What Is A Power Line Interference Filter?

A power line interference filter is a primary tool available to the designer of electronic equipment to control conducted RFI both into the equipment (potential equipment malfunction) and out of the equipment (potential interference to other system Elements   or RF communication). By controlling the RFI conducted onto the power cord, a power line filter also contributes significantly to the control of radiated RFI.

 A power line filter is a multiple-port network of passive components arranged as a dual low-pass filter; one network for common mode attenuation, another network for differential mode attenuation. The network provides attenuation of RF energy in the stop band of the filter (typically above 10KHz), while passing the power current (50-60Hz) with little or no attenuation.

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How Does A Power Line Interference Filter Work?

Power line interference filters, as passive, bilateral networks, have complex transfer characteristics, which are extremely dependent upon source and load impedance. The magnitude of this transfer characteristic describes the attenuation performance of the filter. In the power line environment, however, the source and load impedance's are not defined. Therefore the industry has standardized upon the practices of verifying filter uniformity through measurement of attenuation with 50 Ohm resistive source and load terminations. This measurement is defined to be the Insertion Loss (I.L.) of the filter:

  I.L. = 20 log *(V(l)(Ref)/V(l))

  I.L. = 20 log *(I(l)(Ref)/I(l))

Where V(l)(Ref) and I(l)(Ref) are measured without a filter and V(l) and I(l) are measured with a filter.

 It is important to note that Insertion Loss does NOT describe the RFI attenuation provided by a filter in the power line environment. In the power line environment the relative magnitudes of the source and load impedances must be estimated and the appropriate filter configuration selected such that the greatest possible impedance mismatch occurs at each termination. This dependence of filter performance on terminating impedances is the basis for the concept of "mismatching networks".

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How Do You Select A Power Line Interference Filter?

The only way to select and qualify a power line interference filter is to test the unit in your equipment. As mentioned above, the performance is highly dependent on equipment load impedance. Filter performance cannot be derived from single impedance (50 Ohm) insertion loss data. Performance is a complex function of filter element impedances and equipment impedances which vary in magnitude and phase over the frequency spectrum of interest. Filter selection testing should be performed in your equipment to your required level of performance for both conducted emission control (FCC, VDE, UL, and TUV) and susceptibility control.

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Why Be Concerned With Safety Agency Requirements?

All components in the AC power system, including power line filters, must be safe from potential fire and shock hazard. The standards set by the various safety agencies, such as UL, CSA, TUV, VDE, and SEV, provide guidelines to assist the designer in specifying  safe and reliable components. Components which carry the compliance symbols from these agencies have been designed and manufactured to comply with these standards.

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What Are The Significant Requirements Of UL?

UL are primarily concerned with high potential withstand capability, temperature rise, to creeping and clearance distances, and material temperature capability at the time of manufacture.

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How Do You Specify A Power Line Filter?

The filter you have selected through system testing can best be specified by the data parameters found on the appropriate filter specification sheet accessible from the Home Page. Combining the product family parameters listed under the "specifications" with the package style and dimensional data from your specific filter will adequately define your selection.

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What Are The Test Methods For Verification Of The Important Specification Parameters?

Some filter specifications may be unfamiliar to you or may require slightly different measuring techniques than you have been using for other components. It is very important that supplier and customer use the same techniques for verification of Electrical specifications, in order to assure an uninterrupted flow of quality components. Three specifications that must be clearly understood are Hi-pot Testing, Leakage Current, and Insertion Loss.

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Understanding Hi-Pot Testing

The term "hi-pot" is an acronym for "high potential." Hi-pot testing stresses the insulation and capacitors of a filter assembly by applying a voltage much higher than is usually experienced in normal operation. The purpose of hi-pot specifications is to a assure safety and reliability.

All the major safety agencies require hi-pot testing for qualification of power line filters, and also require that each production unit undergo hi-pot testing to verify the integrity of the line-to-ground components and insulation. Every Powertek Group filter is hi-pot tested twice; once during assembly and again after completion. Applying hi-pot testing as an incoming inspection procedure requires a thorough understanding of its uses and limitations.

 A variety of hi-pot testers are available from a number of manufacturers. The tester shown should have at least a 500VA rating.

 The following precautions MUST be observed to ensure the safety of the operator and the validity of the test:

1. THESE VOLTAGES CAN BE LETHAL - use the utmost safety precautions to protect the test operator.

2. The possibility of high surge currents and oscillatory over-voltage during sudden application of the test voltage requires some method of limiting the applied current or increasing the voltage comparatively slowly.

3. For AC hi-pot tests, use an oscillograph to monitor the applied voltage. The current limiting circuit may react with the filter circuit to distort the 60Hz waveform. This may produce a peak voltage that exceeds the expected peak value of a sinusoidal voltage having the specified rms value. The peak voltage should be 1.414 times the rms value. Higher voltages may cause unwarranted failures due to the peak currents exceeding the trip setting.

4. For line-to-line hi-pot testing, remember that most filters have a bleeder resistor (typical value 100k Ohm to 10M Ohm) to discharge the line-to-line capacitors. Be sure to set the trip point of the hi-pot tester above the current level that will flow through the bleeder resistor: 10mA is usually a safe value.

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Understanding Leakage Current

Leakage current is an important specification of power line filters. There has always been an undeserved negative connotation to this term. Leakage current is not a function of the quality of the components, but 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.

Why, then, do safety agencies specify a maximum allowable leakage current? This is done in order 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. However, 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. Conducted - RFI is conducted over the AC power system in two modes. Common mode (asymmetrical) RFI is present on both the line and neutral current paths with reference to the ground or earth path. Differential mode (symmetrical) RFI is present as a voltage between the line and neutral leads.

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How Do You Specify A Power Line Filter?

The filter you have selected through system testing can best be specified by the data parameters found on the appropriate filter specification sheet accessible from the Home Page. Combining the product family parameters listed under the "specifications" with the package style and dimensional data from your specific filter will adequately define your selection.

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Are There Other Parameters That Need To Be Specified?

There are three additional requirements that are often specified. The CORCOM recommended values are:

1. Insulation Resistance: 6000M Ohm @ 100VDC

2. Current Overload: 6 X rated current for 8 seconds

3. Humidity: 21 days at 40 degrees C 95% RH

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What Are The Test Methods For Verification Of The Important Specification Parameters?

Some filter specifications may be unfamiliar to you or may require slightly different measuring techniques than you have been using for other components. It is very important that supplier and customer use the same techniques for verification of Electrical specifications, in order to assure an uninterrupted flow of quality components. Three specifications that must be clearly understood are Hi-pot Testing, Leakage Current, and Insertion Loss.

Capacitive Current Limits

Limits for Class 1

Since the largest component of leakage current is usually from the power line filter, it is prudent to set a maximum leakage current limit for the filter itself. There has been a tendency in the industry to specify the minimum leakage current to comply with all agency requirements, usually 0.5mA. This specification decision should not be made arbitrarily, because often the size and cost of the filter can be reduced by allowing a greater maximum leakage current.

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Understanding Insertion Loss

What is Insertion Loss?

Insertion Loss is the ratio (expressed in dB) of the signal voltage transferred from source to load without a filter, to the signal voltage transferred from source to load when the filter is inserted. As discussed above ("How Does a Power Line Interference Filter Work?"), insertion loss is not a measure of filter performance in the power line equipment environment.

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How Is It Measured?

If the terminating impedances are standardized, then it becomes meaningful to measure insertion loss, but the results so obtained can be applied only to an identical circuit. The most popular set-up is to make the source and load impedances each 50 Ohm, resistive.

The most important aspect of insertion loss measurement is consistency. It is particularly critical that supplier and user employ the same measurement techniques. The standard method of insertion loss measurement used by Powertek Group is given below.

 Insertion Loss is easily measured with a spectrum analyzer or tuned receiver and a tracking generator. A zero dB reference is established without the filter. Then the filter is inserted, and the attenuation provided over the desired frequency range is recorded.

 For a power line filter we are interested in signal attenuation in two different modes:

◎ Common Mode (CM) - signals present on both sides of the line (hot and neutral) referenced to the ground.

◎ Differential Mode (DM) - signals present on one side of the line, referenced to the other.

Accordingly, we may deal with CM insertion loss or DM insertion loss, or both.

For the common mode, the line and neutral terminals are at the same potential (same magnitude and phase) and may be considered as being in parallel. CM current circulates between this pair and the common (ground) lead. CM insertion loss is measured by s trapping the line and neutral terminals together on both sides of the filter (Figure 3). All CM insertion loss data published in the Powertek Group catalog is measured this way.  

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What Is EMI?

The simple definition for Electro-Magnetic interference (EMI) is noise problems existed in the rapid growth of computers, telecommunication, medical equipment industrial controls, sports equipment etc.  Because of EMI, causing both into and out of the equipment malfunction, have became more severe in the recent decades.  The frequency range of EMI noise is 10KHz to 30MHz by conduction through wires and 30MHz to 1GHz by radiation.

The tips for selecting proper EMI filter are as following:

A) The effectiveness of noise attenuation is the primary concern for selecting an EMI filter.  The capability in this aspect usually refers to the reading of insertion loss which is derived from the following formula:

    Insertion loss (dB) = 20 log v1/v2

    Wherein V1=EMI voltage without filter

    V2=EMI voltage with filter

B) According to the insertion loss data that power line and load have the same impedance and all such data are in practice generated from a 50 Ohm circuit.  However, the said condition seldom exists in actual application.  Therefore, insertion loss readings are not supposed to represent actual performance of noise suppression but a reference for comparison among different units or evaluation of products conformity in incoming inspection.  To verify actual effectiveness in noise suppression, a filter has to be mounted in the equipment for conducted emission test in a shield room.

C) The best way to select an EMI filter is to test the unit in equipment. As mentioned above, filter performance cannot be derived from single impedance (50 Ohm) insertion loss data.  Performance is a complex function of filter element impedances and equipment impedances which vary in magnitude and phase over the frequency spectrum of interest.  Filter selection testing should be performed in equipment to your required level of performance for both conducted emission control (CE, TUV, and UL) and susceptibility control.

   Except the above statement, the following elements should be included considerations.

   1 Safety approval

   2 Dimensional data and terminal configuration

   3 Rated current, Rated voltage, Hi-pot testing, Leakage current.

   4 Circumstance requirements, such as temperature, vibration and humidity