Traditional MOSFETs have had a reputation for unexplicable
failure in use. These failures are often attributed to such things as over
voltage, over current, over temperature etc... although after the event it
is often difficult to pinpoint the true reason. Careful design of the
MOSFET circuit can help enormously but if there is scope for the load
conditions to change (eg driving a different motor) then there is also
scope to take the MOSFET near to or exceeding its design specifications. This
has lead to the alternative meaning of the MOSFET acronym ..
Obliterated, Smoke and Fire Emitting Transistor'. What was needed was a MOSFET that included some of the device protection circuitry normally
present on the circuit board to protect it. This need was met by the
"Intelligent MOSFET" sometimes referred to as the "Omnifet" (although this
is actually a manufacturers product name).
A block diagram of such a device is shown below. It can be seen that the
device still has the outward appearance of a standard 3-terminal MOSFET
even down to the traditional T0-220 package (as illustrated).
Inside the device, however, there are a number of integrated protection
facilities that serve to protect the device from damage. These are.....
During normal operation, the Input pin is electrically connected to the
gate of the internal power MOSFET. The device then behaves like a standard
power MOSFET. Overtemperature is based on sensing the chip temperature and
is not dependent on the input voltage. The location of the sensing element
on the chip in the power stage area ensures fast, accurate detection of
temperature. Overtemperature cutout occurs, typically, at a minimum of
150oC. When detected the signal from the input pin is disconnected from
the internal MOSFET gate and effectively turns it off. The device is then
automatically restarted when the chip temperature falls below 135oC.
Overvoltage Clamp Protection
This can be important when driving inductive loads such as motors and
solenoids. As the MOSFET is turned off, voltage spikes can be generated
that can cause damage. Under these conditions an internal voltage clamp
operates protecting the MOSFET.
Linear Current Limiter.
If the current through Drain - Source exceeds the device specifications it
is automatically reduced and limited. This operation takes the device into
its linear operating region and will inevitably cause an increase in power
dissipation within the device. However, even if this extra power
dissipation causes the device to overheat the over temperature protection
will then come into play, as described above, and protect the device.
Short Circuit Protection
Connecting different loads to any MOSFET circuit can easily lead to short
circuits. In this situation the device is protected again by the
overtemperature facility disconnecting the signal to the internal gate and
turning it off until the short circuit has been rectified.
Although you can never say that a MOSFET is 100% protected from abnormal
conditions, the above protection facilities address and minimise the risks
from the most common sources of damage and make such devices extremely
reliable in use. The intelligent MOSFETS are usually more expensive than
their ordinary equivalents but this cost can easily be offset against the
increased reliability (and consequent life expectancy) of any application