Low-Voltage MOSFETs Aim to Build Better Buck Converters

Apr 1, 2008 12:00 PM
By David Morrison, Editor in Chief

Power semiconductor suppliers continue to advance the state-of-the-art in power MOSFETs by enhancing their process technology and packaging to suit the needs of popular power-supply applications. Recently, there have been numerous MOSFET introductions aimed at low-voltage dc-dc converter designs (typically buck converters) like those found in notebook computers, desktop PCs, servers and telecom/networking equipment.

Seeking to improve power-supply efficiency in these applications, semiconductor manufacturers have developed parts in the latest generations of their proprietary process technologies, while also offering higher performance packaging.

The new low-voltage devices, which carry 20-V to 40-V ratings, feature reductions in values of R_DSON and gate charge. Frequently, the devices are optimized for best performance as either the control FET or the synchronous in a buck converter design. These improvements are being made to increase dc-dc converter efficiency, but also allow migration to higher switching frequencies to reduce the size of magnetics and passive components.

Recent Developments

Numerous low-voltage MOSFETs have been introduced in the past few months for use in point-of-load converters (POLs). For example, STMicroelectronics (www.st.com/pmos) announced two 30-V power MOSFETs — the STD60N3LH5 and STD85N3LH5 — intended for dc-dc converter applications in notebook, server, telecom and networking applications. These devices use the latest version of the company's STripFET technology (STripFET V) to deliver low conduction and switching losses and to achieve a low figure of merit. In addition, the MOSFETs allow operation at higher-than-normal switching frequencies.

With a gate charge (Q_G) of just 8.8 nC and an R_DSON of 7.2 m at 10 V, the STD60N3LH5 is suited for use as a control FET in buck converters. On the other hand, the STD85N3LH5 features an R_DSON of 4.2 m at 10 V and a Q_G of 14 nC, making it appropriate as a synchronous FET. Both devices are produced in DPAK and IPAK packages, but will soon be available in other package options including SO-8, PowerFLAT 3.3 × 3.3, PowerFLAT 6 × 5 and PolarPAK.

STripFET technology makes use of very high “equivalent cell density” and smaller cell features to achieve low R_DSON and losses, while using less silicon area. STripFET V is the latest generation of this technology, achieving approximately 35% improvement in the critical indicator of silicon resistance and active area, plus some 25% reduction in total gate charge per active area, compared to the earlier generation.

Both devices are in full production. In quantities of 2500, the STD60N3LH5 is priced at $0.65 each, while the STD85N3LH5 is priced at $0.95 each.

Another new offering is Toshiba America Electronic Components's (www.toshiba.com/taec/) family of 30-V MOSFETs based on UMOS VI-H, the sixth-generation trench process in the Toshiba fast switching series. This process enables a significant reduction in gate switch charge and on-state R_DSON.

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