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Unformatted text preview: of –ve bias during turn-off is
ensured. IXAN0009 2.4
PROTECTION FOR UNDERVOLTAGE In Transfer Characteristics (I D vs. VGS ) of a
MOSFET, one can see that for values of VGS
below VGS(th), the drain current is negligible, but
in this vicinity, the device is in its linear (Ohmic)
region and concurrent application of large values
of V DS could cause considerable amount of
localized heating of the junction. In short, when
a MOSFET is being used as a switch, any
operation in its linear region could cause
overheating or device failure. Bringing the
MOSFET quickly into its saturation from its offstate is the Driver’s job. And, if Vcc is below the
minimum required value, linear operation can
ensue to the detriment of MOSFET. I hasten to
add, however, that most PWM ICs, controller
ICs and microcomputer ICs have this protection
feature built-in and, if sharing the same Vcc bus,
the Driver IC gets the benefit of this function
being implemented elsewhere in the subsystem.
Nevertheless, Driver ICs having this feature is
an added benefit. The need of UV protection
applies equally well to IGBTs.
2.5OVERLOAD/SHORT CIRCUIT ROTECTION
Any operation of MOSFET/IGBT outside the
Safe Operating Area (SOA) could cause
overheating and eventual device failure and
should be prevented by an electronic active
monitoring and corrective arrangement.
Load or current sensing could be done by either
a Hall Effect Sensor or by a Shunt resistor in
series with source/emitter terminal. The voltage
picked up, which is proportional to current, is low
pass filtered and then compared to a preset
limit. The comparator output could initiate turnoff of MOSFET/IGBT. A circuit to detect
overload/short circuit is shown in Fig. (12),
where the output FAULT will go low when it
All IXDD series of Drivers have an ENABLE pin,
which, when driven low, say, by the FAULT
output from this comparator, puts the final NChannel and P-channel MOSFETs of the IXDD
Driver in its TRISTATE mode. This not only
stops any output from the Driver, but also
provides an envi...
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This note was uploaded on 01/15/2014 for the course ECE 624 taught by Professor Staff during the Winter '08 term at Ohio State.
- Winter '08