Low-Voltage MOSFETs Aim to Build Better Buck Converters

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

The two MOSFETs — the TPCA8028-H and TPC88035-H — are intended for use in dc-dc converter applications such as notebook PCs where the ‘8028 can serve as a low-side FET and the ‘8035 as a high-side FET. The two devices achieve lower R_DSON than was possible with the previous-generation technology, as well as fast switching, enabled through lower gate charge and lower gate resistance (Table 1). The use of aluminum strap connections instead of conventional wire-bonds further reduces RDSON.

Samples of the TPCA8028-H and TPC8035-H are available now, priced at $0.65 and $0.60, respectively.

ON Semiconductor (www.onsemi.com) introduced 12 new power MOSFETs intended for use in CPU, graphics processor unit and system rail dc-dc conversion in computing applications. These 25-V and 30-V devices are single and dual n-channel MOSFETs offering low R_DSON, low gate charge and low gate-charge ratio.

Four of the new devices (Table 2) are offered in SOIC-8 packages for use in notebook computers. These are priced between $0.35 and $0.53 per unit in quantities of 2500. The other eight devices (Table 3) are offered in DPAKs for use in desktop computers and gaming boxes. Pricing for these devices is between $0.20 and $0.47 per unit in quantities of 2500.

Another 30-V MOSFET was introduced by Vishay (www.vishay.com). The Si7192DP is the first device in a third-generation TrenchFET power MOSFET family that delivers low on-resistance and a low figure of merit (on-resistance times gate charge). Offered in a PowerPAK SO-8, the n-channel device features a maximum on-resistance of 2.25 m at a 4.5-V gate drive. On-resistance times gate charge is 98, which the company claims is an industry record for a V_DS = 30 V, V_GS = 20 V device in an SO-8 footprint.

A 40-V MOSFET designed for use as a synchronous rectifier was recently introduced by Fairchild Semiconductor (www.fairchildsemi.com). Aimed at power supplies such as dc-dc bricks where efficiency and footprint are critical, the FDMS8460 is said to provide 20% lower figure of merit than competing 40-V MOSFETs. Built in the company's PowerTrench process, the FDMS8460 features an R_DSON of 2.2 m max at V_GS = 10 V and I_D = 25 A and a Q_GD of 10 nC typ. Now sampling, the 5-mm × 6-mm MLP-packaged device is priced at $0.85 each in quantities of 1000.

Meanwhile, Zetex Semiconductors (www.zetex.com) unveiled a leadless 20-V MOSFET, the ZXMN2F34MA, which provides a pc-board footprint 50% smaller than industry-standard SOT-23-packaged devices, along with low thermal resistance. The part's 2-mm × 2-mm DFN-322 package, with an off-board height of just 0.85 mm, is engineered for space-starved applications such as external switches in buck/boost POLs.

At typical gate-source voltages of 4.5 V and 2.5 V, the respective R_DSON values are only 60 mΩ and 120 mΩ. The low reverse-recovery charge reduces switching loss and EMI problems, critical for low-voltage, portable electronics requiring longer recharge intervals. Available now, the ZXMN2F34MA is priced at $0.10 in 10,000-piece quantities.

Another vendor to adopt the DFN for new MOSFETs was Alpha & Omega Semiconductor (www.aosmd.com). It introduced versions of its MOSFETs in a 3-mm × 3-mm DFN package including what the company claims is the first monolithically integrated Schottky diode + MOSFET (SRFET) in this package. These devices target space-constrained applications requiring high-efficiency dc-dc conversion.

The DFN3×3EP package's exposed copper lead-frame permits efficient heat transfer into the pc board. Although only requiring one-third the board area of an SO-8, the 3-mm × 3-mm DFN maintains the same 60°C/W thermal performance as SO-8 devices, according to the vendor.

The new devices include the AON740x family of fast-switching high-side MOSFETs and the low-side AON770x SRFET family (Table 4). The SRFET device's Schottky body diode features a low-forward voltage drop of 0.4 V. These devices are available now.

Click here for the enhanced PDF version of this article