1 Introduction

Power industry commonly used monitoring instruments compared with traditional electrical parameters of the transmitter, and gradually to the intelligent, integrated, multi-functional direction, and the electromagnetic compatibility can also have high demands (EMS and EMI tests are related to Claim). How to choose the right designer EMC design, the success of product design play a decisive role. This article on how to make power monitoring instrument EMC design a comprehensive elaboration.

2 Standard Interpretation

2.1 Criteria

Combined with heavy industrial products common standard, with power monitoring power meter to meet the needs of EMS, EMI and project evalsuation level shown in Figure 1.

2.2 Standard Interpretation

Continuous interference and disturbances are usually divided into two types of transient interference. Such as radio, mobiles phones signals, walkie-talkies, etc. of a continuing interference. Since the switch, motor brake caused by fluctuations in the power grid, such interference we call transient interference. Figure 1 transient interference comprising: Surge SURGE, electrostatic ESD, electrical fast burst EFT / B, voltage dips, short interruptions and voltage variations DIPS; continuous interference comprising: conducting sensitivity CS, radiation sensitivity RS.

"Performance is not reduced," said the Class A judge is, after applying interference, no hardware damage, no interference is applied during the crash, reset, data dropped frames or high error rates and other issues, as if no interference is applied to the same product. Usually persistent interference level evalsuation are used in this evalsuation grade. Transient interference to occur by chance, and due to grid disturbances is not long, it temporarily reduced performance, which is the evalsuation grade B.

2.3 EMS pilot project and interfere with substantive analysis

(1) Surge SURGE: waveform 1.2 / 50μs, 8 / 20μs, a pulse width of several tens μs pulse is a conductive interference, because it carries a strong pulse of energy, so the need for all port function to do the appropriate level of protection, otherwise it will cause permanent damage to internal hard circuit elements.

(2) electrostatic ESD: waveform rising edge 0.7-1ns, a pulse width of several tens ns pulse, its peak voltage in the range of thousands of first million volts, it also has some of the pulse energy, shall be in doing port protection. Due to its rising edge is very steep, so the high-frequency harmonics that carry very rich, up to 500MHz, so static electricity in the instrument all exposed metal parts (including terminals, screws, etc.) or contact discharge apertures (including LED indicator openings, various cooling and observation hole), or when the coupling plate respectively horizontal and vertical coupling plate indirect discharges, discharge points will be at a high frequency electric field is formed instantaneously through space on the circuit interference, which is co mode interference. Thus, the static design should pay attention to two aspects of port protection and space high-frequency radiation field.
(3) electrical fast burst EFT / B: rising waveform is 5ns, several cycles of waveform burst combination, very low energy. Interference and electrostatic properties as is the common mode interference both conduction path also includes radiation.

(4) conducted susceptibility CS: common-mode interference, interference bands from 150KHz to 80MHz. Projects to test during which the interference signal source to the cable length and frequency interference meter (30MHz) 1/4 compared to the corresponding wavelength λ, so the interference voltage is applied to the modulation frequency of 30MHz over time, due to the skin effect, interference The main way to space radiation signal appears (below 30MHz, mainly to conduction disturbances).

(5) radiation sensitivity RS: common-mode interference, interference band from 80MHz to 1GHz. It should be noted, drag outside of the cable act as receiving antennas, electromagnetic interference for the far field.

2.4 EMI interference substantive analysis of the pilot project and

EMI test contains conduction and radiation emission CE emission RE. CE visits band of 150kHz ~ 30MHz, RE examined frequency band 30MHz ~ 1GHz, usually in a Class A device. For electricity meters, the main study its internal power supply (usually switching power supplies), crystal (including active and passive crystal oscillator) and other major source of harassment through an antenna (from the outside to act as tow cable) conduction and radiation formed in design should pay special attention paid to the source of the harassment process.

3 EMC design methodology

3.1 EMC design of the basic idea

EMC problems occur, there must be interference source, coupling path and sensitive equipment three elements, the lack of any link, can not constitute EMC problems. Thus, for EMC problems, the design is the "blocking" and "sparse" categories for the three elements of one or several technical measures to limit or eliminate its effects, the basic idea can be divided. "Blocking" is by increasing the common mode filter, optocoupler isolation or cable sets and other magnetic and other ways to increase the common mode impedance Z; "Shu" is formed by a high-frequency capacitance path, the common-mode interference introduced a lower impedance earth (PE) or a metal shell. An EMC design can often be both "blocking" and "sparse" way, in the small increase in cost, achieve better EMC performance.

3.2 EMC solving means

Shielding, grounding and EMC filter are three methods to solve. Described in detail hereinafter.

4 schematic level design

After EMC test project and determine the level meter to be met, in schematic design when it is necessary for the relevant pilot project designed to minimize the risk of electromagnetic compatibility and save project development time.

4.1 port design

Instrumentation includes a power port and a port signal port, the EMC test project for the test port include surge SURGE, electrostatic ESD, electrical fast burst EFT / B, conducted susceptibility CS, conducted emissions CE, voltage dips, short interruptions and voltage variations DIPS. Therefore, the design should be followed sequentially after the first surge protective isolation / common mode filtering performed.

(1) Surge protection design

According to the definition of the meter terminal, surge into the differential mode and common mode surge surge two kinds. Inter forward line and the return line as a signal port (also contains the power supply port) for differential mode surges, circuit feed line and the return lines are ground (ground terminal) to the common mode surge.

Surge suppression device is the most common surge suppression devices, such as gas anti-power tube, varistors, TVS. Different ports according to their function, use of different combinations of programs surge protection. For example, when the instrument is a three-phase four-wire input, because the voltage is high impedance input port, the surge level is not too high, generally do not use varistors and gas discharge tubes.

(2) Common Mode Filter Design

In the vicinity of port design common mode filter, common mode interference bypass. To filter out common mode interference can also be taken to design components such as isolation increase the common mode impedance to ground or through the capacitor (if there is a ground terminal port design should meet the appropriate safety requirements) way to achieve.

Common mode filter design, should make clear the common mode frequency interference, in order to select suitable inductance, capacitance parameters. If you need while suppressing low, medium and high frequency common mode interference, and sometimes can be low and high frequency common mode filter in series to solve.

Power meter port often used switching power supply, since the switching power supply is an important external source of interference, we need to port design EMI filter. In addition, from the viewpoint of EMS due to the presence of a large distributed capacitance, high frequency common mode interference between the input and output isolation transformer can be no attenuation from the input coupled to the output, and therefore need to design a filter in front of the switching power supply.

Power supply basic port filtering circuit structure shown in Figure 2. When there is no PE, the common mode capacitors omitted. Common mode choke coil in the winding will produce about 1% of the leakage inductance can be directly used for differential mode filtering, to strengthen the differential mode filtering can be an increase in differential mode choke inductor design after. It should be stressed that the filter shown in Figure 2 during the PCB layout, should be placed in a position close to the port, and traces traces should be noted that the control loop area, allowing the filter to obtain the maximum insertion loss .

4.2 Design of sensitive circuits and components

It should be noted in the design of circuits and devices sensitive to electromagnetic design for easy to receive electromagnetic interference. Maximize the use of devices with high noise immunity in the case of function meet, try to reduce the frequency of the crystal, try to choose more moderate the rising edge of the device.

Capacitance, parasitic inductance of the non-ideal components at high frequencies will greatly affect its filtering effect, so that the interference signals of different frequencies should choose a different filtering parameters. Capacitive example, its frequency impedance curve shown in Figure 3. It should be emphasized that when the leads of such devices is too long, it will reduce the high frequency parasitic own resonant frequency, it is recommended to maximize the use SMD devices. A common practice is to choose the parameters of 100-fold difference between the capacitance in parallel, in order to ensure a wide frequency range in which the capacitance characteristics remain.

Digital chip decoupling design should be done, in particular, carry a rich harmonic digital power pins, usually 1nF capacitor in parallel with 0.1μF capacitor. For critical digital signal chip structure due to objective factors, the transmission path and other effects of the design should be done decoupling, the decoupling should be careful not to affect the normal signal transmission.

For particularly sensitive circuit unit costs and structural design should allow full consideration, choose a metal such as aluminum or copper for radiation pilot project (RS and RE) shielding materials, in order to ensure adequate shielding effectiveness should ensure low ground impedance design, this will not be described in detail.

5 structure level, PCB-level design

The need to consider the structure of an electrostatic discharge, radio frequency electromagnetic radiation, EMC radiated emission three pilot projects, mainly due to the following structural constraints, some of the problems often occur in PCB design were analyzed.

A common problem, because the instrument itself is compact structure, often by a few internal PCB constituted by pins, Internet cable connection between the PCB and so on, how EMI protection?

These are the most vulnerable parts of EMC, the length of the connection when the wavelength of the interfering frequency comparable, both easily received outside interference, but also easy to internal interference with the product, causing EMI exceeded.

The design can be solved from the following three aspects: ① the signal pin passed filtering; ② minimize pin, the length of the interconnection cable (from the perspective of the process can be bundled); ③ to increase the number of pins, the best a "ground - ground signal 1-- - signal ground 2-- - 3-- signal ground" approach to reduce the circuit area of ​​the signal, reducing Zgnd different PCB.

Frequently Asked Question two, for the liquid crystal display, LED lights, and other apertures how static ESD protection?

Recommendation on the LCD screen in a transparent insulating material handling, or to increase the internal circuits discharge distance in the design.

PCB layout should pay attention: ① Do not let the filter design to separate input and output, to avoid coupling, preferably "a" or "L" type; ② The key chip sensitive signal decoupling, decoupling capacitors should be close to its pin, to reduce the loop area; ③ sensitive signals can not pass through the crystal from the bottom, not from the meter near the port, long distance transmission, should pay attention to the use of packet mode; ④ to shorten the signal transmission path critical distance to use with design, increased attention to the number of vias; ⑤ be careful not to apply exist to separate the case; ⑥ by increasing the distance to reduce believe spatial resolution between the channels coupled; ⑦ orthogonal to solve the interaction PCB top and bottom signals; ⑧ analog and digital grounds at one point connection, A / D is generally regarded as analog Devices.

FAQ Third, how to "ground"? A product is only one ground. Thus, the ground is very important to select the locations, the design should ensure that ground is located in the interference signal injection port and has a low impedance ground, through capacitor common mode interference signals can be bypassed. If the design of the instrument ports reserved a ground terminal, may be the front-end phase four-wire input voltage (La, Lb, Lc, N) of the protected PE, it may be its exchange supply (L, N) of protected areas PE or there may be RS485 communication (a, B) of the protected PE / shield to FG.

It should also be designed to ensure that this land is clean, i.e. EMC described in the "quietly." When local is not clean, the common-mode interference signals can be caused by anti-channeling incoming signal contamination from the ground, so the structural design and PCB design, the common practice is to reserve a block of copper in the port, and let other internal signals separated, and leave some distance (considering safety requirements).

6 Conclusion

When carrying out the functional design of electronic products, we need to consider the electromagnetic compatibility, power monitoring with power meter is no exception. EMC design needs to be considered in terms of schematics, PCB and structure, and mutually adjusted according to actual situation, so as to reduce the maximum risk EMC, EMC rectification reduce costs, and ultimately meet the relevant standards.