3. Parameters and Measure for Power Line Filter

1. What are the Important Specification of EMI Filter?

It is very important that supplier and customer use the same techniques to verify Electrical specifications. In this way, we can assure an uninterrupted flow of quality components. Three specifications that must be clearly understood are Hipot Testing, Leakage Current, and Insertion Loss.

2. How is Insertion Loss Measured?

The most popular set-up is to make the source and load impedances each 50 ohms resistive. The most important aspect of insertion loss measurement is consistency. The standard method of insertion loss measurement is given below.
Use a spectrum analyzer or tuned receiver and a tracking generator to test. First establish. a zero dB reference without the filter. Then insert the filter and record the attenuation provided over the desired frequency range. For a power line filter we are interested in signal attenuation in two different modes:
Common Mode (CM) - signals present on both line (L)-to –earth (E) and neutral line (N)-to-earth (E).
Differential Mode (DM) - signals present on line (L) to neutral line (N).
Since EMI filter can control both interference signal of CM and DM, it has CM and DM insertion losses.
When CM insertion loss is measured, one method is that terminals of L and N are connected together, the signal from the source is applied between these terminals and the ground, then the attenuated signal is measured by a receiver, shown in Fig.6. When tested DM insertion loss, an unbalance-balance converter is inserted in front of the EMI filter, and a balance-unbalance converter is inserted in rear of the EMI filter, shown in Fig.7.
The insertion loss data shown in this manual is measured according to the above mentioned.
It would be emphasized that the EMI filter in your system controls the interference signal to the level that is not equal to the insertion loss data shown in this manual, sometimes it is much difference. This is because the data shown in this manual is measured when source impedance is equal to load impedance of 50Ω. But in practice they are not equal to 50Ω in your system. This is the essential reason that causes the difference.

3. What Can Insertion Loss Data be Used For?

Standardized insertion loss data will not accurately get a filter's performance in your equipment. But it can be used as an important tool for verifying product consistency through incoming inspection.
The criterion for acceptance is that the measured insertion loss must either meet or exceed the published data in this catalog when tested in the standardized manner. That is to say, "typical" insertion loss data is not meaningful. The data to which you test should be minimum values. All the insertion loss data published in this catalog are guaranteed minimums, and as such can be tested for a positive indication of component consistency.

4. Hipot Testing

The term "hipot" means "high potential." Hipot testing stresses capacitors of a filter and the insulation assembly by applying a voltage much higher than is usually experienced in normal operation. The purpose of hipot specifications is to a assure safety and reliability. All the major safety agencies require hipot testing for qualification of power line filters, and also require that each production unit undergo hipot testing to verify the integrity of the line-to-ground components and insulation. Each filter is hipot tested twice; one is during assembly and another is after completion. Applying hipot testing as an incoming inspection procedure requires a thorough understanding of its uses and limitations. Hipot test voltages are applied from each line (both lines tied together for VDE) to ground and from line-to-line. The line-to-ground voltages are always higher. Test voltages may be specified as AC or DC, with the DC voltages at least 1.414 times the A C voltages. For incoming inspection testing, we recommend using the voltages given as "hipot rating" for each filter in the catalog.

According to the international Safety Standard, the procedure for test voltage is following: 1) Without the load on the output terminals. 2) Gradually applied the voltage to the appointed value in some rate. 3) Hold this voltage in the specific time. The EMI filter shall not be destroyed.

Something should be reminded: 1) THESE VOLTAGES CAN BE LETHAL - use the utmost safety precautions to protect the test operator. 2) Filter might be destroyed if this voltage between terminals is applied many times. This voltage has been applied into each AERODEV’s EMI filter twice. According to China Communication Standard YD/T777-1999 terms 5.5 and some international safety requirements if the test voltage is checked and applied by users, at least 25% voltage value should be reduced. 3) Test voltage should be applied and gradually increased into the appointed value. It might destroy EMI filters if the voltage is applied suddenly. 4) For the type test, the test voltage is applied and kept for 60s, however, it is only 2-5s for the production line test. 5) For line-to-line hipot testing, most filters have a bleeder resistor (typical value 100kohms to 10Mohms) to discharge the line-to-line capacitors. Be sure to set the trip point of the hipot tester above the current level that will flow through the bleeder resistor: 10mA is usually a safe value. 6) Test voltage for 3 phases’ EMI filter is applied the same as the above.

5. Understanding Leakage Current

Leakage current is an important specification of power line filters. Though it is not a function of the quality of the components, but it is a direct function of the line-to-ground capacitance value. The larger the capacitance, the lower the impedance to Common Mode currents, and the greater the Common Mode interference rejection. Hence, leakage current is a measure of filter performance--the higher the better.
The reason why safety agencies need to specify a maximum allowable leakage current is to limit the magnitude of expected ground return currents. The line-to-ground capacitors provide a path for 50/60Hz current to flow to the chassis. As long as the equipment is grounded, these currents will flow in the ground circuit and present no hazard. But we shall know, in the unlikely but always possible circumstance where the ground circuit is faulty, the earth connection may be established by the body of a person . If this should occur, the maximum leakage current specification limits the ground return current to a safe value, typically 0.5 to 5.0mA. The limits set by safety agencies are based on end user equipment specification, such as those given below.

Country	Standard	Limits for grounded equipment (level 1)
USA	UL1950	3.5mA,120V.60Hz
Canada	C22.2 No.950	3.5mA,120V.60Hz
Europe	EN60950	3.5mA,250V.60Hz

EMI filter’s leakage current is tested as the following: 1) Without load on the outlet. For safety, it is suggested to insert a isolation transformer between the power-line system and the tested filter. 2) The ampere-meter is connected between switch S and terminal E. A switch installed can connect with terminals L,N alternatively 3) Applied the rated voltage, as the switch S is connected with L or N alternatively, 2 numerical values can be obtained from the ampere-meter. Both values should be less than the appointed leakage current.

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