MOTO.SEP–Motorola–wy
Bipolar power transistors still dominate in many areas
Applications continue to expand despite strong pressure from power
MOSFETs
BY DANIEL ARTUSI and RANDY FRANK Motorola Semiconductor Products
Sector, Phoenix, AZ
Contrary to popular belief, the bipolar transistor is alive and well and
is still the choice for many applications. This is true even though the
use of power MOSFETs has continued to grow at a fast pace over the past
decade. It's true partly because much of the growth of MOSFETs has
resulted from new applications not previously addressed by bipolar or any
other semiconductor technology. But it's also true because bipolar
transistors are simply better suited from a performance and/or cost
standpoint in certain applications.
High-voltage applications are one example. At high voltages, the cost
per ampere for a bipolar device is much lower than for competing
transistor technologies. This can be very important in very cost-sensitive
applications, such as in high-voltage switching power supplies for use in
consumer and small office-automation equipment (like fax machines and
copiers) and electronic lamp ballasts. Audio amplifiers are another
example. Bipolar transistors are still the transistor of choice for most
commercially available amplifiers. In fact, a bipolar transistor is a good
choice for any application that requires operation over a linear range.
In the past, designers have had concerns about consistency in the
parametrics of bipolar transistors. However, because of much improved
fabrication techniques, today's devices exhibit tighter specifications and
device-to-device consistency. This is particularly true in the areas of
gain and switching characteristics. For example, the gain of Motorola's
MJE18004 bipolar power transistor is between 14 and 18 with a collector
current of 1 A. Figure 1 shows the consistency of inductive switching with
collector current ranging from 0.5 to 3 A and at both room temperature and
125 degreesC. These much flatter gain curves have tighter parametric
distribution than previous bipolar transistor generations and are fully
characterized at 125 degreesC. These characteristics make the circuit
designer's task much easier than in the past.
Low-voltage bipolar There are still many applications for bipolar
transistors even at the low-voltage end of the power transistor
application spectrum. In these applications, the base drive circuitry can
be simple because a current source is easy to implement, and, in general,
the applications are for low-voltage devices. One of the low-voltage
areas where bipolar power is still very popular is in automotive
applications. A common test condition imposed by automobile manufacturers
is related to jump-start situations where a double battery voltage could
inadvertently be applied with the battery terminals reversed. To survive
reverse-battery tests, a power FET with its intrinsic diode requires an
additional series diode to avoid dissipating excessive power. Furthermore,
if the same voltage drop is then to be maintained, the size of the FET must
be increased to offset the voltage drop across the diode. The problem is
even worse for power MOSFETs used in H-bridge or totem pole
configurations. A reverse battery means a dead short across a several
hundred ampere current source. The bipolar transistor, on the other hand,
has the inherent capability to block the reverse voltage; but in the past,
the limiting voltage rating was the same as the reverse base-emitter
voltage (VBE ). Now, however, a new bipolar process has made possible
a low-voltage Darlington with a Vebo of 26 V or more, high enough to
allow the device to remain off even if a double-reverse battery voltage is
applied. An example of a design that incorporates these changes is a
custom npn Darlington with a BVceo greater than or equal to 80 V, a
minimum signal gain of 2,000 at 15 A, and the ability to withstand 26 V or
more across the emitter-base junction. This bipolar approach is simpler
and more cost-effective than the MOSFET approach. The bipolar device can
be used as a single load-control driver or as an element in a totem pole
where it can be used with a power MOSFET and provide protection for a
reverse-battery condition. Another application for low-voltage power
bipolar devices is in high-side switching. A pnp transistor is easier to
drive as a high-side switch than an n-channel MOSFET because the MOSFET
must have the gate voltage enhanced either 5 V (logic level) or 10 V above
the source. With a grounded load, a charge pump must be used to provide this
drive voltage, which makes the MOSFET design more complicated and more
expensive. Alternatively, a p-channel MOSFET might be used to solve the
drive problem, but this would result in a still higher cost than the
bipolar approach. The combination of pnp transistors as the high-side
elements in an H-bridge and the newer high-efficiency power MOSFETs like
Motorola's MTB75N05E (50-V, 9.5-milliohm) devices as the lower elements
can provide an extremely efficient motor control with easy to drive top
and bottom drivers, along with reverse direction control and inherently
protecting against reverse polarity.
Smart bipolar power transistors Bipolar power transistors (like power
MOSFETs) are now available with integral voltage clamps to provide on-chip
protection. For example, the circuitry of Motorola's Z-switch transistor
is shown in Fig. 2. A zener diode is the control element for the integral
24-V clamp. The internal design features include a temperature
compensation circuit to minimize variation in the voltage clamp over its
operating temperature range. The internal zener bias is set to a current
level well above the zener's knee point for typical applications like
automotive switching. This provides the device with noise immunity at any
temperature and eliminates undesirable oscillations during turn-on or
turn-off. To keep the clamp voltage within the specified values over a
-40 degrees to +150 degreesC temperature range, the chip includes a
proprietary compensation circuit. This circuit improves the thermal
coefficient from the normal 100 mV/ degrees to 6 mV/ degreesC, making
device operation very stable in virtually any application.
An assortment of packages Packaging is a particular bright spot in the
bipolar arena. A wide assortment of well-known plastic and metal packages
including surface-mount types are available. Older plastic packages that
continue to be available in high volume are the TO-220, TO-218, and the
TO-247. The fully isolated TO-220 is very popular because of its simple
requirements for mounting hardware. The recent consolidation of TO-218
packages, which have both a rounded and square tab to a single square tab
design, has standardized this package for worldwide use. Also, the
transition from TO-218 package to the newer TO-247 package for some newer
products has been completed to take advantage of the washerless
capabilities of this package. The venerable TO-204 (formerly the TO-3)
metal package, has been discontinued by many suppliers, with only a few
suppliers remaining. Specifications for a variety of TO-style packages are
given in the accompanying table. And as new packages are developed, their
applicability to bipolar power devices is being continuously evaluated. A
wide variety of surface-mount packages are now available for bipolar power
transistors. Surface-mount DPAK and D2PAK (see Fig. 3) packages for
bipolar transistors are allowing users to make the transition from
through-hole to surface-mount assembly without having to do off-line assembly for the power transistors. Bipolar products offered in these packages
are npn and pnp types, including Darlington, and range from 45 to 400 V
with current ratings from 0.5 to 10 A. Motorola has recently become the
first non-Japanese supplier to provide products in the TO-3PBL–a package
larger than the TO-247 (see Fig. 4). Complementary bipolar power
transistors MJL21193 and MJL21194 are the first products to be introduced
in this package. These are 16-A, 250-V pnp and npn devices, respectively,
and are designed for use in high-power audio amplifiers, disk head
positioners, and other linear applications. The TO3-PBL is a plastic
package based on leadframe assembly techniques like the TO-220 and TO-247
plastic power packages. it extends the power capabilities of this type of
discrete package to provide a cost-effective solution for large die
without requiring more costly power module packaging. Motorola has applied
for JEDEC registration for the TO-3PBL, and the designation TO-264 has
been assigned to the package.
CAPTIONS:
Fig. 1. Inductive switching time remains essentially consistent over wide
variations in temperature and collector current.
Fig. 2. This bipolar transistor's integral zener diode provides on-chip
protection against overvoltage.
Fig. 3. DPAK and D2PAK are popular surface-mount packages for
bipolar power transistors.
Fig. 4. The new TO-3PBL plastic package is considerably larger than both
the TO-247 and TO-220 plastic power packages.
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