What is the difference between MOS tube and IGBT tube? How to choose?
In Electronic circuits, MOS tubes and IGBT tubes will often appear. They can be used as switching elements. MOS tubes and IGBT tubes are also similar in appearance and characteristic parameters. 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 a MOS tube?
There are two main types of field effect transistors, junction field effect transistors (JFETs) and insulated gate field effect transistors (MOS transistors).
MOS tube is MOSFET, the full name in Chinese is metal-oxide semiconductor field effect transistor. Because 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 N-channel depletion type and enhancement type; P-channel depletion type and enhancement type four categories.
▲ 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:
1. The parasitic diode of the MOSFET is used to prevent the MOS tube from being burned out in the case of overvoltage of VDD, because before the overvoltage damages the MOS tube, the diode will first reverse breakdown, and the large current will go directly to the ground, thus avoiding the MOS tube. The tube was burnt out.
2. 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 a reverse induced voltage to prevent the reverse induced voltage from breaking down the MOS tube .
MOSFETs have the characteristics of high input impedance, fast switching speed, good thermal stability, and voltage control of current. In circuits, they can be used as amplifiers, electronic switches, etc.
What is an IGBT?
IGBT (Insulated Gate Bipolar Transistor), insulated gate bipolar transistor, is a compound semiconductor device composed of a transistor and a MOS transistor.
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 capacity. It is widely used in various electronic circuits.
The circuit symbols of IGBT have not been unified so far. When drawing the schematic diagram, the symbols of triode and MOS tube are generally borrowed. At this time, you can judge whether it is an IGBT or a MOS tube from the model marked on the schematic diagram.
At the same time, pay attention to whether the IGBT has a body diode. If it is not marked on the figure, it does not mean that it does not have it. Unless the official information has a special description, this diode exists.
The body diode inside the IGBT is not parasitic, but is specially set up to protect the fragile reverse withstand voltage of the IGBT, also known as FWD (freewheeling diode).
It is not difficult to judge whether there is a body diode inside the IGBT. You can use a multimeter to measure the C pole and E pole of the IGBT. If the IGBT is good and the resistance value measured at the C and E poles is infinite, it means that the IGBT has no body diode.
IGBTs are very suitable for applications such as AC motors, frequency converters, switching power supplies, lighting circuits, traction drives, etc.
Structural characteristics of MOS transistors and IGBTs
The internal structure of MOS tube and IGBT tube is shown in the figure below.
The IGBT is constructed by adding a layer on the drain of the MOSFET.
The ideal equivalent circuit of IGBT is shown in the figure below. 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, for IGBTs and MOSFETs of similar power capacity, the speed of IGBTs may be slower than that of MOSFETs, because IGBTs have turn-off tailing time. Due to the long turn-off tailing time of IGBTs, the dead time will also be extended, which will affect the switching frequency.
Choose MOS tube or IGBT?
In the circuit, the choice of MOS tube as the power switch tube or IGBT tube is a common problem encountered by engineers. If considering the system voltage, current, switching power and other factors, the following points can be summarized:
The conditions for both use can also be seen from the figure below. The shaded area indicates that both MOSFET and IGBT can be selected. “?” indicates the level that the current process cannot reach.
In general, the advantage of MOSFET is that it has good high-frequency characteristics, and the operating frequency can reach hundreds of kHz and up to MHz. The disadvantage is that the on-resistance is large in high-voltage and high-current applications, and the power consumption is large; while IGBT is in low-frequency and high-power applications. It has excellent performance under low on-resistance and high withstand voltage.
MOSFETs are used in switching power supplies, ballasts, high-frequency induction heating, high-frequency inverter welding machines, communication power supplies and other high-frequency power supply fields; IGBTs are concentrated in welding machines, inverters, frequency converters, electroplating electrolytic power supplies, super Audio induction heating and other fields.