Low-Voltage MOSFETs Aim to Build Better Buck Converters
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 RDSON 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 (QG) of just 8.8 nC and an RDSON 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 RDSON of 4.2 m at 10 V and a QG 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 RDSON 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 RDSON.
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 RDSON 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 RDSON, 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 VDS = 30 V, VGS = 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 RDSON of 2.2 m max at VGS = 10 V and ID = 25 A and a QGD 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 RDSON 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.