非标EMC产生的原因都有哪些

Preface

Electromagnetic compatibility (EMC) is an index to test the characteristics of equipment or system in the electromagnetic environment. The EMC level of electronic products needs to pass a series of standardized EMC evaluation 1-2 tests. In order to enable electronic products to reproduce the same EMC test conclusion and its ability to have the same EMC level in similar electromagnetic environment, relevant fields or enterprises have established standardized rules and regulations or specifications for electronic products, including equipment installation and configuration requirements. EMC equipment that conforms to this regulation or specification, such as test level and method, can be called EMC equipment.

When the installation and users of electronic products expand to the global industry, does the equipment still have electromagnetic compatibility because of different installation standards, different user habits and different electromagnetic environment? The self-evident conclusion is doubtful.

1. Reasons for formation of non-standard EMC

Taking the current industrial electronic products as an example, in order to meet the practicability and strong electromagnetic compatibility level, the field EMC including test specifications, EMC assembly equipment and application specifications 5 - have been developed. Generally, it has the characteristics of universality and ideality:

1) EMC laboratory test and evaluation environment;

2) EMC test is that the equipment installation configuration requires good system grounding:

3) The shield layer of shielded cable for sensitive high-speed signal is well grounded.

However, different customers may have one or more deviations between the actual application and the ideal environment due to the actual application cost, limited assembly and other factors:

① There is no grounding point at the equipment installation site;

② The power supply is not grounded or cannot be grounded:

③ Cables are particularly sensitive. High-speed signal cables are not shielded, or shielded but not grounded.

The above and standard EMC non-standard configuration is called non-standard configuration EMC configuration. The EMC evaluation and detection of this non-standard EMC corresponding configuration is called non-standard EMC non-standard test EMC test results are the carrier of the equipment EMC design and rectification measures are called non-standard EMC measures.

Many standard EMC machines operate normally in electromagnetic environment. In non-standard environment, EMC may operate abnormally under equipment, and even cause hardware damage. The resulting customer returns, repairs and even claims have caused huge losses to equipment manufacturers. Therefore, the above non-standard evaluation of the electronic design EMC of the machine should also note that EMC has improved the specific level of electromagnetic compatibility. It is recommended to improve the equipment EMC in the weak configuration and equip it with certain EMC tests. You can refer to one or more of the following worst problems:

① Ungrounded all equipment systems:

② All cables shall be unshielded:

③ Same power supply.

The EMC evaluation and evaluation test for the development of electronic products should first be standardized EMC test, and then non-standard EMC test. If there are major improvement measures during the test, such as capacitance (reference point and location of the internal area of the equipment) PE (bridging Y capacitance between power protection areas, bridging capacitance between different areas) modification of parts or capacitance values, changes of important or sensitive components, etc., Retest 7 in the above order.

This article mainly discusses non-standard EMC equipment after the equipment or cable is not grounded, because the external world has caused greater changes in EMI (interference signal), such as the impact on RE (radiation disturbance), CE (transmission and transmission), so it has no reference significance for the evaluation of EMI test results in non-standard EMO equipment, which is not discussed in relevant EMI evaluation and analysis articles

2. Non-standard EMC dry analysis test

During EMC test, the influence current of lotus syringe can be regarded as water flow. When an electronic product is introduced, it can be discharged from one or more other power supplies or cable sockets. The water flow (influence) can only enter the low altitude, and the water inflow is equal to the water flow. The total water flow is stable, and the total energy affected by EMC is one CU, in which C is the energy storage capacity of the interference source of EMC test instrument; U is EMC test voltage, the same as

Most EMC tests include common mode interference such as electrostatic discharge, electrical transient pulse set, common mode surge, conducted immunity, etc. During the standardized EMC test, the common mode interference current flows back from the ground point to the reference ground plate. Instead of the standard EMC test, the common-mode interference current flows through the cable, and the PCB (printed circuit board) and other facilities return to the reference grounding plate through the parasitic capacitance of the earth. The differential-mode interference, such as the differential-mode surge, standard or non-standard EMC test, is equal, so that it can be introduced into one end under the influence of the test equipment, and then discharged from the other end. The more flat the channel water flow (the lower the impedance or the greater the generated capacitance), the more. Fluctuation on the channel is the root cause of EMC error.

As shown in Figure 1, the influence of current passing through the ground leads to ground impedance fluctuation, i.e. voltage drop AU. If AU equipment is used, it may have a fault exceeding the digital circuit 01 transformation threshold voltage.

The EMC test is standardized because the impedance of the equipment PCB ground plane and the impedance from the plane to the ground point (drain) are the minimum. Most of the water flow (influence) passes through the ground plane, so the fluctuation is usually caused by the current passing through the ground plane. Instead of the standard EMC test, there is no grounding point (i.e., the specification (EMC drain), and the water flow (impact) in the equipment is different from that in the specification.

Figure 1. Drop caused by interference on PCB

In the process of non-standard detection, EMC will affect the introduction of power or signal cables, because there is no direct return ground point, and the affected current will return to the reference floor or ground in the path shown in Figure 2:

Figure 2. Interference current path in non-standard EMC test

① According to the parasitic capacitance from the power or signal cable to the reference floor or ground, this path has no direct relationship with the operation of the machine;

② According to one or more test equipment PCBs, the parasitic capacitance of the reference floor or the earth is coupled to the ground;

③ This method is only applicable to equipment with plastic shell or metal frame. First, the parasitic capacitance of the plastic shell or metal frame is passed through the equipment PCB, and then the parasitic capacitance is connected to the parasitic capacitance through the plastic shell or metal frame;

④ Close the signal or power cable through the PCB and then through the capacitor and the protective device (such as TVS) near the signal or power cable socket, and then through the parasitic capacitance of the cable to the reference floor or the earth. When the cable is in place according to the configuration requirements of the cable EMC test standard higher than the configuration requirements of the reference floor 10 cm insulation support, 1m according to the parasitic capacitance of the ground floor, the long cable is estimated to be about 500. PF Generally, the standard cable length is 5m, and its parasitic capacitance is about 250pF. When the PCB board with the specification of 10cmx20cm is placed 10 cm away from the reference ground plate, the parasitic capacitance between it and the reference ground plate is estimated to be about 10 pF.

When the cable is placed on the insulated support 10cm higher than the reference ground plate according to the configuration requirements of EMC test standard, the parasitic capacitance of 1m long cable to the reference ground plate is estimated to be about 50 pF. Generally, the cable length is 5m, and its parasitic capacitance is about 250 pF. When the PCB board with the size of 10cmx20cm is placed 10 cm high from the reference ground plate, the parasitic capacitance between it and the reference ground plate is estimated to be about 10 pF.

The parasitic capacitance of the power and signal cables to the reference ground plate is far greater than that of the PCB, so most of the interference current flows out from the parasitic capacitance of the cable to the reference ground plate or the ground, and a few of the interference current flows out from the PCB in the way of spatial coupling. Therefore, EMC test needs to consider the fourth way: more interference current flows through the entire PCB, which may cause EMC risk.