优化EMC设计：电路与组件选择最佳实践

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本文档是关于EMC（电磁兼容性）设计的一系列专业指南，由欧英·阿姆斯特朗（Eur Ing Keith Armstrong CEng MIEEM IEEE）撰写，他是Cherry Clough Consultants的合作伙伴和EMC-UK的关联成员。这部分是系列文章的第一篇，着重于电路设计和元器件选择在电子/电气/机械设备硬件设计中的最佳实践。该系列文章面向电子产品的设计师，包括电源供应、单板计算机、电机驱动等基础组件，以及计算机、音频/视频/电视设备、仪器等各种独立或网络化的设备。文章旨在提供实用的EMC设计技巧，帮助设计师了解如何在设计过程中考虑电磁干扰（EMI）和抗干扰（EMS）问题，确保产品符合国际标准，减少电磁辐射对周围环境的影响，并提高系统的稳定性和可靠性。作者在撰写过程中，还参考了多位业内专家的意见和反馈，如Feng Chen、Kevin Ellis、Neil Helsby等，他们对文章进行了修正、修改和补充，部分内容基于与他们的交流和讨论。此外，特别值得一提的是，文章的部分内容被翻译成日语并在Tom Sato的网站上发布，这表明了全球工程师之间的知识共享和跨文化交流。整个系列将在接下来的一年中分期发表，为设计人员提供了深入学习和实践EMC设计的宝贵资源。通过阅读这些文章，设计师们将能够提升他们产品的电磁兼容性，从而提高产品质量和市场竞争优势。

1.3.4 Rectifiers

The rectifiers used for primary flywheels and secondary rectifiers can cause a great deal of noise

(hence emissions) due to their reverse current flow.

Faster-switching devices need less reverse charge (current x time) and can cause less noise. But if

they are hard-switching types they can excite resonances in the switcher components (especially the

isolation transformer) and cause excessive overshoots and emissions.

It is best for EMC to use rectifier types which have fast operation but soft-switching characteristics, as

shown by Figure 6.

1.3.5 Problems and solutions relating to magnetic components

Pay particular attention to closing the magnetic circuits of inductors and transformers, e.g. using

toroids or gapless cores. Iron powder toroidal cores are available for energy-storage magnetics,

these effectively have a distributed air gap and so emit lower fields than gapped cores.

If air gaps have to be used, for instance in C, E or pot cores, an overall shorted turn may be needed

to reduce the leakage fields. ‘Overall’ means that it goes around the entire body of the transformer, so

it is only a shorted turn for the leakage fields.

Primary switching noise is injected via the interwinding capacitance of isolating transformers, creating

common-mode noise in the secondaries. These noise currents are difficult to filter, and travel long

distances, enclosing large loop areas (to keep Mr Kirchoff happy) thereby creating emissions

problems.

Interwinding shields in an isolating transformer can suppress primary switching noise in the

secondaries. One shield is a great help, and should be connected to a primary DC rail. Up the five

shields is not unheard of, but three is more likely. When using three shields, the shield adjacent to the

secondary windings usually connects to the common output ground (if there is one) and the shield in

the middle usually connects to chassis. Be prepared to iterate a prototype to find their best

connections.

PCB-transformers are becoming increasingly popular, and adding shields to these is simply a matter

of adding more PCB layers (making sure that creepage and clearance distances are achieved

despite tolerances in PCB manufacture).

Design techniques for EMC – Part1  Cherry Clough Consultants July 2001 Page 16 of 26

Another powerful technique is to provide a local return path for these currents with small (safety

approved!) capacitors connected between the secondary ground and one of the primary power rails.

Make sure that these capacitors don’t cause the total earth leakage current to exceed the

specification in the relevant safety standard.

These capacitors also help any filters on the secondaries to work much better, by reducing the source

impedance of the emissions so that common-mode chokes can function effectively.

The above two techniques also reduce the secondary switching noise which appears at the input, via

the isolating transformer’s interwinding capacitance. The primary to secondary capacitor also makes

filtering at the input more effective.

Figure 6B shows a simple switcher with a single interwinding shield and a primary-secondary

bridging capacitor.

1.3.6 Spread-spectrum clocking for switch-mode

‘Spread-spectrum clocking’ techniques as described in 1.1.5 above can also be used with some

switch-mode topologies to spread the emissions spectrum of the individual harmonics so that they

measure less on an EMC test. Commercial and industrial conducted emissions tests use a 9kHz

bandwidth from 150kHz to 30MHz, so spreading a harmonic by ±90kHz can give reductions of more

than 10dB.

The spreading range can often be much larger than 1 or 2%, and some high-power converter

manufacturers use almost white noise.

1.4 Signal communication components and circuit design

1.4.1 Non-metallic communications are best

The best communications for EMC purposes are infrared or optical, via free-space (e.g. IRDA) or

fibre-optics. Their transmitters must not emit too much, and the receivers must be immune enough,

but these are usually easier to control than the EMC of a long cable. Metal-can shielded transmitters

and receivers are now readily available. It is often possible to bring metal-free fibre-optic cables right

through the walls of shielded enclosures to PCBs or modules inside, without compromising the

Design techniques for EMC – Part1  Cherry Clough Consultants July 2001 Page 17 of 26

enclosure shielding, whereas metallic wires and cables need to be filtered and/or 360

shield bonded

at the points where they cross shielded enclosure boundaries.

Wireless communications are another alternative, but because they use the radio spectrum they

sometimes cause interference with nearby electronics, and they can also be interfered with by

electromagnetic disturbances.

Wires and cables may appear at first sight to be more cost-effective, but by the time their EMC

problems have eventually been solved at the end of a project the non-metallic alternatives would

often have been preferable for reasons of cost and timescale. Another reason for using non-metallic

communications is that galvanic isolation to very high values is automatically achieved, improving

product reliability and greatly easing the risks of failing EMC tests.

Wires and cables are usually cost-effective within a fully shielded product enclosure, but even then

‘internal EMC’ problems and the slow propagation velocity in cables can make infra-red or optical

alternatives more attractive. (Don’t forget to take account of the delays in the infra-red or optical

transceivers themselves into account.)

1.4.2 Techniques for metallic communications

Single-ended signal communication techniques have very poor EMC performance for both emissions

and immunity, and are best restricted to low frequency, low data rate, or short distance applications.

They are usually all right as long as they remain on a PCB with a solid ground plane under all the

tracks and don’t go through any connectors or cables, which means that the single-PCB product is

often the most cost-effective.

High-frequency or long-distance signals should be sent/ received as balanced signals (sometimes

even on PCBs) for good signal integrity and EMC, and this is going to be a main issue in this sub-

section.

Figure 8 shows examples of good and bad practices when connecting a millivolt output transducer to

an amplifier via a cable.

In general, connecting a cable shield to a circuit’s 0V is very bad practice, as is the use of pigtails and

grounding cable screens at one end only. Some older textbooks divide cables up into low and high

frequency types, with different shield-bonding rules for each. But the electromagnetic environment is

now so polluted with RF threats (and as was shown earlier, even ‘slow’ opamps will demodulate

Design techniques for EMC – Part1  Cherry Clough Consultants July 2001 Page 18 of 26

>500MHz), and so many signals are polluted with RF common-mode noise from digital processors

inside their products, that all cables should now be treated as high-frequency.

The three schemes in figure 8 show a hierarchy from a poor system for connecting to a transducer,

through a better one, to a good system. Fitting an A/D converter in the transducer enclosure and

sending high-level encoded data (with error-correction) over the cable to the product for decoding

would be better than the best shown opposite. A perfect system would send the digital data over a

fibre-optic instead of a metallic cable, and such systems are increasingly used in industry.

Concerns about cable shield heating in large or industrial premises are best dealt with by running the

communications cable over a parallel earth conductor (PEC) to divert the majority of the heavy low-

frequency currents (which will prefer to follow paths with lower resistance) and not by ‘lifting’ a shield

connection at one end – which ruins the cable’s shielding benefits at that end. Fitting a capacitor in

series with the shield at one end is also not recommended as a design technique, although it may be

useful as a remedial technique, because it is very difficult to make a capacitive bond work effectively

over the full range of frequencies. PECs and other installation cabling and earthing techniques are

discussed in detail in [2] [3] and [4].

For low frequency signals (say, under 100kHz) higher voltage levels in the communication link are

better, for reasons of immunity. Where signal frequencies are above 10MHz (say) high voltages can

lead to high levels of emissions – lower voltages are often preferred as the best compromise (e.g. as

used by ECL, LVDS, USB). The signal frequency at which lower voltages are preferred depends on

the length of cable and its type and EMC performance (especially its longitudinal conversion loss)

and the design of the transmit and receive circuits.

Transmission line techniques may be essential for high-speed analogue or digital signals, depending

on the length of their connection and the highest frequency to be communicated (see Part 5 of this

series). Even for low-frequency signals, immunity will be improved by using transmission line

techniques for their interconnections.

The best type of cable for EMC usually has a dedicated return conductor associated with each signal

conductor, and any cable shields are used only to control interference. Co-axial cable is generally not

preferred. Some cables need individually shielded signal pairs. It is very important to achieve a good

balance over the whole frequency range, as this means a good common-mode rejection ratio

(CMRR) and hence improved emissions and immunity. Balanced send / receive ICs are good, but

isolation transformers have the benefit of adding galvanic isolation (up to the point where they flash-

over) and also extending the common-mode range well beyond the DC supply rails.

Balanced construction twisted-pair or twinaxial cables usually give the best and most cost-effective

emissions and immunity performance and very small differences in twist (and even the dielectric

constants of the pigments used to colour their insulation) can be important. Balance is so important

that in high-performance circuits even a physically balanced (mirror-image) PCB layout will be

needed, using the same PCB layers.

Transformers and balanced send/receive ICs all suffer from degraded balance at RF. They generally

require a common-mode choke in series to maintain good balance over the whole frequency range of

interest. The CM choke always goes closest to the cable or connector at the boundary of the product.

Transformer isolation, balanced drive and receive, and CM chokes, all help to get the best EMC

performance from a cable.

Figure 9 shows two examples, both equally applicable to providing good emissions and immunity for

digital or analogue signalling (communications) of any speed or frequency range.

Design techniques for EMC – Part1  Cherry Clough Consultants July 2001 Page 19 of 26

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