provides a sufficient safety margin for 12V-input synchronous buck converters and improves performance at the same time by minimising conduction, switching, driver and output losses compared with the 30V mosfets traditionally used in this application.
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In desktops and servers, the raw AC mains input is converted to a 12V DC output using a conventional AC/DC convertor.

This 12V DC-regulated input is then converted down to the voltage required by the CPU using a synchronous buck converter.

Synchronous buck converters demand high-efficiency power components in order to provide ultra-clean DC for today's power-hungry GHz-class microprocessors.

For example, when the 30V mosfets in a manufacturer's two-phase buck converter for a Pentium 4 system are replaced with IR's new 20V synchronous FET (IRFR3711) and the new 20V control FET (IRFR3704), test results show a 4% efficiency increase.

An efficiency increase of 4% at 20A is significant.

As circuit board space is at a premium, power management systems must provide increased power in the same or smaller footprint.

The improved circuit using IR's application-tuned mosfets delivered 78% overall efficiency at 1.7V and 20A per phase, operating at 220kHz.

When frequency is increased to 410kHz in the same two-stage convertor, IR's mosfets deliver 74% efficiency at full load, while the competitions' devices go into thermal runaway.

Higher performance processors with additional functionality may require faster current transient response from the DC/DC convertor as the CPU ramps up from idle to full power.

This can be achieved by increasing the operating frequency of the converter.

IR's new 20V mosfets will enable this frequency increase while the buck converter continues to operate in the tight thermal envelope required by manufacturers.

The new 20V mosfets come in a variety of surface mount and through-hole packages.