Application of field effect transistors in low voltage, wide voltage and dual voltage circuits

Application of field effect transistors in low voltage, wide voltage and dual voltage circuits

FET Low Voltage Applications:

When using a 5V power supply, if the traditional totem pole structure is used at this time, because the be of the triode has a voltage drop of about 0.7V, the actual final voltage applied to the gate is only 4.3V. At this time, there is a certain risk in choosing a field effect transistors with a nominal gate voltage of 4.5V.

The same problem also occurs when using 3V or other low voltage power supplies.

FET wide voltage application:

The input voltage is not a fixed value, it will change with time or other factors. This change causes the driving voltage provided by the PWM circuit to the field effect transistors to be unstable.

In order to make the FET safe under high gate voltage, many FETs have built-in regulator tubes to forcibly limit the amplitude of the gate voltage. In this case, when the provided driving voltage exceeds the voltage of the Zener tube, a large static power consumption will be caused.

At the same time, if the gate voltage is simply reduced by the principle of resistive voltage division, when the input voltage is relatively high, the field effect tube works well, but when the input voltage is reduced, the gate voltage is insufficient, resulting in insufficient conduction, thereby increasing power. consumption.

FET dual voltage application:

In some control circuits, the logic part uses a typical 5V or 3.3V digital voltage, while the power part uses 12V or even higher voltage. The two voltages are connected in a common way.

This puts forward a requirement that a circuit needs to be used so that the low-voltage side can effectively control the FET on the high-voltage side, and the FET on the high-voltage side will also face the problems mentioned in 1 and 2.

In these three cases, the totem pole structure cannot meet the output requirements, and many off-the-shelf FET driver ICs do not seem to include a gate voltage limiting structure.

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