Selecting network noise suppression filters..
Blandova Ekaterina Sergeevna, Doctor of Technical Sciences
SELECTING NETWORK NOISE SUPPRESSION FILTERS
Requirements for noise suppression filters are regulated by standards developed by international, regional and national organizations:
- Russian standards – GOST 13661-92, RD 11 0956-96;
- standards used in Europe – VDE, CISPP, VG;
- US standards – MIL-F-15733, MIL-T-28861, MIL-STD-4610, FCC 79-555.
Developers of noise suppression filters or equipment that includes these filters must be guided by the standards of the region where they are supposed to be used. All these standards and recommendations cover a wide range of issues related to limiting interference in power supply circuits generated by electronic devices.
Network noise suppression filters (NSF) are included in the general class of noise suppression filters, therefore the requirements for these products also apply to NSF.
These requirements can be divided into the following groups:
- frequency range requirements;
- load requirements;
- leakage current requirements;
- requirements for attenuation of impulse interference;
- requirements for resistance to external influences;
- requirements for filter design.
The correct formulation of the requirements imposed on the SPF is the basis for the criterion for selecting these filters.
Since the main purpose of the SPF is to limit the passage of the high-frequency part of electromagnetic oscillations, only low-pass filters from 0.01 kHz to 10 GHz are considered. The frequency range is determined by the equipment for which the filter will work and the spectrum of interference that it generates.
Of course, the attenuation value that the filter must provide must be consistent with the requirements for the equipment, taking into account the actual attenuation of signals, which show to what extent interference propagating along wires is attenuated on its way from the source of interference to the receiving device and through free space.
Another main criterion for choosing a filter is the current and the nature of the load.
The noise sources and the receivers have different characteristics. The impedance between two symmetrical wires is low, and the asymmetrical impedance between the supply wire and the ground potential is high. The symmetrical impedance is determined mainly by the load impedance — the noise source and the receiver. On the other hand, the asymmetrical impedance is determined by the coupling capacitances with respect to the ground (metal case) inside the devices. Therefore, the most suitable is an asymmetrical LC filter, supplemented by symmetrically connected capacitors of large capacity (several μF) on the connection side of the noise source or receiver.
Noise suppression filters provide the greatest attenuation when they are matched to the impedance of the noise source, receiver or line. Taking this aspect into account, Fig. 1 shows filter circuits for different impedances of the line and the noise source (receiver).
The simplest high-impedance «low-pass» filter is a noise-suppressing capacitor, and the simplest low-impedance filter is a noise-suppressing inductor.
In power delivery systems, line impedance depends on the length of the line, the number and type of loads involved, and the resonant characteristics of the line.
Thus, it can be said that power supply systems have relatively low impedance. The same is true for the impedance of the wire pairs and between the wire pairs and the ground bus (since the neutral is usually grounded), hence it follows that it is advisable to start the SPF with an inductance (L).
Fig. 1. Filter circuits and their impedance
For some types of equipment, an important parameter when choosing a filter is the leakage current — this is the conduction current between the electrodes of the capacitor, characterizing, among other things, the quality of the interelectrode insulation (dielectric). Of course, the required leakage current is determined by safety requirements. You should just remember that the leakage current is inversely proportional to the filter size. This parameter is determined by the formula:
Iyт=2pf*U1*C1*10-3,
where U1, B is the supply voltage;
f, Hz is the supply frequency;
C1, μF is the filter capacitance between the wire and the ground.
From this formula it follows that, for example, in a 50 Hz 220 V network, if you set the leakage current to 0.5 mA, then capacitors with a total capacity of more than 0.007 μF cannot be placed in the filter circuit between the wire and the ground, and at Iout = 10 mA, C <0.25 μF. If the leakage current is not specified, then this value is clearly specified by the VDE 0875 standard. According to this standard, for non-stationary devices, the leakage current should not exceed 0.75 mA, while capacitors with a capacity of 2x0.0025 μF are installed in the SPF to suppress asymmetrical interference. For stationary devices with a plug connection, the leakage current should not exceed 3.5 mA, while capacitors with a maximum capacity of 2x0.0035 μF can be used in the SPF.
The criteria for selecting a filter for attenuating impulse interference should be discussed separately. Since the filter coil's own capacitance and the series inductance of the capacitor terminals transform the low-pass filter into a band-stop filter, the insertion losses decrease at high frequencies. Therefore, it is necessary to select a filter whose stopband at least covers frequencies up to I/Dt or I/tA (Dt is the pulse duration for short pulses, tA is the front time for long pulses).
For example, at Dt = 1 μsec, the stopband should be up to 10 MHz. The low-pass filter reduces the amplitude of short pulses and stretches the front of longer pulses. In any case, the high-frequency part of the interference spectrum decreases to a non-critical value.
However, the product of the amplitude and the pulse duration should not exceed a certain value acceptable for a given filter, since otherwise the filter choke core will become saturated, which will lead to a sharp decrease in the noise suppression effect.
The operating conditions of the technical specifications for each type of filter usually indicate the value of the permissible pulse interference. Thus, for the FPBM filter in TU 6346-007-11496205-96 it is written: It is allowed to use filters when exposed to pulse interference with a duration of no more than 10 μsec. And a nominal AC voltage of up to 1000 V. In this case, the attenuation is reduced by 10 dB.”
Figure 2 shows the graphs of the dependence of the attenuation of pulse interference on the pulse duration and the cutoff frequency of the low-pass filter, and Figure 3 shows the dependence of the maximum permissible pulse amplitude on the pulse duration and the amplitude-duration product. These graphs help in the optimal selection of the filter required by the customer.
It is necessary to take into account that here the attenuation of the amplitude of the impulse interference is understood as the logarithm of the ratio of the amplitude of the impulse at the filter input to the maximum amplitude of the transient process measured at the filter output.
From the consumer's point of view, the question of connecting the filter to the load is also important: should all sources of interference and receiving devices be connected through one filter (Fig. 4) or not?
The prerequisite for the first is the shielding of all connecting lines. This seems more economical, but in this case the filter must be rated for the full current consumption of the system, and therefore the choke (in order not to be too bulky) must have a small inductance. But then a capacitor of higher capacity is needed, which will lead to higher currents through the shielded lines. Therefore, it is advisable to protect each load with a separate filter or a set of filters in case the load current exceeds the filter current (Fig. 5). Usually, each TU provides for parallel operation of filters.
When selecting filters by the consumer, in addition to the main parameters (frequency, attenuation, operating range, load current, leakage current, dimensions, weight, design features, etc.), it is necessary to be guided by the operating conditions in which the filter will operate, since not only its performance, but also the specific volume and specific gravity characteristics depend on this. Requirements for resistance to external factors are set out in GOSTs for electronic products of the RV 20.39.414.1 (2) -97 series, which generally correspond to the requirements of international regulatory documents in terms of “climatics, mechanics, reliability, storability, etc.”
Fig. 2. Dependence of the attenuation of impulse interference on the pulse duration and the filter cutoff frequency
Fig. 3. Dependence of the maximum permissible pulse amplitude on the pulse duration and the amplitude-duration product
Fig. 4. The principle of collective interference suppression
A, B, C, D – interference sources;
E – load;
F – interference suppression filter for the total current.
Fig. 5. The principle of individual interference suppression
A, B, C, D – interference sources;
E – load;
1 – connection block;
2, 3, 4, 5 – interference suppression filters.
The method for measuring the main filter parameter – the attenuation value – is described in GOST 13661-92, and the publication IEC-540 (1988) provides guidance on the use of capacitors, resistors, inductors and filters for suppressing radio interference.
Since measurements of filter parameters are carried out not only at low but also at high frequencies, this requires special devices provided by GOST 11001-80 (Devices for measuring industrial interference, technical requirements and methods of radio testing).
As for the SPF certification, in the author's opinion, the filter should undergo it as part of the product, since, firstly, it itself is an element that does not contain active elements, does not amplify and cannot read information, but is a passive element in the information protection scheme (system).
Since information protection is an additional function for the noise suppression filter, not provided for by the TU, the «Regulation on the certification of information protection equipment according to information security requirements», which provides a list of elements subject to mandatory certification, does not include network noise suppression elements. However, this does not exclude voluntary certification, but, naturally, for compliance with the requirements of the TU and the requirements of STR-97 in terms of acoustic and electromagnetic effects.
From the point of view of quality control of manufacture and incoming inspection at the consumer, the filter must be tested strictly for compliance with the requirements of the technical conditions for them.
Conclusions
1. In order to effectively use network noise suppression filters, their selection is made by the consumer based on the main parameters of the filter (network parameters, type and nature of the load, operating conditions, frequency range, leakage current, etc.).
2. The filter, like any electronic element, must withstand climatic and mechanical influences provided for by the set of standards «Climate».
3. Taking into account the devices available for incoming inspection, the consumer selects a measurement technique for one of the main parameters of the SPF (attenuation value in a given frequency range) in accordance with GOST 13661-92.
Literature
1. Network noise suppression filters
2. Hans-Werner Schuiz, Si-Neue Filterreihen fur die EMV und Funk-Entstorung. Siemens Components 21 (1983) Heft 2.
3. New in Electromagnetic Compatibility (Bulletin of Scientific, Technical and Commercial Information) Moscow No. 1, 1994.
4. STR-97.
5. GOST 13661-92.
6. E.S.Blandova. Noise-suppressing products. Recommendations for selection and use.