# The world of MOS tubes

MOS tube, metal oxide semiconductor, also known as field effect transistor. MOS tube has three poles, namely gate (G pole), source (D pole) and drain (S pole). It is a voltage controlled device, and the conduction cutoff of the device is controlled by controlling G pole.

MOS tubes are divided into Nmos tubes and Pmos tubes, which have different ways of driving, mainly because of the differences in their structures. The driving mode of Nmos tube is to apply positive voltage on GS pole, that is, VGS>0V. At the same time, when the voltage exceeds the opening voltage, that is, VGS>VGS(ON)>0V, the tube begins to conduct. On the contrary, Pmos tube applies negative voltage on GS pole, and when VGS<0V<vgs(on), the="" tube="" begins="" to="" conduct.<="" span=""> The following uses Nmos tube as an example</vgs(on),>

Generally, the opening voltage of the MOS tube is marked in the data manual of the device, and there are basically two voltages, VGS=4.5V or VGS=10, respectively. The difference between these two voltages is that the internal resistance of the MOS tube is different after opening. The higher the VGS is, the lower the corresponding internal resistance RDS is. The more thoroughly the channel is opened, the lower the internal resistance, but this voltage is not infinitely high, generally not more than ±20V.

Through the above simple introduction, in fact, the selection of MOS tube will have the corresponding understanding, MOS tube in the selection, the most basic is to look at VGS (open voltage), RDS (conduction internal resistance), ID (allow through current), VDS (voltage). MOS tube voltage is applied in the DS, by controlling the VGS, to make the DS conduction, flows through the current, therefore, internal resistance of the device itself is seem to be very important, if the resistance is too large, you can through the P = I ^ 2 r to calculate the loss of the province is very high, the loss through the form of heat to send out, so the device will be very hot, damage to last more than heat resistance. Therefore, when the current is very large, the device with very small internal resistance will be selected to use. In general, these parameters can be clearly seen in the data book of the device.

In application, MOS tubes usually require a pull-down resistor between the GS poles.

Plus the resistance because of the MOS pipe between polar and polar this parasitic capacitance, the capacitance will cause no work when G in dangling uncertain state, and when there are static, or to the DS applied voltage, can give a parasitic capacitance to charge, if no pulldown resistor to release energy, will result in higher G voltage, When the voltage is applied to the G pole at this time, it is easy to cause damage. After adding the pull-down resistance, it can form a loop through the resistance to the ground to demonstrate the energy and protect the pipe.

At the same time as a result of the existence of capacitance, when use, because the capacitance will be given to charge, so, in fact, there will be a voltage, voltage, called, at this point, the MOS tube will be put into a municipality, the pipe will continue to heat, the final damage, so in control G extremely, drive current and not too small, when the current is big, can accelerate the capacitor charging time, In this way, the tube in the enlarged state is relatively reduced, and the tube heat is reduced. It is the same when the device is turned off. Therefore, when the oscilloscope is used to measure the output waveform of the MOS tube, a waveform as shown in the figure above will be obtained.