Electromagnetic interference of power supply

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Electromagnetic interference of power supply [Copy link]

Abstract: Based on the definition of electromagnetic compatibility and its test methods, this paper introduces the general methods of suppressing electromagnetic interference and the existing problems. Finally, the characteristics of the new electromagnetic interference suppression devices - FTS series group pulse suppressors and LSA series lightning surge absorbers are introduced.

Keywords: electromagnetic interference, electromagnetic compatibility, electromagnetic compatibility test, new anti-electromagnetic interference device

1 Introduction

Electromagnetic compatibility (EMC) refers to the ability of a device or system to work properly in its electromagnetic environment and not cause unbearable electromagnetic disturbance to anything in the environment. As electronic products increasingly adopt low-power, high-speed, and highly integrated LSI circuits, these devices are more vulnerable to electromagnetic interference than ever before. At the same time, the increase in high-power home appliances and office automation equipment, as well as the widespread use of mobile communications and wireless paging, have greatly increased the sources of electromagnetic disturbance. These changes have forced people to pay attention to electromagnetic compatibility as an important technical issue. In particular, the European Community has incorporated the electromagnetic compatibility requirements of products into technical regulations and enforced the 89/336/EEC Directive, which stipulates that from January 1, 1996, electrical and electronic products must comply with EMC requirements and be affixed with the CE mark before they can be sold in the European Community market. This has promoted governments to attach great importance to electromagnetic compatibility technology from the perspective of international trade.

　 In order to meet the requirements of international trade and technological development, the State Technical Supervision Bureau is also preparing to carry out mandatory EMC certification for 10 categories, including sound and television broadcasting equipment, information technology equipment, household and electric heating, electric tools, power supplies, lighting appliances, spark ignition engine drive devices, financial and trade settlement electronic equipment, security electronic products, and low-voltage electrical appliances. For products that do not meet EMC standards, the relevant departments will have the right to hold the manufacturers and sellers of the products accountable. The State Entry-Exit Inspection and Quarantine Bureau and the Ministry of Foreign Trade and Economic Cooperation jointly issued the "Notice on the Implementation of Mandatory Electromagnetic Compatibility Testing for 6 Imported Goods" in December 1998, stipulating that mandatory EMC testing will be implemented for 6 imported goods, including personal computers, monitors, printers, switching power supplies, televisions and audio equipment, from January 1999. This indicates that the electromagnetic compatibility performance of electrical and electronic products has begun to become a key indicator in my country's commodity import and export inspection.

The so-called electromagnetic compatibility (Electromagnetic Compatibility) refers to the coexistence state of equipment (subsystems, systems) that can perform their respective functions together in a common electromagnetic environment. This contains two meanings: the electromagnetic emission generated during its operation must be limited to a certain level; in addition, it must have a certain anti-interference ability. This is the compatibility problem that must be solved in the development of equipment.

2 EMC test

In the past, people paid more attention to the testing and suppression technology of electromagnetic emission to protect the communication, broadcasting system or other devices from interference. Recently, the testing and countermeasure technology of electromagnetic sensitivity has become a hot topic in the field of EMC, and various industries have also added anti-interference performance regulations to industry standards. For example, in the power system, multi-function watt-hour meters, user centralized meter reading systems, and remote terminal relay protection devices have all put forward anti-interference requirements.

The Ministry of Electric Power Industry standard DL/T614 "Multi-function Electric Energy Meter" stipulates the following anti-interference test requirements:

(1) Electrostatic discharge immunity test

According to IEC61000-4-2, and under the following conditions:

Contact discharge;

Severity level: 4;

Test voltage: 8kV;

Number of discharges: 10.

After the electrostatic discharge, the instrument should not be damaged or have information changed, and can work normally. The register should not produce a change greater than X (kW·h), and the test output should not produce a pulse signal greater than X (kW·h). The calculation formula of X is as follows:

X=mUnImax×10－4（1）

Where m – number of measurement units;

Un——reference voltage V;

Imax——maximum current, A.

(2) High frequency electromagnetic field immunity test

According to IEC61000-4-3, and under the following conditions:

voltage and auxiliary circuit plus reference voltage;

Frequency range: (80～100) MHz;

Severity level: 3;

Test field strength: 10V/m.

Under the action of high-frequency electromagnetic field, the instrument should not be damaged or have information changed, and should be able to work normally. The register should not produce a change greater than X (kW·h). The test output should not produce a pulse signal greater than X (kW·h). The calculation formula of X is the same as formula (1).

Under the conditions of load current Ib, power factor cosφ of 1, and being at sensitive frequency or main oscillation frequency point, the error change should be within the limits specified in Table 1 (this item is only applicable to electronic multi-function watt-hour meters).

(3) Electrical fast transient pulse group test

According to IEC61000-4-4, and under the following conditions:

The test voltage should be applied in common mode;

Severity level: 4;

Test voltage: 4kV;

Test time: 60s.

Under the action of the pulse group, the instrument should not be damaged or have information changed, and can work normally. The register should not produce a change greater than X (kW·h), and the test output should not produce a pulse signal greater than X (kW·h). The calculation formula of X is the same as formula (1).

(4) Surge test

According to IEC61000-4-5, and under the following conditions:

Severity level: 4;

Test voltage: 4kV;

Waveform: 1.2/50μs;

Polarity: positive/negative;

Number of tests: 5 times for positive and negative polarity respectively;

Recapture rate: once per minute.

Under the action of surge, the instrument should not be damaged or have information changes and can work normally. The register should not produce a change greater than X (kW·h), and the test output should not produce a pulse signal greater than X (kW·h). The calculation formula of X is the same as formula (1)

(5) Radio interference test

When the frequency is in the range of (0.15～3) MHz, the allowable values of conducted interference voltage are shown in Table 2.

When the frequency is in the range of (30～1000) MHz and the measurement distance is 10m, the allowable value of radiated interference is shown in Table 3.

(6) External magnetic field influence test

Under normal working conditions, an external magnetic field with the same frequency as the reference voltage of the multi-function electric energy meter, sinusoidal variation over time and intensity of 0.5mT (400A/m) is applied and carried out in the most unfavorable direction and phase. During the test, the program should not be disordered, the memory data should not be lost, and the error change should meet the requirements of relevant standards.

(7) Harmonic influence

Apply 10% of the 3rd and 5th harmonic interference sources to the voltage circuit of the multi-function electric energy meter respectively. The change of the measured indication error should not exceed 0.2%. The program should not be disordered and the memory data should not be lost.

Although the State Administration of Technical Supervision does not require anti-interference testing for power supply products, various interferences are often transmitted to electronic devices through the power supply, thus causing damage to these devices. Switching power supply manufacturers should pay enough attention to the anti-interference problem. A power supply with good anti-interference design can enable users to design products without considering the anti-interference problem caused by the power supply, greatly speeding up the user's product development cycle and saving development costs.

3 Methods and types of interference

Power supply interference can exist in "common mode" or "differential mode". "Common mode" interference refers to the potential difference between the power supply and the earth, or the neutral line and the earth. Sometimes also called longitudinal interference, asymmetric interference or ground interference, this is the potential difference between the current-carrying conductor and the earth.

"Differential mode" interference exists between the power phase line and the neutral line, and for three-phase circuits, it also exists between the phase lines. It is sometimes also called normal mode interference, transverse mode interference or symmetrical interference.

The distinction between the two interference modes is very important, because common-mode interference cannot be solved using differential-mode methods, and vice versa.

Interference types can range from short-duration spikes to complete power outages, including voltage changes (such as voltage drops, surges and interruptions), frequency changes, waveform distortion (voltage or current), continuous noise or clutter, and transients.

Among the several types of interference in Table 4, the main ones that can be transmitted through the power supply and cause damage to the equipment or affect its operation are electrical fast transient pulse groups and surge shock waves. As for interference such as electrostatic discharge, it will not cause any impact on the electrical equipment caused by the power supply as long as the power supply equipment itself does not produce vibration stop, output voltage drop and other phenomena.

A good power supply design should enable the power supply to work normally in a harsh electromagnetic environment, and at the same time have a good suppression effect on various pulse interferences in the power line.

4 Methods of suppressing interference

The general interference suppression methods are as follows:

(1) Add a line filter to the input of the power supply, as shown in Figure 1.

The coils of L1 and L2 are wound in the same direction on the same magnetic core. The magnetic flux generated by the two inductors for the differential mode current and the main current cancel each other out, so it will not cause the saturation of the magnetic core. For the common mode current, it can be reflected as a large inductance to obtain the maximum filtering effect, so it is also called common mode inductor.

The Cx capacitor is used to attenuate differential mode interference, the CY capacitor is used to attenuate common mode interference, and R is used to eliminate static electricity accumulation that may occur in the filter.

Power supply filters are mainly used to suppress noise in the frequency range below 30MHz, while for pulse interference, its harmonic frequency is often as high as hundreds of megahertz, and its effect is often not obvious in actual use. For example, a research institute tested the ability of 20 power supply filters to suppress surge waves, and only 4 of them exceeded 20dB, and some even produced oscillations at the output end.

(2) Use a transformer with a shielding layer

Since common-mode interference is a kind of interference relative to the earth, it is mainly transmitted through the coupling capacitance between the transformer windings. If a shielding layer is inserted between the primary and secondary and is well grounded, the interference voltage can be bypassed through the shielding layer, thereby reducing the interference voltage at the output end. The shielding layer has no adverse effect on the energy transmission of the transformer, but it affects the coupling capacitance between the windings. Figure 2 shows the common-mode interference path of the isolation transformer with a shielding layer. From Figure 2, it can be seen that in order to make the common-mode attenuation large, as long as the grounding impedance of the transformer shielding layer is small, it will work. In theory, a transformer with a shielding layer can achieve an attenuation of about 60dB. However, after actual use, it can be found that the peak interference is suppressed, and the effect is not very obvious.

(3) Wave-absorbing devices such as varistors, gas discharge tubes, TVS tubes, and solid discharge tubes. These devices have a common feature, that is, they exhibit high impedance below the threshold voltage, and once the threshold voltage is exceeded, the impedance drops sharply, so they have a certain inhibitory effect on the peak voltage, but they also have their own limitations. For example, the response speed of gas discharge tubes is slow, the current absorption capacity of varistors is not large enough, and the threshold voltage of TVS tubes and solid discharge tubes is generally only 300V to 400V.

5 FTS series group pulse suppressor and LSA series lightning surge absorber

The FTS group pulse suppressor series and LSA lightning surge absorber series designed by Shanghai Sanji Company have high-speed response level, high withstand voltage and stronger surge absorption current.

FTS series group pulse interference suppressor is a new type of anti-interference device developed based on the feedback from users that electronic products are difficult to pass the IEC61000-4-4 standard (the standard for electrical fast transient pulse interference).

Electrical fast transient pulses are a type of pulse group with a rising edge of 5ns, a half-slope width of 50ns, and a repetition frequency of 2.5kHz or 5kHz. Their harmonic frequencies can reach 100MHz, and ordinary power supply filters cannot suppress them.

Ordinary power supply filters are composed of some lossless reactance components, which can prevent other signals outside the frequency band from passing through and reflect them to the signal source. Therefore, when the impedance is not matched, part of the useful signal will be reflected and returned to the signal source, which will increase the interference level instead of reducing it. The group pulse interference countermeasure overcomes the above shortcomings of the power supply filter and adopts the principle of combining absorption and reflection.

The absorption part adopts a composite structure composed of a special ferrite material customized from Japan and a new semiconductor clipping device produced by PROTEK, an American company. Due to its own characteristics, ferrite has almost no power loss for DC or low-frequency signals, but has a strong absorption effect on high-frequency noise above 1MHz, and releases this energy in the form of heat, rather than reflecting it back to the signal source or radiating it out. In the customization of ferrite materials for group pulse antagonists, we require a large saturation flux density, and the ratio of unit volume loss to saturation flux density must be high enough, so that the device can have a strong suppression effect on interference when the volume is small enough. For the semiconductor device absorption part, we use PROTEK's absorption device, which has an extremely high response speed, and the measured response time is less than 1ns.

For the reflection part, the FTS series uses TDK's high saturation flux density material as the core of the conjugate coil and a special winding method to improve the suppression of common-mode interference and reduce the distributed parameters of the device. For the selection of internal filter capacitors, FTS uses high-frequency response capacitors.

Due to the differences in internal semiconductors and capacitors, the working voltage of the antagonist is also different. The one with a high working voltage is suitable for the AC input end, while the one with a low working voltage is suitable for the DC input or output end.

Figure 3 is the test wiring diagram without and with FTS inserted under the IEC61000-4-4 level 3 standard. Figures 4 and 5 are the waveforms measured at the output end when FTS is not inserted and inserted respectively. It can be seen from the figure that the insertion of FTS plays an important role in the amplitude reduction of the pulse train with a 5ns rising edge, thereby reducing the energy of the interference.

In short, after repeated tests in many aspects such as parameter design, device selection, and circuit layout, the FTS series has finally become a countermeasure device with low distributed parameters, high attenuation frequency, low external radiation, and extremely strong suppression effect on fast transient pulses.

Compared with other components, LSA lightning surge absorber series has high-speed response (nanosecond level) and stronger surge absorption capability, and the maximum absorbed surge current can reach 4000A.

The specific parameters of the FTS group pulse suppressor series and the LSA lightning surge absorber series are listed in Table 5 and Table 6 respectively .

6 Conclusion

Practice has proved that the switching power supply using the above-mentioned anti-interference devices can enable the user's products to pass the EMC test smoothly after being used by the user.