Triode is a flow control device , It can't drive devices with too much power , Because at this time C Extremely high current , and CE The pressure drop is 0.3V about , The power consumed on the triode is very large , It's also prone to fever . So pressure controlled pressure type MOS Tube was born .

characteristic

• Give to at first GS There is current in the process of charging the terminal capacitor , When MOS After the tube is completely connected , The grid has little current .

• MOS There is an equivalent capacitance between the poles of the tube due to the process , Here we introduce the concept of three capacitors ：
  （1） Input capacitance C_iss = C_gd + C_gs
  （2） Output capacitance C_oss = C_gd + C_ds
  （3） Miller capacitor C_rss = C_gd

• With the triode CE The end conduction voltage drop is fixed differently ,MOS Tubular DS The terminal is equivalent to a variable resistor R_dson,MOS When off, the resistance is infinite , When conducting, the resistance is infinitely small , So even when it turns on I_D It's big , This power consumption is also very small .

• I_D The current is determined by the load

• High pressure MOS The tube is equivalent to multiple MOS Tubes in series , low pressure MOS The tube is equivalent to multiple parallel
  （1） High pressure MOS Tubular R_dson Big ( Load of the same power , High voltage , The current is small , High equivalent resistance , Generally tens of milliohms ),GS Small capacitance ( Series connection , So the conduction is fast ).
  （2） low pressure MOS The tube is the opposite (R_dson For a few milliohms ).

• MOS Tubular DS There is a diode between , It is associated with I_D In the opposite direction , Its pressure drop is 0.7V about , As the current increases , This pressure drop will also increase , Such as 100A when , May reach 1V More pressure drop . The current of the body diode is related to I_D Is close to or equal to . It consumes a lot of power , This loss is called freewheeling loss .

• To make sure GS There is a discharge circuit between the capacitors , Usually in GS And a resistor at the end , In this way MOS When the tube is closed, no matter how the front stage circuit is designed , There are discharge circuits .
  （1） This resistance is equivalent to the pull-down resistance , Avoid high resistance state , And prevent electrostatic damage MOS tube
  （2） resistance ： Too small power consumption , Too large is not conducive to preventing static electricity , Generally choose 10K~100K. For high pressure MOS Complementary output circuit , You can choose smaller ones according to the actual situation , Prevent two MOS Turn on at the same time .

Conduction process and Miller capacitance

GS Capacitor charging process （ The grid is connected in parallel with a resistor ）：

1. GS The internal resistance of the capacitor is just 0, Almost all the current goes from the capacitor
2. GS The capacitor is not full , The current flows from capacitance and resistance respectively , Due to the high resistance , The current still mainly goes from the capacitor
3. GS The capacitor is full of , The current does not flow from the capacitor , Only a small current flows from the resistor

Conduction process ：
（1）t1 moment , The voltage reaches MOS Tubular V_th,MOS The pipe is open ,I_D There's starting to be electricity , The current is very small , But then it will gradually rise
（2）t2 moment ,I_D The current reaches the maximum , remain unchanged , But it is not completely conductive at this time . Due to the transfer characteristics , The grid voltage is also constant , So at this time, the current mainly goes from Miller capacitor ( Grid -> Miller capacitor -> Drain electrode -> Source pole ), Not from GS Capacitor walk .

• MOSFET Transfer characteristics ： Gate voltage and drain current maintain a proportional relationship
• The size of Miller capacitance is related to the drain voltage , The higher, the bigger , So high voltage MOS Tube is more afraid of Miller effect .

（3）t2~t3 moment , This period of time is Miller platform . The grid voltage remains unchanged , Drain current max ,MOS The tube is in an enlarged state ,R_dson Start at infinity and get smaller ,V_DS smaller . although R_dson It's getting smaller , But it's still big , therefore At this time, the power consumption is high and the heat is high , So we hope this period of time is very short .
（4）t3 moment ：MOS Tube saturation conduction , Miller platform is over , The inherent transfer characteristics disappear .R_dson Become small , The drain voltage decreases , Miller capacitance becomes very small , Basically does not exist . The current continues from GS Capacitive current .V_GS Gradually increase to the voltage provided by the grid .

Reduce the time of Miller platform

• increase I_GS electric current ： Reduce the grid resistance
• Increase the grid drive voltage ： No more than MOS The limit value of the tube

and There will also be some problems in reducing the time of Miller platform

• High pressure pipe , Generally, the load current is small , namely I_D Small , and V_DS Big , If the Miller platform gets smaller , When on V_DS We should reduce from very high to very low in a shorter time
• Low pressure pipe , Generally, the load current is large , namely I_D Big , If the Miller platform gets smaller , When it is turned off I_D In a shorter period of time, it should be reduced from large to 0

In this way, the rapid change of voltage or current will make it unchanged on the Miller platform V_GS There's an oscillation .
Reference design
stay V_GS When the voltage is determined ：

• High pressure MOS The grid resistance is taken as 100~330Ω, Miller platform is 200ns~1μs, commonly 300ns
• low pressure MOS The grid resistance is taken as 10~100Ω, Constant access 33Ω、51Ω. Miller platform is 90ns~300ns

The above suggestions are for reference only , The specific time needs to be observed V_GS Waveform oscillation .

in addition , For high voltage applications , Suggested choice V_GSTH high MOS tube ; Low voltage and high current are also recommended V_GSTH high MOS tube . Low voltage and small current can be a little lower ( Consumer goods such as toys ). At the same time, the pull-down resistance of the grid can be smaller .