Electromagnetic compatibility testing of solid-state relays

Electromagnetic compatibility (EMC) is an important technical indicator in electronic products. This article introduces the main factors, test methods and judgment criteria that affect the EMC of solid-state relays (SSRs). At the same time, it puts forward precautions during testing and gives a set of results with practical value.

1 Introduction

There are many factors that affect the electromagnetic compatibility (EMC) of solid-state relays (SSRs), such as the selection and matching of devices, circuit principles, PCB wiring and structure, etc. Among them, the influence of AC optocouplers (photocouplers) on EMC parameters is often ignored. The main reason is that there is little understanding of their application environment and requirements, and at the same time, there is insufficient understanding of EMC standards, equipment use and test methods. Therefore, when using poor-performance devices and unreasonable structures, the EMC performance of solid-state relays often fails to meet some international standards (such as CE, etc.).

2 EMC standards for solid-state relays

Electromagnetic compatibility includes electromagnetic radiation (Emission) and electromagnetic interference immunity (Immunity). For devices, European customers do not require electromagnetic testing (except for some customers with special requirements), and usually only require testing according to the EN50082-2 standard, that is, 1995V EN61000-6-2. However, the most important tests are ESD (EN61000-4-2); EFT/B (EN61000-4-4) and SURGE (EN61000-4-5). It should be noted that if the customer requires electromagnetic radiation testing, it should be tested according to the EN50081-2 standard, that is, 2001V EN61000-6-4. This article only tests and summarizes the electromagnetic interference resistance of SSR. In our many years of communication with European customers, the most common requirements they put forward are as follows:

(1) Require EMC to perform electromagnetic interference resistance testing according to common standards, that is, the above-mentioned ESD, EFT/B and Surge. The voltage levels are 4kV contact discharge and 8kV air discharge, 1kV/5kHz fast group pulse disturbance and 2 kV surge impact, and no other requirements are made;
(2) EMC is required to do the general standard electromagnetic interference resistance test to reach level 3, that is, ESD and Surge are the same as above, and EFT/B is required to reach 3kV/5kHz;
(3) Special requirements: ESD requires 6kV/10kV and EFT/B is 4kV
(4) Other requirements: In addition to electromagnetic interference (Immunity), electromagnetic radiation (Emission) test is also required.

3. Main factors affecting the EMC of solid-state relays

In order to find out the main factors and combined components that affect the EMC of solid-state relays, it is necessary to understand the circuit principles that make up SSR. We have combined the circuits of major domestic and foreign SSR manufacturers and obtained the following common circuit structures using AC optocoupler solutions. These different circuit structures and the same circuit structure with different device combinations are tested to find different EMC performance.

3.1 Circuit Principle
The three circuits of Figure 1, Figure 2 and Figure 3 can realize the function of solid-state relay. Figure 1 is the simplest circuit schematic of solid-state relay, in which A1 is AC optocoupler and A2 is bidirectional thyristor. It is often used in small SSR. When using it, users often add devices outside to improve the performance. Figure 2 adds RC absorption circuit and anti-shock resistor R2 on the basis of Figure 1. R2 is to protect the optocoupler and suppress the impact current flowing through the optocoupler at the moment of conduction. After adding these devices, the EMC characteristics of SSR and the impact performance of surge on optocoupler are greatly improved. Therefore, this circuit is often used in occasions with high reliability. Figure 3 is a special circuit, which is often used in SSR design with high reliability and high EMC level. The diode D1 in the figure can prevent input reverse polarity. The constant current circuit composed of Q1, Q2, R1 and R3 ensures that the current of optocoupler A1 is constant when the input voltage changes. The functions of the RC circuit C1, R7 and R6 are the same as those of Figure 2. A varistor RV is also added to this circuit. RV not only enhances the overvoltage protection capability of the SSR, but also greatly improves its EMC performance.

Carry out the following tests according to the above three different circuits and different structures:

3.2 Tests
Two tests were performed using different samples (see below): group pulse test (see Table 1) and surge test (see Table 2).

The test conditions are as follows:
A. Input voltage: 0V or nominal value. Load: 40W bulb;
B. "Burst test frequency: 5kHz at 2kV;" Burst test frequency: 2.5kHz at 2kV.
C. Samples: N0.1, N0.2 and N0.3 have different structures, but the same PCB; the optocouplers of N0.4 and N0.5 are different, but the others are the same. (See "Product/Component Model" in Table 1)
D. The test results are shown in Table 1.

a. "1" in the table means pass, "0" means fail. Judgment criteria: "1": bulb does not flicker; "0": bulb flickers.
b. L in the table: live wire; N: neutral wire.

It is not difficult to see from the test:
(1) Comparing the two relays of Company A's product, their circuit structure and component parameters are exactly the same, except that the former has an additional heat sink, but its burst test can only reach 2kV, while the second one can reach 4kV. It can be seen that changes in the internal structure of the product will affect the EMC performance;
(2) RC will greatly affect the EMC performance of the solid-state relay; the structure without RC circuit has the worst EMC performance, see Company B's product;
(3) Using different optocouplers will greatly change its EMC performance, see the test results of N0.4 and N0.5;
(4) Different optocouplers and different thyristor combinations also have a great impact on EMC performance;
(5) Selecting an appropriate RC combination can also improve the Burst and Surge immunity. When the C value in RC is constant, the SSR's anti-interference ability to Burst is inversely correlated with the R value, but when R is small to a certain value, this relationship is no longer obvious;
(6) Increasing the RV varistor can greatly enhance the Surge's anti-interference ability.

4 Test methods and precautions

In order to correctly obtain the test data, the test circuit must be correctly designed and the wiring must be correct as required. Then set the method and steps, and finally obtain the data according to the judgment criteria. 4.1 Test circuit Figure 4 shows the test circuit. For SSR, there is a signal source (regulated power supply), a connection less than 1 meter, a load and related instruments; Figure 5 shows the waveform of the power supply signal superimposed with the interference signal.

4.2 Test standards and requirements and judgment requirements

Table 3 shows the test standards and requirements of EMC anti-interference standards: EN61000-4-4, EN61000-4-5 and EN61000-4-2. For SSR, its working state requires that the sample is in the normal off and on state, add the interference source, and observe whether the sample fails.

5 Test conclusions

(1) Changes in the internal structure of the product directly affect the EMC performance. For example: the layout design of PCB or DCB usually requires several adjustments, such as changing the routing position between input and output, the placement of components, etc., to achieve the best state;
(2) Among all the tested optocouplers, the performance of VISHAY optocouplers is relatively outstanding, and the Burst test of most of its optocouplers can reach 4kV. Optocouplers from other manufacturers rarely reach 4kV (this mainly depends on the internal structure of the optocoupler);
(3) The varistor RV has little effect on the Burst performance, but has a great impact on the Surge performance; therefore, RV should be added in situations where there is a large surge voltage impact. The size of RV depends on the blocking voltage of the thyristor; (
4) From the test data, it can be seen that in terms of resistance to electrical pulse group impact, the optocoupler has a greater impact on the relay (see the second point of the conclusion), and different optocouplers have different impact resistance performance; and in terms of Surge resistance, the thyristor has the greatest impact on the relay (poor thyristors such as dv/dt is too low will be broken down);
(5) Different combinations will have different EMC capabilities. If an optocoupler with better EMC is used with a poor thyristor, it will result in poor EMC anti-interference capability. The same result can be obtained in the opposite case;
(6) Without RC, the ability of most optocouplers to resist group pulses is lower than 500V; basically, it cannot meet the CE standard. Therefore, designers must change the circuit structure and component parameters to meet customer requirements and CE standards. Practical applications have shown that the dielectric loss angle and temperature characteristics of capacitor C have a great influence on the absorption circuit. In addition to its power and thermal stability parameters, the resistance value of resistor R also has a great influence on EMC performance. Usually C is selected to be 10-22nf, and R is usually 10-100 ohms;
(7) There is no necessary connection between the blocking voltage of the optocoupler and its ability to resist group pulses. However, the blocking voltage of the thyristor has a great relationship with the ability to resist surge voltage.

6 Existing problems

Due to the different electrical properties of the optocoupler's pulse resistance, when the SSR relay is connected to the forward and reverse circuit of the motor, and the existence of interference voltage (which can be viewed with an oscilloscope), the SSR will be misconnected and even burned. The same is true for zero-crossing relays. Theoretically, the interference voltage is twice the square root of the sum of the back electromotive force and the load voltage, but in reality the interference voltage can reach 3-5 times the load voltage, and sometimes 10 times. The reason is that the distributed parameters of the circuit produce LC parallel resonance. Although the energy of the resonant voltage is small and the duration of the peak is only microseconds, it will cause the SSR to be mis-conducted, that is, the optocoupler will fail. Therefore, it needs further discussion.