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EMI filter parameters and test methods
Resource:EMI Filter | power filter low-pass filter phase noise filter noise filter Power line filter EMI filter socket Chengdu MengSheng Electronics Co., LtdEMI Filter | power filter low-pass filter phase noise filter noise filter Power line filter EMI filter sock       Pubic Date:2015-10-09   Hits:6321

EMI Filter parameters and testing methods

1. What is the important specification parameter of power filter?

For suppliers or customers, it is important to use the same technology to verify electrical parameters in order to ensure that the use of qualified components not disturbed. Three parameters that must be clearly understood: high voltage test, leakage current and insertion loss.

2. How to measure insertion loss?

The most common setting is to make the power and resistive load impedances equal to 50 ohms. The most important point in the measurement of insertion loss is consistency, specific testing methods are as follows:

Use spectral analyzer, or FM receiver or tracking generator to set up a zero dB reference point without a filter. Then, the filter is inserted to record the attenuation provided in the required frequency range.

For power line filters, were interested in two different modes of attenuation:

Common mode (CM) interference signal -- phase line (L) signal between Earth (E), neutral line (N) and ground (E). That is, U1 and U2 in figure 5.

Differential mode (DM) interference signal between phase line (L) and neutral line (N) signal. That is, U3 in figure 5.

Because the power filter can suppress common mode EMI signal and suppress differential mode EMI signal, the insertion loss should also have common mode insertion loss and differential mode insertion loss.

When measuring the common mode insertion loss, the L and N of the filter power source are connected together, and the signal source is connected between the power supply end and the ground end. At the same time, the L and N of the filter load end are also connected together, and the receiver is connected between the power supply and the ground terminal, as shown in figure 6.

When measuring the insertion loss of the differential mode, the unbalanced balanced converter and the balanced unbalanced converter are respectively accessed at the signal source and the receiver respectively, as shown in figure 7.

The common mode and differential mode insertion loss given in this manual are measured in accordance with the above specifications. There are other methods for measuring insertion loss. See the relevant data.
It is necessary to note that the insertion loss of the EMI filter provided by this manual is not equivalent to the attenuation of the interference signal used by the actual filter, sometimes its a long way off. This is because the insertion loss given in this manual is measured in a 50 ohm system, In practical application, the impedance of the EMI filter is not 50 ohms, this is the root cause of the difference.

3. What is the effect of insertion loss?

The standard insertion loss data cannot accurately obtain the performance of the filter in the device, but it can be used as an important means to verify the product conformity in the feed inspection.

The criterion is that the insertion loss measured in the standard way must meet or exceed the data on the sample. That is to say, "Typical" insertion loss data is meaningless. The data you measured should be the minimum. Most insertion loss data on a sample is its minimum guaranteed value, this value can be tested to illustrate the conformance of the components.

4. What is high voltage testing?

The high voltage test is aimed at the capacitor component and the insulation part of the filter, which is tested by exerting a much higher voltage than the normal operating voltage. The purpose of high voltage test is to ensure the safety and reliability of the filter.

All major safety mechanisms require high voltage testing of power line filters, it is also required that each product be tested at high pressure during production, to verify the integrity of line to ground components and insulators. Each filter passes two high voltage tests: one is in the assembly process, the other is after loading the finished product. It is necessary to fully understand the use and limitation of high voltage testing as a feed inspection procedure.
In high voltage testing, voltage is applied to each line (to VDE, two wires tied together) to ground and line to line, line to ground voltage is usually higher. The test voltage can be AC or DC, and the DC voltage is at least 1.414 times that of the AC voltage.
For the feed inspection test, we propose to use values specified in the "test voltage" column in explanation page of each filter.

According to the international safety standard, the test procedure of the test voltage is:

1) the load of the EMI filter is not connected to the load

2) the test voltage applied to the filter terminals should be gradually increased to the specified amplitude at a certain rate

3) keep the voltage constant within the specified time. In this process, the filter should not be broken down.

What need to be noticed by the user include:

1) The voltage may be fatal, please use the security measures to protect the test operator.

2) The test voltage cannot be repeated several times on the filter, otherwise the filter will be damaged. The EMI filter produced by our company has been tested 2 times in the factory load test, therefore, according to the provisions of paragraph 5.5 of Standard of communications industry of the Peoples Republic of China YD/T777-1999, if the user accepts the filter to apply the test voltage again, the voltage amplitude should be reduced by 25%;

3) Test voltage must be gradually increased at a certain rate, and finally reach the specified amplitude. Direct voltage test method cannot be used for testing, that is, to increase the test voltage from zero to the specified amplitude in a very short period of time, for it will damage filter.

4) When the quality of the EMI filter is evaluated, the test voltage is applied for 1 minutes, and the test voltage is applied in the production test;

5) Line to line high voltage test: most filters have a leakage resistance (typically 100K to 10M) achieve discharge to line to line. To ensure that the tripping point of the high voltage tester is set higher than the current flowing through the leakage resistance: 10mA is usually a safe value.

6) The test method of three-phase EMI filter test voltage is the same as that of single phase EMI filter.

5. What is leakage current?

Leakage current is an important parameter of power line filter. Although it is not a function of product quality, it is a direct function of line to ground capacitance value. The larger the capacitance is, the smaller the impedance of the common mode current is, and the greater the common mode interference suppression. Therefore, leakage current is an index of filter performance, the higher the better.
The safety mechanism specifies the maximum allowable leakage current to limit the expected ground return current value. The line to ground capacitance provides the channel for the 50Hz/60Hz current to the chassis. As long as the equipment is grounded, the current will flow in the grounding circuit, and it will not cause danger. But you need to know that even the grounding loop is reliable, there is also a minimal probability of failure, at this pint, the grounding loop communication may pass through the human body. In order to avoid this situation, the maximum leakage current parameters must be limited, the return current shall be limited to a safe value, typically 0.5 to 5.0mA. The limitations made by safety organizations are based on the parameters of the end-user equipment, as shown in the following table:



Restriction on grade-1 earthing equipment





C22.2 No.950





Since the largest source of leakage current is usually the power line filter, so for the filter itself, be careful when you set the maximum leakage current limit. There is a tendency in industry to specify minimum leakage current to meet all safety requirements, usually 0.5mA. Such rules should not be arbitrarily determined, for allowing a larger leakage current may reduce the cost and shape of the filter.

The leakage current measurement steps of EMI filter are as follows:

1) The load end of EMI filter is not load. For testing security, it is recommended to insert the isolation transformer at the power terminal.

2) The SPDT Switch S is connected to the L and N terminals of the filter, and milliammeter is connected between S and E.

3) Load rated voltage at the power supply end of EMI filter, when the S is connected to the L and N terminals respectively, 2 leakage current values can be read from the milliammeter, generally, these two values must be smaller than the specified leakage current value.

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