Power Electronics

P-Channel MOSFET Optimized for Synchronous Buck Converter

With every new generation of handheld electronics such as cell phones, digital cameras, and PDAs, manufacturers are striving to deliver additional features while reducing the size of the portable products. That means the latest portable electronic systems are demanding improved thermal performance and prolonged battery life. One area that continues to garner interest in these applications is power conversion. Therefore, designers continue to investigate architectures, components, layout, and packaging to squeeze every bit they can from dc-dc converters used in such products. Each percentage point improvement in efficiency translates into longer battery life and/or better thermal management, which results in greater long-term reliability for the product.

Since power MOSFETs are key components in dc-dc converters in terms of losses, they offer the best opportunity to improve converter efficiency. According to Vishay Siliconix, some 60% to 70% of the total losses are in the MOSFET for a 60W power converter. Thus, more efficient power MOSFETs are needed that offer both reduced conduction and switching losses at higher frequencies. Faster turn-on and turn-off characteristics are needed as well. Obviously, improved packages are necessary to support new performance requirements.

To address these issues for low-current applications, Siliconix, a subsidiary of Vishay Intertechnology, Malvern, Pa., has developed a p-channel MOSFET employing its advanced stripe TrenchFET process technology (see the figure). Advancements in silicon technology have likewise reduced on-resistance for p-channel MOSFETs to levels comparable with n-channel devices, according to Serge Jaunay, vice president of market development at Vishay Siliconix. To ensure efficient operation at high frequencies, this p-channel MOSFET, labeled Si3867DV, has been optimized for low switching losses. It also offers a low on-resistance of 0.051Ω at room temperature with a very low gate charge. Typical gate charge for this p-channel transistor is 7 nC. As a result, the Si3867DV delivers an on-resistance times gate charge figure of merit (FOM) of 0.36Ω.nC. At this rate, the Si3867DV is 22% better than the comparable p-channel MOSFET on the market today.

In addition to low FOM, Siliconix's p-channel MOSFET has been optimized with a low Qgd/Qgs ratio of 0.7 to eliminate secondary turn-on effects. According to Siliconix, the Si3867DV is the industry's first p-channel power MOSFET optimized for high-side use in synchronous buck converters. Since the p-channel MOSFET can be turned on with a gate drive that is lower than the battery voltage, it eliminates the need for extra bootstrap circuitry, simplifying design and lowering costs.

Although the first member of this PWM optimized p-channel MOSFET family is a 20V device, future entries in this series will include 12V and 30V breakdown voltages, and an optional on-board Schottky diode in a range of LittleFoot and PowerPak packages. Single and dual versions are planned in these thermally enhanced packages. Also, 40V p-channel parts are in the works, and are expected to be released in the fourth quarter. In comparison, the LittleFoot is a small-outline leaded package, whereas PowerPAK is a leadless thermally enhanced version. In PowerPAK, the bottom of the die is attached to a lead frame, which is exposed to provide a direct, low-resistance thermal path to the substrate on which the device is mounted. In other words, the main thermal path is no longer through the leads but through a large area of copper pad.

As shown in the Si3867DV datasheet, the continuous drain current in the steady state for the 20V transistor is 3.9A, which degrades to 2.8A at 85°C ambient temperature. The 20V Si3867DV comes in miniature surface-mount LittleFoot TSOP-6 and is in production. In 100,000-piece quantities, it's priced at $0.29.

For more information, visit www.vishay.com or CIRCLE 349 on Reader Service Card

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