If that happens approach described in fig 10 can be

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Unformatted text preview: and fall times will incur more power loss in driver due to increased Vcc and Qg. If that happens, approach described in Fig. (10) can be employed. Or possibility of using a 30 Amp driver can be investigated by power calculation as shown above. 8. High overall efficiency of gate drive is possible. 6.0 ISOLATED MOSFET/IGBT Almost all possible configurations of bi-directional switches employing IGBTs and FREDs (Fast Recovery Epitaxial Diodes) are shown in Fig.(15). When connected in series with the IGBT, the FRED gives reverse blocking capability to the bi-directional switch. When connected inversely across the IGBT, the FRED provides a path for the line current to flow in the reverse direction. Both techniques prevent reverse voltage application to the emitter of IGBT. GATE DRIVE FOR Many applications of MOSFETs and IGBTs require an isolated Gate Drive Circuit. For example in H-B ridge and 3 -Phase Bridge inverters, the upper MOSFETs and IGBTs require an isolated gate drive, because the Source/Emitter of upper MOSFETs/IGBTs are not at the ground potential. Similarly in matrix converters, all bi-directional switches require isolated gate drive circuits. Basically there are two popular techniques available to implement isolated gate drive. Fig.15 shows a method to implement an isolated gate drive, using Gate Drive Transformers. There are several advantages in using the gate drive transformers: 1. If properly wound and built, they can give adequate galvanic isolation. 2. Depending on the drive current and voltage required, they give step-up or step-down facility. 3. They are immune to dv/dt transients 4. They experience no propagation delays. 5. Using modern high permeability cores, tiny Gate Drive Transformers are now available that meet most stringent design specifications. 6. There is no need to have an isolated power supply. 7. Large duty cycle range of pulse widths is possible, say, from 1% to 99%. The two disadvantages are: 1. Gate drive transformers are not suitable for DC and very low frequency signals. 2. It is difficult...
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