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disadvantage of using an opto-coupler is that
separate isolated power supply is required to
feed the output side of the opto-coupler and the
driver connected to it. However, isolated DC-toDC Converters with few thousand Volts of
isolation are readily available. These can be
used to supply isolated and regulated +ve 15 V
and –ve 5V to the output side of the optocoupler and the Driver IC for driving Upper
MOSFET/IGBT as is shown in Fig. (12) and Fig.
(13). As can be seen, identical chain of optocoupler, Driver and DC-to-DC Converters are
used for even lower IGBTs. This is to guarantee identical propagation delays for all signals so that
their arrival time at the gate of IGBT bears the same
phase relationships with one another as when they
originated in the DSP.
3.3 USE OF TRANSFORMERS TO OBTAIN
GALVANIC ISOLATION IN DRIVING UPPER
Using transformers to achieve galvanic isolation is a
frequently used technique. Depending on the range
of frequencies being handled and power rating
(voltage and current ratings and ratios), transformers
can be designed to be quite efficient. The gate drive
transformer carries very small average power but
delivers high peak currents at turn-on and turn-off of
While designing or choosing a Gate Drive
transformer, the following points should be kept in
1. Average power being handled by the transformer
should be used as a design guideline. Margin of
safety should be taken into account, keeping in
mind maximum volt-second product and allowing
for worst-case transients with maximum duty ratio
and maximum input voltage possible.
2. Employing bifilar winding techniques to eliminate
any net DC current in any winding. This is to avoid
3. If operation in any one quadrant of B-H loop is
chosen, care should be taken for resetting the
Advantages of employing transformers for Gate
1. There is no need for any isolated DC-to-DC
Converter for driving an upper MOSFET/IGBT .
2. There is practically no propagation delay in a
transformer to carry signals fro...
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- Winter '08