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Unformatted text preview: to implement overload/short circuit
protection for the upper MOSFET/IGBT.
As can be seen in Fig. (15), gate drive transformer’s
primary is fed from the output of a Driver IC. For 1:1
transformer, the peak output current and voltage on
the secondary is the same as the Driver IC’s output
values. As explained in section 5.0 before, knowing
the switching frequency, rise and fall times desired,
gate resistor used, total gate charge for the
MOSFET/IGBT and the voltage swing, one can
calculate the peak currents in the transformer
primary and secondary. Calculating the average
values for these currents will enable one to either
design the gate drive transformer or to enable one to
select the appropriate commercially available gate
drive transformers. These bi-directional switches form nodes for matrix
converters or for A.C. to D.C. convert ers. Driving
IGBTs in these bi-directional switches can be easily
implemented using the gate drive transformers, as
shown in Fig. (15). As can be appreciated, R , R2,
R3 and R4 help wave shaping by facilitating core
resetting. Z1, Z2, Z3 and Z4 protect the gate of IGBT
from voltage spikes above 18.7 volts.
Another method uses opto-couplers, as illustrated in
Fig. (16). As can be seen, opto-couplers need
isolated power supply. However, they facilitate D.C.
to several Mbits/sec of pulse rate and do provide
kilovolts of isolation. Keeping the same chain of
opto-coupler and Driver IC in each complementary
signal path will nullify the effect of slight propagation
delay through these opto-couplers and Driver ICs. It
is assumed here that difference in propagation
delays between two same type opto-couplers is
Opto-couplers have the following features: IXAN0009 1.Adequate galvanic isolation is possible. Many
opto-couplers are U/L listed.
2.Most opto-couplers are compatible with
TTL/CMOS/HCMOS inputs. Their outputs
depend on Vcc of isolated power supply. They
do need isolated power supply.
3.They are not immune to severe dv/dt
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- Winter '08