In electronic circuits, MOS tubes and IGBT tubes often appear. They can both be used as switching elements. The appearance and characteristic parameters of MOS tubes and IGBT tubes are also relatively similar. So why do some circuits use MOS tubes and some circuits use IGBT tubes?  
Let’s take a look at the difference between MOS tubes and IGBT tubes!  
                 

What is MOS tube

There are two main types of field effect transistors, namely junction field effect transistor (JFET) and insulated gate field effect transistor (MOS tube).  
MOS tube is MOSFET, and its full Chinese name is metal-oxide semiconductor field effect transistor. Since the gate of this field effect tube is isolated by an insulating layer, it is also called an insulated gate field effect transistor.  
   
MOSFET can be divided into four categories: N-channel depletion type and enhancement type; P-channel depletion type and enhancement type.  
    MOSFET types and circuit symbols  
Some MOSFETs have a diode inside, which is a body diode, or a parasitic diode or a freewheeling diode.  
  There are two explanations for the role of parasitic diodes:  

• The parasitic diode of the MOSFET is used to prevent the MOS tube from burning out when the VDD overvoltage occurs, because before the overvoltage causes damage to the MOS tube, the diode reversely breaks down first and directs a large current to the ground, thus preventing the MOS tube from being burned out.
• It prevents the MOS tube from burning out when the source and drain of the MOS tube are reversely connected. It can also provide a path for the reverse induced voltage when the circuit has reverse induced voltage to prevent the reverse induced voltage from breakdown of the MOS tube.

MOSFET has the characteristics of high input impedance, fast switching speed, good thermal stability, voltage control current, etc. In the circuit, it can be used as an amplifier, electronic switch, etc.  
                 

What is IGBT

IGBT (Insulated Gate Bipolar Transistor) is a compound semiconductor device composed of a transistor and a MOS tube.    
As a new type of electronic semiconductor device, IGBT has the characteristics of high input impedance, low voltage control power consumption, simple control circuit, high voltage resistance, and large current tolerance, and has been widely used in various electronic circuits.  
   
The circuit symbols of IGBT are not unified yet. When drawing the schematic diagram, the symbols of triode and MOS tube are usually borrowed. At this time, you can judge whether it is IGBT or MOS tube from the model marked on the schematic diagram.  
At the same time, you should also pay attention to whether the IGBT has a body diode. If it is not marked on the picture, it does not mean that it does not exist. Unless the official data specifically states otherwise, this diode is present.  
   
The body diode inside the IGBT is not parasitic, but is specially set up to protect the fragile reverse withstand voltage of the IGBT. It is also called FWD (freewheeling diode).  
It is not difficult to determine whether there is a body diode inside the IGBT. You can use a multimeter to measure the C and E poles of the IGBT. If the IGBT is good and the resistance values ​​measured at the C and E poles are infinite, it means that the IGBT does not have a body diode.  
IGBT is very suitable for applications in fields such as AC motors, frequency converters, switching power supplies, lighting circuits, traction drives, etc.  
                 

Structural characteristics of MOS tubes and IGBTs

The internal structure of MOS tube and IGBT tube is shown in the figure below.  
   
An IGBT is constructed by adding an additional layer to the drain of a MOSFET.  
The ideal equivalent circuit of IGBT is shown in the figure below. IGBT is actually a combination of MOSFET and transistor. MOSFET has the disadvantage of high on-resistance, but IGBT overcomes this shortcoming. IGBT still has low on-resistance at high voltage.    
In addition, for IGBTs and MOSFETs with similar power capacities, the speed of IGBT may be slower than that of MOSFET, because IGBT has a turn-off tail time. Due to the long turn-off tail time of IGBT, the dead time is also lengthened, which will affect the switching frequency.  
                 

Choose MOS tube or IGBT

In a circuit, whether to use MOS tubes as power switching tubes or IGBT tubes is a question that engineers often encounter. If factors such as the voltage, current, and switching power of the system are taken into consideration, the following points can be summarized:
 

You can also see the conditions for the use of both from the figure below. The shaded area indicates that both MOSFET and IGBT can be selected, and "?" indicates a level that cannot be achieved by the current process.  
   
Generally speaking, the advantage of MOSFET is that it has good high-frequency characteristics and can operate at a frequency of several hundred kHz and up to MHz. The disadvantage is that it has a large on-resistance and consumes a lot of power in high-voltage and high-current situations. IGBT, on the other hand, performs well in low-frequency and high-power situations, with a small on-resistance and high withstand voltage.  
MOSFET is used in switching power supply, ballast, high-frequency induction heating, high-frequency inverter welding machine, communication power supply and other high-frequency power supply fields; IGBT is mainly used in welding machine, inverter, frequency converter, electroplating electrolytic power supply, ultrasonic induction heating and other fields.

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