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Unformatted text preview: igh Temperature cut-off, a bridge circuit is used
with the CBI module’s thermistor. Comparator U3
compares voltage drop across the thermistor to the
stable voltage from the Zener diode. P1 can be used
to preset the cut-off point at which the comparator’s
output goes low. This is fed into the DSP as
Short circuit protection is provided by continuously
monitoring the voltage drop across a SHUNT. Note
that one end of SHUNT is connected to the power
supply ground GND1. The voltage picked up from
this SHUNT is amplified by a low noise Op Amp and
is then compared to the stable voltage from the
same Zener. When short circuit occurs, the
comparator output (FAULT) goes low. 1% metal film
resistors are used throughout in both these circuits
to ensure precision and stability. C3 and C4 help in
offering low pass filtering to avoid nuisance tripping.
Principle of DESAT sensing for detecting overload
on an IGBT has been explained before in section 2.5
above. In the case of AC Motor Drive, each IGBT
has to be protected from overload using separate
DESAT sensing. Fig. (12) and Fig. (13) show the
connection for each IGBT. DESAT sensing is done
on the isolated side of each opto-coupler, while the
resultant FAULT signal is generated on the common
input side with respect to GND1. Each FAULT signal
is open collector type and hence can be tied IXAN0009 together with other FAULT signals from other
opto-coupler or from other comparators. The
DSP will stop output drive signals when either
FAULT or OVERTEMP signal goes low. When
this happens, notice that IXD_414 offers a -ve
bias of -5V to guarantee turn-off conditions,
even in presence of electrical noise. -5V is
applied to gate of each IGBT during turn-off
even under normal operating conditions. After
fault is cleared, the DSP can issue a RESET
signal for resuming normal operation.
5.0 PRACTICAL CONSIDERATIONS
When designing and building driver circuits for
MOSFET/IGBT, several practical aspects have
to be taken care of to avoid unpleasant voltage
spikes, oscillation or ringing and false turn-on.
More often than not, these are a result of
improper or inade...
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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