Components Expert

What is the difference between a field effect transistor and a thyristor?

These two electronic devices will be encountered in many circuits. For example, FETs can be used as switch oscillating tubes in induction cookers, and as switch tubes in battery car charger circuits. FETs are more commonly used in computer motherboards; For thyristors, we commonly call thyristors. The first time I came into contact with this device was the use of thyristor converter technology. As I learned later, I found that thyristors are also used very much, such as in the circuits of some speed-regulating fans. You can see SCRs, SCRs that are also used in smart dimming circuits, and SCRs can be used in washing machine circuits. Let’s talk about the difference between these two devices.

The first point is that the control method of FET and thyristor is different

1. Field effect tube

With the continuous advancement of electronic technology, there are several types of field effect transistors. Our most common types are junction field effect transistors (FET tubes) and metal oxide semiconductor field effect transistors (MOSFET tubes). These two types of field effect transistors have a common feature. They both use the electric field effect in the semiconductor to control the current of the field effect transistor. Below I use metal oxide semiconductor field effect transistors, which are commonly known as MOS tubes. Explain the problem. For example, in the N-channel field effect transistor shown in the figure below, when the Ugs voltage between the gate and the source is greater than a certain value, and Uds is greater than zero, the equivalent resistance of Rds is very small, and a larger current can flow from the drain. It flows to the source, as if they are connected by a wire.

Transistor

When the UGS voltage between the gate G and the source S is small to a certain value, or when the Ugs voltage is equal to zero, then the equivalent resistance between the drain D and the source S is very large, just like a disconnected wire In any case, there is no current through. It can be seen that the FET only controls the connection between the drain and the source after a certain voltage is established between the gate and the source.

2. Thyristor

Let’s talk about this thyristor again. It is a power semiconductor device developed on the basis of a diode and can be used to rectify semiconductor devices. Let’s talk about how to control its on and off. To make the thyristor turn on, two conditions must be met. The first is to add a trigger signal to the gate G of the thyristor, that is, between the gate G and the cathode K of the thyristor. Add a large enough forward current and voltage in between. At the same time, the anode of the thyristor is higher than the cathode, so that the equivalent resistance between the anode A and the cathode K is very small, just like connecting with a wire.

Thyristor

When the thyristor is to be turned off, as long as we reduce the current between the anode A and the cathode K to a certain value, the anode A and the cathode K of this thyristor are disconnected, and no current can flow. Or we can turn off the thyristor by setting the voltage of the anode A of the thyristor lower than the voltage of the cathode K.

Thyristor

Through the above analysis, we can know that the control methods of FET and thyristor are different. FET is a semiconductor device whose on-off is controlled by voltage. We call it a voltage-controlled device; the control method of thyristor is determined by a certain The current value triggers the turn-on of the thyristor. We commonly call it a flow-controlled device and it is a semi-controlled semiconductor device, which means that the gate electrode G of the thyristor can only control the turn-on of the thyristor but not the turn-off of the thyristor.

The second point is that the input resistance of the FET and the thyristor is different

The DC equivalent input resistance of the field effect tube is very high, and its resistance can reach 10 ninth power ohms. For MOSFET tubes, it can reach 10 fifteenth power ohms. In view of this characteristic, it is composed of field effect tubes. The circuit power consumption is relatively small, its stability and anti-interference ability are very strong, so now many integrated chips are used in integrated circuits composed of field effect transistors, and some operating voltages can be as low as 2V.

For the circuit composed of thyristors, it is relatively low in terms of input DC equivalent resistance, which determines that its power consumption is very large, and its anti-interference ability is far inferior to that of field effect transistors. This also explains the circuit composed of thyristors. The stability is not as good as the field effect tube.

Thyristor

The third point is that the role of field effect transistors and thyristors are different.

We can see from their structure that it can amplify the signal for the field effect tube, so it can be used as an amplifier in an amplifying circuit, and it can also be used as a high-speed electronic switch to control the on and off of the load, such as in a switching power supply. The field effect tube performs this function. At the same time, the use of the field effect tube can also realize the speed control, such as the modulation output of the PWN wave, the external dimming circuit, and the temperature control circuit can all be used.

Judging from the working process of the thyristor, it cannot be used to amplify the signal of the circuit, so it cannot be used as a power amplifier. Thyristors are generally used in rectifier circuits and control large load circuits. When it is used as an electronic switch, its working frequency is not as high as a field effect tube, and it can generally only be used in low-speed control situations.

Thyristor

The fourth point is the difference in integration between field effect transistors and thyristors

It can be seen from the structure of the field effect tube that its structure is relatively simple, especially in the manufacturing process is much simpler than the thyristor, coupled with its low power consumption, low noise, thermal stability is also very good, and anti-radiation ability Also strong. It is used in large-scale and ultra-large-scale integrated circuits, which integrates many advantages, but it is impossible for thyristors to do this.

The fifth point is that the “short board” of FET and thyristor is different

FETs have very high requirements for static electricity during normal storage and storage. Because the input impedance of the MOS tube is very high, the three electrodes of the gate G, source S and drain D must be short when the MOS tube is not in use. Connected together, this can prevent the field effect tube from being damaged due to the high voltage of the electrostatic field. Therefore, the weakness of FETs is that they have strict requirements for static electricity. Therefore, the soldering iron we use when welding the field effect tube must have an external ground wire, which can shield the AC electric field and prevent the field effect tube from being damaged. When I weld the field effect tube, especially the MOSFET tube, I will First, cut off the power supply of the soldering iron, and then use the residual heat of the soldering iron to solder the FET. As for the thyristor, its “soft underbelly” is its relatively poor overcurrent capability. Many protection links must be designed when forming a circuit, which is relatively troublesome to use.