In switching power supply circuit, high-power MOS transistor has more advantages than high-power crystal triode

High input impedance and low driving power - due to the silicon dioxide (SiO2) insulation layer between the gate and source, the DC resistance between the gate and source is basically SiO2 insulation resistance, generally up to about 100m Ω, and the AC input impedance is basically the capacitive reactance of the input capacitor. Due to the high input impedance, there will be no voltage drop on the excitation signal, and it can be driven with voltage, so the driving power is very small (high sensitivity). General crystal triode must have base voltage VB and then generate base current IB to drive the generation of collector current. The driving of crystal triode requires power (VB × Ib)。

2) Fast switching speed - the switching speed of MOSFET is closely related to the capacitive characteristics of the input. Due to the existence of the capacitive characteristics of the input, the switching speed slows down, but when used as a switch, it can reduce the internal resistance of the driving circuit and speed up the switching speed (the input is driven by the "perfusion circuit" described later to speed up the capacitive charge and discharge time). MOSFET only depends on multi carrier conduction and there is no minority carrier storage effect. Therefore, the shutdown process is very fast. The switching time is between 10-100ns and the working frequency can reach more than 100kHz. Due to the storage effect of a few carriers, the switch of ordinary crystal triode always lags behind and affects the improvement of switching speed (at present, the switching power supply using MOS transistor can easily achieve the working frequency of 100k / S ~ 150k / s, which is unimaginable for ordinary high-power crystal triode).

3) No secondary breakdown - because the common power crystal triode has the phenomenon that when the temperature rises, the collector current will rise (positive temperature current characteristic), and the rise of collector current will lead to the further rise of temperature and temperature, which will further lead to the vicious cycle of the rise of collector current. The withstand voltage Vceo of the crystal triode decreases gradually with the increase of the tube temperature, which leads to the continuous rise of the tube temperature and the continuous decline of the withstand voltage, and finally leads to the breakdown of the crystal triode. This is a kind of ring breaking thermoelectric breakdown phenomenon, also known as the secondary breakdown phenomenon, which leads to the damage rate of the TV switching power supply tube and line output tube accounting for 95%. MOS transistor has the opposite temperature current characteristics to ordinary crystal triode, that is, when the tube temperature (or ambient temperature) increases, the channel current decreases. For example; For a MOS FET switch with IDS = 10a, when the VGS control voltage remains unchanged, IDS = 3A at 250C. When the chip temperature rises to 1000C, IDS decreases to 2A. This negative temperature current characteristic that the channel current IDS decreases due to temperature rise will not produce a vicious cycle and thermal breakdown. That is, there is no secondary breakdown of MOS tube. It can be seen that the damage rate of MOS tube is greatly reduced by using MOS tube as switch tube. It is also an excellent proof that the damage rate of MOS tube is greatly reduced after using MOS tube to replace the past ordinary crystal triode in TV switching power supply in recent two years.

4) After MOS transistors are turned on, their conduction characteristics are pure resistance - ordinary crystal triodes are almost straight through when they are saturated, and there is an extremely low voltage drop, which is called saturation voltage drop. Since there is a voltage drop, that is; Ordinary crystal triode is equivalent to a resistance with very small resistance after saturation conduction, but this equivalent resistance is a nonlinear resistance (the voltage on the resistance and the current flowing cannot comply with Ohm‘s law). While MOS transistor is used as a switch, there is also a resistance with very small resistance after saturation conduction, but this resistance is equivalent to a linear resistance, The resistance value of the resistance, the voltage drop at both ends and the current flowing comply with Ohm‘s law. When the current is large, the voltage drop is large, and when the current is small, the voltage drop is small. Since it is equivalent to a linear element after conduction, the linear element can be applied in parallel. When the two resistors are connected in parallel, there will be an automatic current balance. Therefore, when the power of a MOS tube is not enough, It can be used in parallel without additional balancing measures (nonlinear devices cannot be directly used in parallel).

Compared with ordinary crystal triode, MOS transistor has the above four advantages, which is enough to completely replace ordinary crystal triode in the switching state. The current technology MOS pipeline VDS can achieve 1000V, which can only be used as the switching tube of switching power supply. With the continuous progress of manufacturing technology and the continuous improvement of VDS, it can also be realized in the near future to replace the line output tube of picture tube TV