Power MOSFET performance continues to improve

I-REC.SEP–International Rectifier–wy

Power MOSFET performance continues to improve

Lower on-resistance, more package types, and on-chip protection lead the
way

BY DAN KINZER and GENE SHERIDAN International Rectifier Corp. El
Segundo, CA

Since its introduction more than 15 years ago, the power MOSFET has
continued to improve in performance, reliability, and cost. It now
provides some fundamental advantages over bipolar devices that often make
it the choice in traditional power conversion, motor control, and power
management applications. Furthermore, a substantial part of the rapid
growth of MOSFET usage results from design-ins into entirely new
applications not suited for bipolar device use. One of the biggest
advantages of MOSFETs is their need for only very simple drive circuitry,
a result of the MOSFET being a voltage-controlled device with high input
impedance. MOSFETs also achieve nanosecond switching times at high current
levels. They also exhibit a high degree of ruggedness because of the
absence of the second breakdown failure mechanism present in bipolars.
Then, too, the MOSFET's gain stability and response time over a wide
temperature range are excellent. These features in particular lead to
rapid acceptance of MOSFETs, especially in high-frequency switching power
supply designs. Particularly below 200 V, the forward drop, and
consequently the conduction efficiency, of MOSFETs have proven better than
those of bipolars (see Fig. 1). In addition to their use in power
supplies, MOSFETs are finding increasing use in automotive systems, portable
and consumer electronics, and electronic lighting ballasts. And
evolutionary performance demands from these and other advanced
applications have themselves created even more demanding device
requirements. These demands include improved power efficiency, faster
switching, lower input power losses, self-protection, reduced size, and
lower cost. Probably the most notable of power MOSFET trends today is the
continued reduction in on-resistance R_DS(on) , as shown in Fig. 2.
R_DS(on) is the predominant measure of efficiency and performance for
a MOSFET because it determines the power loss for a given drain current.
The lower a MOSFET's R_DS(on) , the higher its current-handling
capability. All current flow is majority carrier, and resistance is
largely determined by the resistively and thickness of the lightly doped
drain region. Since these parameters also influence the device breakdown
voltage, optimizing the silicon for the voltage required is the best way
to reduce R_DS(on) . As more and more applications arise in 3 to 12-V
systems, MOSFET manufacturers are offering devices optimized for 30-, 20-,
and even 10-V ratings. Optimization is not limited only to low-voltage
devices, however. For example, International Rectifier offers intermediate
voltage levels MOSFETs such as 450-V devices, optimized for specific line
voltage and topology. Improved VLSI processing tools and techniques are
also helping to lower MOSFET R_DS(on) for a given chip size. These new
tools and techniques help to significantly reduce MOSFET feature sizes,
junction depths, and oxide thicknesses, which in turn, drives down the
R_DS(on) . Cell density itself, however, is not the best measure of
MOSFET performance. That's because every part of the device that conducts
current has resistive drops that may be significant. These parts include
package leads, source wires, top metallization, and the silicon substrate.
Other factors are the metal to silicon contact resistances, the source
diffusion resistance, and the resistance of the MOS inversion channel,
which actually becomes the largest single contribution of resistance drops
when the voltages are very low. MOSFET supplier success in reducing
on-resistance is reflected by specifications in the 2-milliohm range or so
for devices in today's emerging products. These include the International
Rectifier IRF1010 HEXFWT, and MOSFETs with similar characteristics from
companies like Motorola, Harris, and Siliconix. P-channel MOSFETs are
very attractive for applications like automotive, telecommunications,
computer, and office automation. This is because a p-channel device is
very convenient to use as a high side switch in applications where its
desirable to connect the load to ground. These transistors are driven with
a negative gate voltage, which eliminates the need to generate an
over-rail supply. To date, applications have been limited by the
inherently higher R_DS(on) of p-channel MOSFETs because of their lower
hole mobility in silicon. But as voltages get lower, parasitic resistances
unrelated to mobility become a larger fraction of a MOSFET's total
R_DS(on) . This reduces the difference between the two types and should
greatly increase p-channel use for power management and load switching in
the 10 to 30-V range. Both channel types are now starting to become more
and more available from several MOSFET sources. In some applications,
particularly high-frequency power conversion, the switching performance of
a MOSFET can be more critical than its R_DS(on) . Switching times of
MOSFET transistors are much faster than those of bipolar devices of
comparable size, primarily because they do not have minority carrier delay
times. The most straightforward benchmark for comparison of MOSFET
devices is total gate charge (Q_g ). This is the amount of charge
pumped in and out of the gate during each switching cycle. It depends on
the gate source and gate-drain capacitances, and usually determines the
switching speed of the MOSFET. Internal resistance and inductance of the
gate circuit do not significantly limit a MOSFET up to several megahertz
in frequency, as does the external impedance of commonly used drive
circuits. A good benchmark for MOSFET switching performance is the
R_DS(on) x Q_g product, which is independent of die size for a
given voltage rating but can vary among vendors. The switching performance
of a just-introduced International Rectifier family of 400 to 600-V
devices specifically optimized for low gate charge and high frequency is
compared to non-optimized earlier devices in Fig. 3. Some MOSFET
manufacturers–such as International Rectifier, Motorola, and Harris
Semiconductor–provide smart or intelligent power MOSFETs. These smart
devices contain on-chip active and passive elements to protect the MOSFET
and its associated circuitry. For example, International Rectifier's
recently introduced IRSF3010 50-V, 70-milliohm MOSFET includes on-chip
protection against overcurrent, short circuit,
overtemperature-overvoltage, and electrostatic discharge. Because smart
MOSFETs contain all their circuit protection features on the same chip,
rather than requiring separate protection components, pc-board space and
assembly time are considerably reduced. So, too, are inventory needs. A
wide variety of packaging styles is available for power MOSFETs (see Fig.
4). These include a variety of surface-mount packages like TO-220, DPAK,
SOT-223, and SOT-89. Where board space is limited and power dissipation is
limited to a few watts maximum, the small size of these packages has real
advantages. Users are finding ways to increase power in these devices such
as by using extra copper on the board or mounting the devices on insulated
metal substrates. The SO-8, available from International Rectifier and
Siliconix, is an innovative, small-footprint, eight-leaded surface-mount
package device capable of housing one or two die, either n- or p-channel
or both. This package has one of the best ratios of silicon area to board
area of any package. It has the added convenience of two independent
transistors designed to dissipate up to 2 W by heat conduction through the
drain leads of the package. As an example, the IRF7101 has two 20-V, 0.
1-ohm n-channel devices rated at 3.5 A. With the extremely low
R_DS(on) of the latest MOSFETs, a truly remarkable current capability
of 5 A or more is achievable. A pervasive trend in assembly technology
for higher MOSFET power levels is the use of isolated packages, such as
International Rectifier's FullPAK. This package is completely encapsulated
front and back with insulating thermally conductive epoxy and holds
similar die sizes to the popular TO-220. It has the benefit of eliminating
the need for insulating pads in the assembly process, to isolate the drain
from the heat sink.

CAPTIONS:

Fig. 1. The power MOSFET's forward voltage drop is superior to that of a
bipolar device.

Fig. 2. MOSFET manufacturer's have continued to reduce R_DS(on) and
increase cell density, as typified by these four generations of devices
from International Rectifier.

Fig. 3. Today's low gate charge (Q_g ) optimized MOSFETs provide
significantly faster switching time than previously available MOSFETs.

Fig. 4. Power MOSFETs are available in a wide assortment of package
types.

OVERLINE:

Power MOSFETs