Gate and Base Drives2

Gate and Base Drives
Gate or base drive circuits are designed to supply the control terminals proper currents or voltages to cause the devices to switch and can mainly be divided into two parts, one is the signal progressing circuit by which the desired timing and wave shapes are produced, the other is the drive circuit, the output stage, which basic function is to amplify the low level signals created by the first part into the required levels of voltages and currents for the main device and to transform them into a special shape during the switching transient to make the device work better. Considering the design for the former part is too flexible in various applications, we discuss the latter part，the output stage chiefly here.
To design a drive circuit, many problems are common, three of them listed as follows:
Isolation is very important if not more important than other two problems. Most drive circuit without isolation may not work in order. It has two meanings which are the isolation between the drive circuit and the main circuit to reduce noises and disturbances and the isolation between each output in case the cathodes of the devices are not at the same potential to produce the trigging pulses separately.
In practice, to obtain desired isolation usually involves the use of a pulse-transformer or an opto-coupler. But they both have some disadvantages. The opto-coupler has the danger of unexpected turn-on of the device, e.g. a thyristor, by the parasitic capacitance between the optically isolated components, so a Faraday shield is provided by some manufactures to reduce the capacitance. For the pulse-transformer, the interwinding capacitance which plays the same role as the input-output capacitance in the opto-couple, may add some noise on the signal side of the transformer and what needs be paid more attention is that the magnetization in the core must be reset to prevent the transformer from saturating.
Power loss is another problem we must think much of. Considering the main usage of the drive circuit, the switching power
loss is mainly discussed
here. Mentioned in chapter
one, a power electronic
device is not an ideal
switch and power loss can
not be avoided during the
switching time. One
possible way to solve the
problem is to shape the
waveforms of the control of
signal, especially for the current-control device. The figure shows the ideal base current waveform, a typical pedestal and porch shape. High current pulse is employed so the turn-on time for the device is minimized and therefore switching loss is reduced. Many other ways are possible too, for example, using a snubber circuit which will be studied in the next chapter.
The third problem is the switch frequency that is crucial in some cases. Usually adopted is the way to add some clamp circuit to pervert the devices working in a deep saturated on-state, while the penalty of increased on state losses will occur simultaneously, so a clever designer must make a balance between them with the practical requirement. Another serviceable way is to reverse biased voltage, to produce reverse base current, so the charge stored in device can be dissipated much more quickly, therefore the turn-off time is much cut down. Sometimes negative gate voltage is needed.
Apart from the main problems mentioned above, many other cases must also be attached importance to on the basis of different characters of the different devices to drive. For instance, because of the parasitic capacitance, when the MOSFET are used in parallel operation, small resisters must be placed in series with the gate to avoid the possible oscillation; the gate current can be removed completely if the device is thyristor, but a small continuous current must be kept during the whole time the GTO is on, and others are not listed now one after another.。