Improvements in silicon processes have steadily produced more-efficient power MOSFETs. These MOSFETs have propelled dc-dc converters, such as the synchronous buck converters used to power microprocessors, to higher levels of power density. But as MOSFETs have gotten better, the package's electrical characteristics—in particular, surface-mount styles—have become much more significant.
As a result, over half of the RDS(ON) of the best available MOSFETs is now attributed to the package rather than the silicon. And because existing MOSFET packages introduce significant thermal resistance as well, they also limit the power dissipation of MOSFETs, because of their inability to transfer heat efficiently away from the chip.
International Rectifier's DirectFET is a surface-mount package that improves MOSFET performance by lowering both the package's electrical and thermal resistance. It does so with a design that permits direct attachment of the die to the customer's pc board via solderable pads on the chip and through attachment to a copper drain clip that allows double-sided cooling (Fig. 1). The latter feature is an industry first for a surface-mount power package, according to Carl Blake, an IR marketing manager. He notes that DirectFET differs from previous package innovations, which produced relatively minor improvements in performance. Says Blake, "This is the first power package developed from the ground up."
DirectFET is being introduced in an SO-8-sized footprint, which naturally warrants its comparison with that industry-standard package. Because of its improved electrical and thermal design, IR's new package can carry twice the current of a standard SO-8. The DirectFET boasts a die-free package resistance that is 86% less than the SO-8. In terms of RDS(ON), one of the first DirectFET packaged devices—the IRF6603—measures just 3.8 mΩ at VGS = 10 V, a value that is 45% less than IR's best SO-8 MOSFET, the IRF7822.
At the same time, DirectFET's thermal resistance is much less, with values of 3°C/W junction-to-case on top and 1°C/W junction-to-pc board. For the SO-8, these same parameters would be 18°C/W and 20°C/W, respectively (Fig. 2). So given the same power levels, a DirectFET-packaged MOSFET operating under full load should run more than 35°C cooler than an SO-8.
Although the bottomless SO-8 developed by Fairchild also achieves a 1°C/W thermal resistance from junction to pc board, its thermal resistance from junction-to-case on top is similar to a standard SO-8. Consequently, it transfers all of its heat to the pc board, which can complicate board design. Double-sided cooling also means better thermal performance for the DirectFET when compared to other existing surface-mount packages (Fig. 3).
To achieve these performance levels, DirectFET trades plastic packaging and wire bonds for solderable die pads and a copper clip, while eliminating overmolding in favor of passivation (Fig. 1, again). Gate and source connections are made directly to the pc board (without solder balls) via wide-area pads on the die. These pads provide the good thermal interface needed to achieve the 1°C/W noted previously, while also reducing electrical resistance.
At the same time, the drain connection is made through a copper clip that simultaneously provides low electrical resistance to the board and low thermal resistance from the top side of the chip to the case. That in turn permits greater heat dissipation up top with the option for heatsinking and forced-air cooling.
Eliminating wirebonds not only lowers the RDS(ON) associated with packaging, it also permits the package to house a 30% larger die than an SO-8. Another benefit of the DirectFET construction is its low package height. It stands 0.7 mm tall, versus 1.75 mm for the SO-8.
The primary innovation in DirectFET is a proprietary passivation scheme that serves two purposes. The passivation acts as a solder mask when the device is mounted to the pc board, preventing the gate and source from shorting. It also seals the die against potential sources of contamination—humidity, solder flux, board cleaners, and others—while leaving the die pads exposed for soldering.
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