Another disadvantage of using an opto coupler is that

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Unformatted text preview: ler adds its own propagation delay. Another 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 MOSFET/IGBT 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 MOSFET/IGBT. While designing or choosing a Gate Drive transformer, the following points should be kept in mind: 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 core saturation. 3. If operation in any one quadrant of B-H loop is chosen, care should be taken for resetting the core. Advantages of employing transformers for Gate Drive are: 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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