In electronic circuits, MOS tubes and IGBT tubes often appear, and they can both be used as switching elements. MOS tubes and IGBT tubes are similar in appearance and characteristic parameters. Then why do some circuits use MOS tubes and some circuits use IGBT tubes?
Let's take a look at the difference between MOS tube and IGBT tube.
What is MOS tube?
There are two main types of FET, junction FET (JFET) and insulated gate FET (MOS).MOS is MOSFET, the full name of which is metal-oxide semiconductor field effect transistor in Chinese. Because the gate of this field effect transistor is isolated by an insulating layer, it is also called insulated gate field effect transistor.
MOSFET can be divided into N-channel depletion type and enhancement type; P-ditch depletion type and enhancement type.
MOSFET种类与电路符号
Some MOSFET have a diode inside, which is a body diode, or a parasitic diode or a freewheeling diode.
There are two explanations about the role of parasitic diodes:
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The parasitic diode of MOSFET is used to prevent the MOS tube from burning out when VDD is over-voltage, because before the over-voltage causes damage to the MOS tube, the diode first breaks down in reverse, and the large current goes directly to the ground, thus preventing the MOS tube from burning out.
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It can prevent the MOS tube from burning out when the source and drain of the MOS tube are reversely connected, and can also provide a path for the reverse induced voltage when the circuit has reverse induced voltage, so as to prevent the reverse induced voltage from breaking through the MOS tube.
MOSFET has the characteristics of high input impedance, fast switching speed, good thermal stability, voltage control current, etc. It can be used as amplifier, electronic switch, etc. in circuits.
What is IGBT?
IGBT (Insulated Gate Bipolar Transistor), insulated gate bipolar transistor, is a composite semiconductor device composed of transistor and MOS transistor.
As a new type of electronic semiconductor device, IGBT has the characteristics of high input impedance, low power consumption for voltage control, simple control circuit, high voltage resistance, high current tolerance and so on, and is widely used in various electronic circuits.
The circuit symbols of IGBT are not unified up to now. When drawing the schematic diagram, the symbols of triode and MOS tube are generally borrowed.
At this time, it can be judged whether it is IGBT or MOS tube from the model marked on the schematic diagram.At the same time, pay attention to whether IGBT has a body diode. If it is not marked on the figure, it doesn't mean that it doesn't. Unless the official data has special instructions, this diode exists.
The internal body diode of IGBT is not parasitic, but specially set to protect the fragile reverse withstand voltage of IGBT, also known as FWD (Free-wheeling diode).
It is not difficult to judge whether there is a body diode inside the IGBT. The C and E poles of the IGBT can be measured with a multimeter. If the IGBT is good and the resistance values of the C and E poles measured are infinite, it means that the IGBT has no body diode.IGBT is very suitable for application in fields such as AC motor, frequency converter, switching power supply, lighting circuit, traction drive, etc.
Structural characteristics of MOS transistor and IGBT
The internal structure of MOS tube and IGBT tube is shown in the following figure.
IGBT is formed by adding a layer to the drain of MOSFET.The ideal equivalent circuit of IGBT is shown in the following figure.
IGBT is actually a combination of MOSFET and transistor triode. MOSFET has the disadvantage of high on-resistance, but IGBT overcomes this shortcoming, and IGBT still has low on-resistance at high voltage.
In addition, the speed of IGBT and MOSFET with similar power capacity may be slower than that of MOSFET, because there is a turn-off tailing time of IGBT, and the dead time of IGBT will be prolonged due to its long turn-off tailing time, thus affecting the switching frequency.
Choose MOS tube or IGBT.
In the circuit, whether to choose MOS tube as power switch tube or IGBT tube is a problem that engineers often encounter. If the factors such as voltage, current and switching power of the system are taken into consideration, the following points can be summarized:
It can also be seen from the following figure that the conditions for both of them are used. The shaded area indicates that both MOSFET and IGBT can be selected, "?" Indicates the level that the current process can't reach.
Generally speaking, MOSFET has the advantages of good high-frequency characteristics, which can work at a frequency of several hundred kHz and up to MHz, but has the disadvantages of large on-resistance and high power consumption in high-voltage and high-current situations. However, IGBT has excellent performance at low frequency and high power, with 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 widely used in welding machine, inverter, frequency converter, electroplating electrolytic power supply, ultrasonic induction heating and other fields.
