Design of EMC protection device TVS diode and gas discharge tube GDT

TVS designed by EMC devices

TVS can be said to be the best device in EMC protection devices. It has fast response speed and large traffic flow. It can play a very good role in the protection of most application circuits. The key lies in some application details, improper application. The problems that come with it abound, and my understanding and summary probably talk about some application points of TVS.

First, the interpretation of the characteristic curve

The above figure is the characteristic curve of TVS. TVS uses the avalanche breakdown principle of semiconductor: when the reverse voltage is too large, the drift of the minority is accelerated, and the neutral particle is hit to cause an avalanche effect, and a large current passes instantaneously. Thermal power does not damage under conditions, and thermal breakdown will damage the device. This curve focuses on several voltage turning points, Vc is the maximum clamping voltage, which ensures that the latter device will not be damaged under this voltage condition, VBr is the voltage value that determines TVS has been turned on, and Vwm is when TVS is completely unconducted. The maximum voltage value.

Reverse leakage current is also a critical parameter! The specific value of the leakage current is related to the withstand voltage value.

Second, application design summary

Before the protection design, we must first understand the characteristics of the protected circuit:

1. Leakage current requirements.

2, the pressure value requirements.

3, the response time requirements, that is, the requirements of the capacitance characteristics.

4. Place it at the interface.

The following are several design cases and descriptions. Generally, TVS will work together with other devices to form multi-level protection.

TVS series fuses to avoid short circuits and high currents.

Signal line protection.

Generally, the protection bleed path to the power supply or ground, there is also a bleed between the differential signals.

summary:

TVS has a wide range of applications. As a comprehensive protection device with optimal performance, the design is unreasonable and cannot effectively protect the circuit, or it is easy to damage due to normal operation. Therefore, the above several key parameters need to pay more attention.

EMC device designed PTC thermistor

The thermistor is a temperature-sensitive resistor. The PTC thermistor is a thermistor with a positive temperature coefficient, that is, the resistance becomes larger as the temperature rises. The high current is an important factor in the cause of our circuit failure. The high power is easy to burn out the circuit and even catch fire. Just use this feature for high current protection.

The following figure is an introduction to the thermistor model name:

First, the interpretation of the characteristic curve

It can be seen that the resistance value has a sudden change at high temperatures.

Second, the application summary

1. Protect the current and estimate the normal working current and the current when abnormal.

2, resistance selection, under normal working conditions, the resistance of the thermistor can not affect the normal operation of the circuit.

3, thermistor is mainly for the protection of continuous high current conditions, non-transient devices.

4, the use of similar fuses, basically interchangeable.

Gas discharge tube GDT is also an important component in EMC protection design. The basic principle is to use high-pressure ionized gas and move ions to induce avalanche effect to form abnormal pressure of channel bleed.

Working state waveform:

Take a product specification as an example to see some basic parameters of GDT.

        

The main parameters are the withstand voltage value, the flow rate value, and the junction capacitance.

application:

The application of GDT is relatively simple, used in interface protection, pay attention to the venting path.

GDT is characterized by large flow, high withstand voltage, and extremely low junction capacitance. The disadvantage is that the performance needs to be replaced after multiple work, and the application of the active interface must pay attention to the maintenance voltage or freewheeling problem, so it is generally used in conjunction with the varistor.

summary:

GDT is one of the first choices for the first-class poor protection in EMC protection devices. Especially its extremely low junction capacitance is widely used in high-speed communication interfaces, playing a very important role in the protection and lightning protection of outdoor equipment. .