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Industry
Num：38
See：2158
2020-10
16
MOS tube structure principle
The advantage of the MOS
1. Great flexibility in circuit design, grid bias can be positive or negative or zero, triode can only work under positive bias, vacuum tube can only work under negative bias; In addition, the input impedance is high, which can reduce the load of the signal source and make it easy to match the front stage.
2. Can be used as an electronic switch;
3. It can be easily used as a constant current source;
4. Can be used as a variable resistance;
5. Very high input impedance is very suitable for impedance transformation and is often used for impedance transformation at the input stage of a multistage amplifier;
6. Can be used for amplification. Because of the high input impedance of field-effect tube amplifiers, the coupling capacitance can be small without the use of electrolytic capacitors;
 
Schematic diagram of MOS tube structure
Structure and symbol (take n-channel enhanced type as an example) -- Two high-concentration N-type areas are diffused on a low-concentration P-type silicon as drain and source, the semiconductor surface is covered with a silicon dioxide insulation layer and an electrode is drawn as a gate.



Other MOS tube symbols:


 
Working principle (take N-channel Enhancement type as an example)
 
  • When VGS=0, no matter what the polarity of VDS is, there is always a PN junction that is inversely biased, so there is no conductive channel
  • VGS = 0, ID = 0
  • VGS must be greater than 0 for the pipe to work
 
 
  • At VGS >, an electric field perpendicular to the semiconductor surface is generated in Sio2 medium, repels multiple subholes in the P region and attracts fewer subelectrons. When THE VGS reaches a certain value, the P-zone surface will form an inverse layer to communicate the N-zone on both sides and form a conductive channel
  • VGS > 0 → g attracts electrons → invert layer → conductive channel
  • VGS↑ → invert layer thickening → VDS↑ → ID↑


  • ↑D↑ → IDS is small: VDS when VT ≥ VGSV
  • VT: Voltage on, VGS when conduction begins under the action of VDS, VT = VGS -- VDS
 
  • increases to a certain value, the channel near the drain is clipped to form a clipping zone.DS is 0 and VGSWhen V
  • VDS↑ → ID stays the same


MOS transistor step-down circuit
In the figure, Q27 is an N-channel MOS tube. The 1-pin of U22A outputs Q27 conduction at high voltage, and the VCC-DDR memory voltage is reduced to obtain the power supply of 1.2V-HT bus, while the 1-pin of U22A outputs Q27 at low voltage, and the voltage of 1.2V_HT bus is 0V.
 
MOS tube applications
1. Industrial field, stepper motor drive, electric drill tools, industrial switching power supply
2. New energy field, photovoltaic inverter, charging pile, UAV
3. In the field of transportation, on-board inverters, car HID stabilizers, electric bicycles
4. Green lighting field, CCFL energy-saving lamp, LED lighting power supply, metal halide lamp ballast


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