The power MOS field effect transistor is called a Power Metal Oxide SEMIconductor FielD Effect Transistor (Power Me MOSFET), which is a voltage control device. Depending on the nature of the carriers, the MOSFET can be divided into N-channel and P-channel types, and the graphical symbols are shown in the figure. According to the conductive structure, the MOSFET has a vertical conductive structure and a lateral conductive structure, and the power MOSFET is almost always composed of a vertical conductive structure, and this transistor is called a VMOSFET.
(a) N-channel type: (b) P-channel type
Graphical symbol of the power MOSFET
The main features of VMOSFET:
(1) The switching speed is very fast. VMOSFET is a majority carrier device, there is no storage effect, so the switching speed is fast. Its general low-voltage device switching time is on the order of 10ns, and the high-voltage device is on the order of 100ns. It is suitable for high-frequency power switch.
(2) High input impedance and low level drive. The input impedance of VM0S device is usually 10 (7) Ω or more, and the DC drive current is of the order of 0.1 μA. Therefore, as long as the logic amplitude exceeds the threshold voltage of VM0S (3.5~4V), it can be directly driven by CM0S, LSTTL and standard TTL devices. The circuit is simple.
(3) The safe working area is wide. The VM0S device has no secondary breakdown, and the safe working area is determined by the peak current of the device, the rated value of the breakdown voltage, and the power capacity, so the work is safe and the reliability is high.
(4) High thermal stability. The minimum turn-on voltage of a VMOS device is determined by the on-resistance, and the on-resistance of the low-voltage device is small, but increases as the voltage between the drain and the source increases, that is, the drain current has a negative temperature coefficient. The tube consumption is self-compensated with the change of temperature.
(5) Easy to use in parallel. VM0S devices can be easily paralleled to increase their current capacity, while bipolar devices require shunt resistors, internal network matching, and other additional protection devices in parallel.
(6) The transconductance is highly linear. The VM0S device is a short-channel device. When the UGS rises to a certain value, the transconductance is basically a constant value, which makes the nonlinear distortion greatly reduced when used as a linear device.
(7) The drain diode is present in the tube. A reversed drain-source diode is parasitic between the internal drain-source of the VM0S device. Its forward switching time is less than 10ns. Similar to the fast recovery diode, it also has a reverse recovery time of the order of 100ns. This diode acts as a clamp and damper in the actual circuit.
(8) Pay attention to anti-static damage. Although the VM0S device has a large input capacitance and is not as sensitive to electrostatic discharge as a general MOS device, its maximum gate-source voltage rating is about ±20V, which is much lower than the electrostatic voltage of 100 to 2500V. Therefore, it is necessary to take anti-static measures, that is, the device should be placed in anti-static packaging or conductive foam during transportation; wear a grounded bracelet when handling the device, preferably on an anti-static workbench; Soldering iron; a resistor should be connected between the gate and the source to keep it low impedance. If necessary, a Zener diode with a regulated voltage of 20V is connected in parallel to protect it. The difference between a power MOSFET and a power transistor is shown in the following table.
High-frequency MOSFETs have become a key component in switching power supplies, and MOSFETs are currently growing at an annual rate of 20%. Nowadays, in the switching power supply products on the market, the switching frequency of the switching power supply using the bipolar transistor is only 30 to 3000 kHz, and the switching frequency of the power MOSFET can reach about 500 kHz.
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