Analysis of the factors affecting the switching speed of MOS transistor Kia MOS transistor

MOS Tube quick switch

No matter what N Type or P type MOS tube , It works the same way .MOS The transistor controls the current of the output drain by the voltage applied to the input grid .MOS Tube is a voltage controlled device, which controls the characteristics of the device through the voltage applied to the grid , There will be no charge storage effect caused by base current when triode is used as switch , Therefore, in Switching Applications ,MOS The switching speed of transistor should be faster than that of triode .

MOSFET The switching speed and Cin Charge and discharge have a great relationship , Users cannot reduce Cin, But it can reduce the internal resistance of the driving circuit Rs Reduce the time constant , Speed up the switch ,MOSFET Conducting electricity only by many electrons , There is no minority carrier storage effect , Therefore, the shutdown process is very fast , The switching time is 10—100ns Between , Working frequency can reach 100kHz above , It is the highest of the main power electronic devices .

When the field control device is static, there is almost no need to input current . However, the input capacitor needs to be charged and discharged during the switching process , Still need a certain driving power . The higher the switching frequency , The greater the driving power required .

In addition to considering the voltage of the device 、 electric current 、 Out of frequency , You must also know how to protect the device in the application , Do not damage the device in transient changes . Of course, thyristors are a combination of two bipolar transistors , Plus the large capacitance brought by large area , So its dv/dt Ability is relatively fragile . Yes di/dt Come on , There is also a problem of the expansion of the conduction region , So it also brings quite strict restrictions .

power MOSFET The situation is very different . its dv/dt And di/dt Every nanosecond （ Not every microsecond ） To measure . But nonetheless , It also has the limitation of dynamic performance . These we can from the power MOSFET To understand the basic structure of .

chart 4 It's power MOSFET And its corresponding equivalent circuit . In addition to the existence of capacitance in almost every part of the device , You have to think about MOSFET And a diode in parallel . At the same time, from a certain point of view 、 It also has a parasitic transistor .（ It's like IGBT Also parasitic with a thyristor ）. These aspects , It's research MOSFET Dynamic characteristics are important factors .

First MOSFET The intrinsic diode attached to the structure has a certain avalanche ability . It is usually expressed by single avalanche ability and repeated avalanche ability . When reverse di/dt When a large , The diode will withstand a very fast pulse spike , It may enter the avalanche area , Once its avalanche capability is exceeded, the device may be damaged . As either PN For junction diodes , Careful study of its dynamic characteristics is quite complex . They are generally understood as PN The simple concept of junction conduction in the forward direction and blocking in the reverse direction is very different . When the current drops rapidly , The diode loses its reverse blocking ability at one stage , The so-called reverse recovery time .PN When the junction requires rapid conduction , There will also be a period of time when the resistance does not show very low . At power MOSFET Once the diode has forward injection , The injected minority carriers will also increase the capacity as a multi carrier device MOSFET Complexity .

power MOSFET In the design process, measures are taken to make the parasitic transistors as ineffective as possible . In different generations MOSFET There are different measures in , But the general principle is to make the transverse resistance under the drain RB As small as possible . Because only at the drain N The transverse resistance under the zone flows through enough current to supply this N When establishing the condition of positive deviation , Parasitic bipolar thyristors began to be difficult . However, under severe dynamic conditions , because dv/dt The transverse current caused by the corresponding capacitance may be large enough . At this point, the parasitic bipolar transistor will start , It is possible to give MOSFET Bring damage . Therefore, when considering transient performance, the power MOSFET Each capacitor inside the device （ It is dv/dt The passage of ） Must pay attention to .

Transient conditions are closely related to line conditions , This aspect should be paid enough attention in application . Have a deep understanding of devices , To help understand and analyze the corresponding problems .

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