MOSFET.NOV–Harris–wy
Intelligent MOSFETs–three-terminal devices with brains and brawn
Although their on-chip “smarts” are limited, intelligent MOSFETs can save
space and cut manufacturing costs in many applications
BY JACK E. WOJSLAWOWICZ Harris Semiconductor Products Div. Somerville,
NJ
Intelligent, or smart, discrete transistors are power transistors that
contain one or more on-chip passive or active control elements to protect
the transistor and associated circuitry. Types of protection provided by
these three-terminal devices include overcurrent, overvoltage, and
thermal. The added elements–resistors, diodes, and small
transistors–require very little chip area, and their inclusion adds
little to the manufacturing process. Intelligent discrete transistors do
away with the need for the circuit designer to add protective components
outboard of the power transistor. This not only reduces assembly time, but
saves valuable board space as well. Both of these factors help keep system
costs down. And the cost of an intelligent discrete, itself, is only
slightly more than that of a comparable discrete without any
intelligence–typically only a few cents at most. (Author: is this number
about right?) Equally important, an intelligent discrete often fits
applications when full smart power devices are rejected because they are
too costly. (Author: Please specify which applications.) The expression
“smart power” has been around for quite some time, and many semiconductor
technologies include so-called “smart devices.” But the term “intelligent”
or “smart” discrete has, in the past, been reserved for the three-terminal
vertical conduction power MOSFET with simple on-chip control elements.
However, insulated-gate bipolar transistors (IGBTs) and lateral conduction
power MOSFETs are now available with simple on-chip control elements.
These can properly be referred to as intelligent or smart discretes.
Intelligent discrete circuits Several types of circuits are being
fabricated in intelligent discrete technology. A list of many of the
manufacturers of these devices is included at the end of this article.
Current-limiting MOSFET. The intelligent discrete power MOSFET (see Fig.
1) includes a bipolar transistor and two resistors as on-chip control
elements to self-limit drain current. Limiting is achieved by monitoring
the voltage across the resistor in series with the MOSFET's source. As
current increases, the voltage bias on the base of the bipolar transistor
increases to the point at which the transistor turns on. This causes the
collector of the bipolar control transistor to drop, which in turn pulls
the MOSFET's gate voltage down to reduce (or cut off) drain current. All
this occurs while the circuit maintains proper bias on the discrete gate.
Note from Fig. 2 that the device cannot operate beyond the design current
limit even when additional gate bias is applied. Drain current is limited
to approximately 2.5 A typ at 25 degreesC junction temperature. Another
feature that makes this device self-protecting is the over-temperature
protection provided by its control elements. As the device becomes
hotter–the result of heat from an external source or, more likely, from
internal heating caused by an abnormal load condition–the control
elements operate to limit the rise in current. The sensing resistor in the
base circuit of the control bipolar transistor has a positive temperature
coefficient, while the transistors Vbe resistance has a negative
temperature coefficient. The combined effect is to self-limit large
current increases as junction temperature rises. As shown in Fig. 3, the
normalized drain current limit as a function of temperature decreases 50%
for this 1-A-rated MOSFET as the junction temperature reaches its rated
value of 150 degreesC. The device illustrated in Fig. 1 was the first
intelligent MOSFET developed by Harris Semiconductor and was intended
primarily for automotive applications. The automotive environment, far
from being a 12-V battery system, may have the worst transient environment
of any electronic system, what with inductive kickbacks, stalled motors,
load dump transient, and the like. The built-in protection provided by
these intelligent devices was well suited. An overstress condition not
protected against, however, was electrostatic discharge, encountered
during manufacturing and often during servicing. The solution is a zener
diode added to the MOSFET chip to provide the necessary protection (see
Fig. 4).
Non-intrusive sensing. A more recent intelligent MOSFET provides a higher
level of current limiting without the need for sensing the high current
level directly. In this device, the vertical power MOSFET structure is
designed to monitor drain current in a ratiometric manner. By bonding off
some of the individual cells, an Nil ratio can be established to monitor
principle current flow (see Fig. 5). The output from this connection is
fed into the base of the control bipolar transistor to provide the
necessary protection. This type of sensing results in a
lower-RDS(on) , higher-current device.
Built-in voltage clamping. An on-chip voltage-changing circuit is another
feature of some intelligent power MOSFETs. On the MOSFET die a string of
zener diodes with electrical connections from the drain to the gate (see
Fig. 6). If the voltage on the drain tries to go beyond the zener value,
the power MOSFET section is gated on, thereby dissipating the load energy
in the active mode of operation.
Intelligent IGBTs, too. Intelligent IGBTs have also joined the ranks of
intelligent, or smart discretes. Available from both Harris and Motorola,
these discrete devices, much like the intelligent power MOSFETs,
incorporate control elements on-chip to provide overvoltage protection.
CAPTIONS:
Fig. 1. Current limiting in the intelligent discrete power MOSFET is
provided by an on-chip circuit consisting of a bipolar transistor and two
resistors.
Fig. 2. The drain current of the intelligent MOSFET remains essentially
constant even when gate bias increases.
Fig. 3. Intelligent MOSFET drain current is limited by junction
temperature.
Fig. 4. An on-chip zener diode provides electrostatic discharge
protection for discrete power MOSFETs.
Fig. 5. Bonding off some cells in the MOSFET's structure allows the
principal current flow to be monitored without having to monitor the
device's full drain flow.
Fig. 6. Gate-to-drain voltage clamping is achieved with a string of
on-chip zener diodes in some intelligent MOSFETs
Fig. 7. IGBTs have joined the ranks of intelligent power transistors,
providing on-chip clamping, electrostatic discharge protection, and more.
Here is a list of many of the manufacturers of intelligent MOSFETs.
Harris; Motorola; SGS; Siemens; IR; IXTS; Philips, and one more.
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