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Design Techniques for EMC Part 3 - Filtering and Suppressing Transients (third part)

By Eur Ing Keith Armstrong C.Eng MIEE MIEEE, Cherry Clough Consultants

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Note: Part 3 has been split over three issues, this is the final part.

 

This is the third in a series of six articles on basic good-practice electromagnetic compatibility (EMC) techniques in electronic design, to be published during 2006. It is intended for designers of electronic modules, products and equipment, but to avoid having to write modules/products/equipment throughout - everything that is sold as the result of a design process will be called a ’product’ here.

This series is an update of the series first published in the UK EMC Journal in 1999 [1], and includes basic good EMC practices relevant for electronic, printed-circuit-board (PCB) and mechanical designers in all applications areas (household, commercial, entertainment, industrial, medical and healthcare, automotive, railway, marine, aerospace, military, etc.). Safety risks caused by electromagnetic interference (EMI) are not covered here; see [2] for more on this issue.

These articles deal with the practical issues of what EMC techniques should generally be used and how they should generally be applied. Why they are needed or why they work is not covered (or, at least, not covered in any theoretical depth) - but they are well understood academically and well proven over decades of practice. A good understanding of the basics of EMC is a great benefit in helping to prevent under- or over-engineering, but goes beyond the scope of these articles.

The techniques covered in these six articles will be:

 

  1. Circuit design (digital, analogue, switch-mode, communications), and choosing components
  2. Cables and connectors
  3. Filtering and suppressing transients
  4. Shielding
  5. PCB layout (including transmission lines)
  6. ESD, surge, electromechanical devices, power factor correction, voltage fluctuations, supply dips and dropouts

Many textbooks and articles have been written about all of the above topics, so this magazine article format can do no more than introduce the various issues and point to the most important of the basic good-practice EMC design techniques. References are provided for further study and more in-depth EMC design techniques.

Table of contents for this article

3. Part 3 - Filters and transient suppressors

In Issue 66, September 2006

3.1 Introduction
3.2 Designing and selecting filters
3.2.1 How filters work
3.2.2 Imperfections in the basic filter circuits
3.2.3 The importance of the RF Reference
3.2.4 Differential-mode (DM) and Common-mode (CM)
3.2.5 Maximising impedance discontinuities
3.2.6 Using soft ferrite cores

In Issue 67, November 2006

3.2.7 Issues with wound components (inductors,transformers, chokes, etc.)
3.2.8 Specifying and designing filters
3.2.9 Problems with real-life supply impedances
3.2.10 Problems with real-life switch-mode converter input impedances
3.2.11 Damping filter resonances that cause gain
3.2.12 Filters and safety
3.3 Filter installation issues
3.3.1 Input and output conductors
3.3.2 Skin effect and the flow of surface currents
3.3.3 The synergy of filtering and shielding
3.3.4 Assembly and installation techniques for filters that penetrate shields
3.3.5 Designing to prevent corrosion
3.3.6 Filters connected in series
3.4 Types of overvoltage transients and surges

In this issue

3.5 Protecting from surges
3.5.1 Protecting insulators from surge overvoltages
3.5.2 Protecting conductors from surge overcurrents
3.5.3 Protecting electromechanical contacts from surges
3.5.4 Galvanic isolation is the best defence against surges
3.5.5 Surge suppression with filters
3.5.6 Suppression with surge protection devices (SPDs)
3.5.7 Types of Surge Protection Device (SPD)
3.5.8 Characteristics and comparisons of SPD types
3.5.9 Minimising the inductance in series with SPDs
3.5.10 Rating SPDs
3.5.11 Combining SPDs
3.5.12 A hierarchy of surge protection
3.5.13 Protecting SPDs
3.5.14 Equipment reliability and maintenance issues
3.5.15 Surge protection products
3.5.16 ’Earth lift’ problems in systems
3.5.17 Data needs error detection/correction
3.6 References
3.7 Acknowledgements

3. Part 3 - Filters and transient suppressors

3.5 Protecting from surges

3.5.1 Protecting insulators from surge overvoltages

Conductors in cables, connectors, transformers, etc., are protected by insulation, which can be overcome by surge voltages, causing interference and even possible safety problems. Solid insulation can also degrade over time, due to repeated surges. Even where a surge causes a spark to an earthed chassis and does not cause any direct interference, the RF noise caused by the spark can be a significant source of interference within a product.

So insulation (whether air or solid materials) must at least satisfy the ’creepage’, ’clearance’ and ’voltage withstand’ requirements of the relevant safety standards (e.g. IEC/EN: 60950, 60335-1, 61010-1, 60204-1, 60601-1, etc.) - which usually provide sufficient performance for EMC purposes. However, environments suffering from especially high levels and/or high rates of surge overvoltages could require increased insulation voltage withstand, unless the surge voltages are limited by other means, such as galvanic isolation or SPDs (see below).

3.5.2 Protecting conductors from surge overcurrents

The high levels of transient currents flowing during a surge can damage conductors themselves, so we need to provide appropriate protection. If conductors have sufficient CSA (Cross Sectional Area) they can handle momentary surges that are many times higher than their continuous current ratings. For copper conductors, the following guide can be used to calculate its maximum current/time before it fuses (melts): I = 290 x CSA / √t, where I is the peak current in amps, CSA is given in square millimetres, and t is the duration of the current in seconds. This guide is only valid for t up to 5 seconds, because thermal convection and conduction effects start to become significant at longer times, requiring a very much more complex equation.

On a PCB, 1/2oz (finished) copper foil has a thickness of 17.5μm; 1oz copper 35μm; 2oz copper 70μm and 3oz copper 105μm. Multiplying the finished copper thickness by trace width gives its CSA, which can be used in the above guide. For example, a trace that is 0.18mm (7 thousandths of an inch) wide in 1oz (finished) copper (35μm), subjected to a rectangular current surge lasting 40μs, would have a maximum transient or surge current (before melting) of around 290A. Of course real surges do not have rectangular current waveforms, so there is some error in using the above guide to estimate real surge capability of conductors - a large ’engineering margin’ is recommended!

Underwriters Laboratories (UL) carried out some tests of their own [15][16], resulting in some guidance on the maximum current handling that would not result in an open-circuited PCB trace. The guidance given by Figures 7 and 8 in [15][16] seems to correspond pretty well with the above formula (but note that the horizontal axes of the graphs in its Figures 7 and 8 say they are trace thickness, when in fact they are trace width).

But the above guide only considers the copper conductor’s transient or surge current carrying capacity before it melts, and such high temperatures will damage wire insulation and PCB dielectrics, possibly causing delamination with consequent reliability problems.

Some suppliers publish transient current data for wires, typically for current surges with a 1s duration. A typical 1s rating for wire with a 0.5mm2 CSA and standard PVC insulation, with a normal operating temperature of 25oC, is 55A. This current will not raise the copper temperature above the standard PVC insulation’s 160oo

The transient current rating (for t <5s) is proportional to the conductor’s CSA and inversely proportional to the square root of the duration, i.e. the current rating for duration t is the 1s rating divided by the square root of t.

Applying this guide to our example trace above - 0.18mm (7 thousands of an inch) wide in 1oz (finished) copper (35μm), subjected to a rectangular current surge lasting 40μs - assuming the normal ambient temperature is 25oo

To protect a trace or PCB from an overcurrent that exceeds the values given by the above guides, we might want to use a fuse. But even the fastest fuses cannot respond in less than 10ms (which is a very slow transient or surge in EMC terms), and all fuses have tolerances on their time-to-opening values, so it is important to design for the slowest of the fuse type that will be used.

When calculating the maximum permissible current for normal operating temperatures above 25oooooHigh-temperature cable insulation materials, and grades of FR4 and other PCB dielectrics are available, and using them allows conductors to carry higher transient or surge currents without damage. On the other hand, some types of PCB dielectrics (especially some low-loss and microwave types, increasingly used in modern PCBs) can have lower short-term temperature ratings than FR4, so they could only withstand lower transient or surge currents than the above guide.

It is best to use ’UL Recognised’ or ’UL Approved’ PCB materials, especially to prevent safety hazards from fire, smoke and toxic fumes, so always obtain and check the UL Approval certificate for the basic PCB material used (or other evidence). Some suppliers don’t always deliver what they said they would, so check that all delivered PCBs have the appropriate UL logo stamped all over them.

3.5.3 Protecting electromechanical contacts from surges

Overvoltages can cause electromechanical contacts to spark-over, and since the resistance of an arc-channel is low, this can apply power to circuits that should be off. The high currents during a surge can also cause electromechanical contacts to weld together, so they might not open when supposed to. Either of these effects can cause erroneous operation or malfunction, even safety risks in some cases.

Spark-over is prevented by using large contact gaps and or SPDs (see below) to limit the maximum voltage. Some types of switches and electrical contactors are available with contact gaps up to 8mm, which will generally withstand nearly 8kV. Most switches and relays will change their mechanical state even when one or more of their contacts are welded, and the solution here is either to divert the surge current away from the contacts using SPDs (see below) so they do not weld, or to use devices with positively guided contacts, which will only change their mechanical state if all the contacts also change state. Automatically operated switches, relays and contactors with positively-guided contacts must be combined with position sensing to detect welded contacts and prevent problems from resulting.

3.5.4 Galvanic isolation is the best defence against surges

Various techniques are available for achieving galvanic isolation that completely prevents surges from entering equipment, including.

  • High-voltage opto-isolators or opto-couplers, fibre-optic links
  • Wireless, infra-red, free-space microwave or laser voice or data communications
  • Motor-generator sets
  • On-line continuous-conversion double-conversion uninterruptible power supplies (UPSs) employing isolation transformers

Of course, the part of the galvanic isolation device that is connected to the surge-exposed circuits must themselves be resistant to the surges they will experience (e.g. the mains input circuitry of a UPS).

The above provide protection against CM and DM surges. Isolating transformers are commonly used for mains power (e.g. in linear or switch-mode DC power supplies) or signals (e.g. Ethernet), but they only provide galvanic isolation for CM surges, as shown in Figure 3AF. Even so, because normal mains transformers are not wound for good balance at RF, there is conversion from the CM surge on the primary to DM surge on the secondary [17].

Keith Slide 30

Figure 3AF CM surge suppression with isolating transformers

DM surges are rare on most types of signal (e.g. analogue signals or digital data carried by twisted-pair cables) but are common on DC or AC power, and can pass straight through an isolating transformer to the protected circuits, although if the transformer is a step-down type, the overvoltage will generally be stepped down too.

 

However, if the DM surge voltage exceeds a level that saturates the magnetic core, this excess will not be transformed to the secondary. ’Constant voltage transformers’ (CVTs) run their cores in saturation (and run hot as a result) so they are effective at suppressing DM surges.

Increased attenuation of CM surges, especially their higher-frequency spectral content, can be achieved through the use of high-isolation transformers, for example using separated primary and secondary windings to reduce the stray capacitance and stray mutual inductance in the air between them, even placing them on separate limbs of the transformer core.

Another technique is to use an interwinding shield, but this needs to be connected to a RF Reference (often misleading called earth or ground), with the shield, Reference and their interconnection all having a very low impedance at the frequencies to be suppressed. The RF Reference for an item of equipment is usually an appropriately-designed chassis or metal enclosure (see 3.2.3 and 3.3 above), but if the equipment is a PCB in an unmetallised plastic enclosure it could be the PCB’s 0V plane (see Part 5 of this series).

[17] gives more details on the use of transformers to suppress surges, and states that ordinary isolating transformers are not very good at surge suppression, even if fitted with electrostatic shields.

3.5.5 Surge suppression with filters

Filters are often found following the SPDs described below, and are not often recommended for suppressing surges on their own. However, they can be quite effective in either application providing issues of resonance and voltage/current/energy handling are taken into account. Figure 3AG shows some examples of low-pass filters used for suppressing surges. The spectra of many surges have high amplitude at low frequencies (e.g. a few kHz), so simple filters like the single-pole types shown in this figure need to have low corner frequencies, often just a few tens or hundreds of Hz, to provide sufficient attenuation of the surge.

Keith Slide 31

Figure 3AG Surge suppression with filters

Filters do not dissipate the energy of a surge - they aim to convert the waveforms from spikes with high peak values, to gentle ’bumps’ with lower peaks that are more easily survived by the protected circuits. The areas under the voltage/time and current/time curves stay much the same, as a surge passes through a filter.

But all LC filters are resonant circuits, and there is the possibility that most of the energy in a surge might emerge from a filter as a burst of damped sinusoidal waves at the resonant frequency. The peak amplitude of such a burst could be much higher, even as much as ten times higher, than the peak of the original surge voltage, and the author has seen examples of this effect in real life. Even where filters are not supposed to be providing surge suppression, if they connect to external cables (especially mains cables) they will always be exposed to surges, and it is important that at least they not make a surge overvoltage worse.

Keith Slide 32

Figure 3AH Filter resonances can increase peak voltages

So all filters that could be exposed to surges, or are intended for surge suppression, should be carefully designed not to exhibit significant resonances in the frequency range of concern (the spectrum of the energy in the surges), and/or be damped (see 3.2.9 and Figure 3Q, and 3.2.10 and Figure 3R) so that at least they don’t increase the overvoltage. RC filters do not suffer from resonances, and can easily be used for surge suppression in low-power circuits. When soft ferrites are used as the L element in filters this dampens down resonances at higher frequencies, but at the lower frequencies they have a significant inductive reactance, so cause resonances.

Like all filters, filters used for surge suppression need RF References, and interconnections to them, that have very low impedances at the frequencies to be suppressed (see 3.2.3, 3.3 and 3.5.5 above).

When a filter is used to suppress surges on AC power conductors, its cut-off frequency should be chosen to avoid excessive levels of power-frequency currents in the filter capacitor, that could cause it to heat up and reduce its reliability. If used to suppress line-to-earth surges the filter capacitor powerline currents should not cause the equipment’s protective conductor or touch current to exceed the levels set by the relevant safety standards. These issues compromise the design of a filter, making it less effective on certain kinds of surges on AC power conductors.

But these corner frequency compromises do not apply when filtering DC conductors, or where AC power is converted to DC in the rectifier of an off-line mains power supply unit (PSU). The storage capacitor that follows a rectifier will provide some DM surge suppression - providing it can handle the surge voltages and currents without reducing its life by too much. But adding an inductor on the AC side of the rectifier, as shown in Figure 3AJ, adds a filtering function that can be designed to improve the attenuation of the surges, whilst also reducing the voltage and current stresses on the storage capacitor.

Keith Slide 33

Figure 3AJ Using the input circuit of a PSU as surge protection, by adding a series inductor

It is quite common in industrial applications to fit such external inductors in the AC power inputs to electronic equipment such as variable-speed motor drives, to improve their reliability by increasing their surge protection as shown in Figure 3AJ. These inductors are usually called ’line inductors’ or ’line reactors’, and they also help reduce the emissions of mains harmonics from the electronic units’ AC power rectifiers (sometimes this is the primary reason for their use, and the surge suppression is a bonus).

When designing a filter for surge suppression, in circuits similar to those in Figures 3AG and 3AJ, the filter components need to be rated appropriately for their high-voltages and high-currents, including.

  • Choose the value of capacitor so that it can absorb the total charge of the highest-energy surge without its voltage rising so much that the following circuit is damaged. A useful guide is CΔV = IΔt, where ΔV is the change in capacitor voltage caused by the current I flowing for time Δt. Because surges never have rectangular waveforms, some assumptions and/or additional calculations are necessary when using this guide.
  • Choose the capacitor voltage rating so it is not damaged itself by its voltage rise during the highest-energy surge
  • Design the inductor and capacitor as a low-pass filter that attenuates even the slowest surge rising edges by enough to protect the following circuits from damage
  • The capacitor’s ESR and ESL should be low enough to maintain good attenuation whilst absorbing the highest surge currents
  • Use a type of capacitor that will handle the highest current and fastest surges expected, it may be necessary to use specially ’pulse-rated’ capacitors, rather than types intended for storage, filtering or decoupling
  • Filter series elements (inductors or resistors) must not spark-over due to the high-voltage (this makes it difficult to use surface-mounted components)

3.5.6 Suppression with surge protection devices (SPDs)

SPDs are high-resistance devices that switch to a low-resistance state above a certain voltage. They are connected in parallel (shunt) with the circuit to be protected to limit the overvoltage it is exposed to, and to divert the surge currents away via a different route. SPD techniques can be used as alternatives to the methods described above, or in conjunction with them.

The choice of SPDs and design of the circuits that use them is far from trivial, with a number of issues to be dealt with to achieve reliable protection at a reasonable cost. For example, SPDs are non-linear devices, so adding them to a circuit can add EMC problems. Some may need filtering to prevent them from demodulating RF, and gas-discharge or spark-gap types create RF noise when they operate, which may need filtering to prevent interference.

Figure 3AK shows some typical examples of the use of SPDs. When used in conjunction with an isolating transformer, the CM surges are suppressed by the primary-secondary insulation (see 3.5.4), and the DM surges are suppressed by the SPD connected between the lines (phases). Three-phase power systems would need three SPDs to suppress their DM surges, connected either in star or delta.

Keith Slide 34

Figure 3AK Some surge protection circuits using SPDs

The lower circuit in Figure 3AK shows SPDs used for both CM and DM suppression of a single-phase AC power supply. An important safety consideration is that all SPDs fail eventually, due to wear-out, and when they do those connected between line and earth/ground (CM suppression) can cause hazardous levels of leakage currents (usually called ’touch currents’ or ’protective conductor currents’ in product safety standards).

So, for safety reasons, SPDs between line and earth/ground are only used (when used at all) in permanently-wired fixed (stationary) equipment, such as mains distribution cabinets, and never in portable or pluggable equipment.

Of course, a surge that is energetic enough to cause an SPD to explode will result in an open-circuit failure, and this can be a consequence of not choosing the right device ratings for their EM environments. Other types of devices can also explode when exposed to surges they cannot handle, and this is why appropriate safety precautions should always be taken when testing with surges - always placing a blast-proof screen around the tested unit so that no-one can be injured by ejecta from the equipment being tested, or their ricochets. It is also a good idea to have a power isolation switch in the blast-protected area, and ready access to fire extinguishers suitable for electrical fires.

It is possible for surges that are below the threshold of activation of the SPDs to cause more damage than surges with higher voltages. Surges below the threshold voltage can source all of their current into to the circuitry being protected, possibly causing damage. Low-voltage surges are much more common than high-voltage ones, so if the SPDs are not activating at low enough voltages the consequences for reliability can be severe.

Most people assume that testing with the most severe surge is the worst-case, and avoids the need to test with lower levels, but this is not the case, and to check that a design is going to be reliable enough it is always recommended to test with surge voltages that are just too small to activate the SPDs.

3.5.7 Types of Surge Protection Device (SPD)

SPDs have a voltage-dependant resistance. When the voltage exceeds their rating, their resistance falls rapidly so they carry some (or all) of the surge current whilst maintaining a low level of voltage between their terminals. They are highly stressed components, and degrade (wear out) and fail after a number of surges, depending on the type of device and the energy and number of the surges - so it is very important to choose appropriate SPD types and ratings for the surges expected in their EM environment.

There are four basic types of SPD: Metal oxide varistor (MOV); Avalanche Diode; Spark gap / Gas Discharge Tube (GDT); and silicon controlled rectifier (SCR), as shown in Figure 3AL along with some of their common schematic symbols.

Keith Slide 35

Figure 3AL Types of SPDs

Metal oxide varistors (MOV)

These are low cost, inherently bi-directional devices that operate in about 1ns, available in a very wide range of energy ratings, from small devices for PCB mounting to very large devices for primary lightning protection. MOVs are available in a very wide range of voltage ratings, from a few tens to hundreds of volts, and as discrete devices (see Figure 3AL) or arrays in IC style packages.

Their natural wear-out mode is to degrade gradually to a low-resistance, even short-circuit, and they can degrade very quickly indeed if not rated correctly. MOVs naturally have a high capacitance, generally measured in nF, making it possible to combine filtering and overvoltage protection in one device, but making them unsuitable for use on RF and high-data-rate signals.

Avalanche diodes

These are like zener diodes but with very high transient current and power ratings. They are available only with low voltage ratings, from a few volts to a few tens of volts. Uni- or bi-directional types exist, so it is important to choose the correct type. They are available as arrays of devices in IC package styles, as well as discrete items such as the type shown in Figure 3AL.

They are the fastest type of SPD, operating in well under 1ns so can also be used for ESD protection, but are relatively expensive and tend to have low current and energy ratings. Like all semiconductors they can fail short or open, and like all SPDs they do wear out eventually. Proprietary types include ’Transil’, ’Transorb’ and ’TVS’. (TVS is often used as a generic name for these devices, but is short for ’Transient Voltage Suppressor’ - so generic a term that it could be used to mean any kind of SPD.)

Avalanche diodes historically have had quite high capacitances, measured in 100s of pFs, but because of the need to protect the recently developed USB2.0, Firewire and similar high-speed datacommunication interfaces, versions with capacitances of 5pF or are now available.

Gas Discharge Tubes (GDT)

These are low cost, inherently bi-directional devices that are available in a very wide range of energy ratings, from small devices for PCB mounting to very large devices for primary lightning protection. They are available with voltage ratings (trigger voltages) from around 100V to kV.

They have very low capacitances, usually just a few pF, which makes them very useful for protecting RF and high-speed signals, and they have been used to protect radio transmitters and receivers from overvoltages on their antennas for many years. They can eventually degrade to a short-circuit, as their electrodes vaporise and plate the inside of their packages, and this can happen very quickly if they are not rated correctly.

GDTs have to reach a trigger voltage before they start to conduct significantly, but once the discharge is ’ignited’ it creates an arc, which has a very low resistance and then the GDT has a very low terminal voltage at any level of current (SPDs are typically measured with pulses of 1kA, or more).

The arc created inside a GDT when it ’strikes’ makes them glow a very pretty violet colour, but arcs are hot and take a little time to cool down enough to restore the high-resistance (’off’) state. This can mean that when used to protect AC power, there might not be enough time for them to cool down between mains cycles - so once turned on, they might never turn off. However, some manufacturers offer GDTs designed for rapid arc quenching, so that they do not restrike after the mains zero-crossing. Most distributors’ catalogues and some manufacturers’ data sheets do not make it clear whether their GDTs are suitable for use on AC mains supplies, so always check.

Another consequence of the arc in a GDT is that when used to protect AC mains supplies, an SPD that is in a low-resistance state carries mains current as well as surge current. In the case of GDTs and spark gaps these currents can continue for up to 10ms (half a cycle of the mains) after the surge has finished, until the arc is quenched by the zero-crossing of the AC mains waveform (but see earlier comments on choosing GDTs suitable for use on AC mains). These are known as ’follow-on’ currents, and the SPD’s conductors need to be sized to cope with them safely and reliably. It is possible for power frequency overvoltages (see 3.4 above, and Figure 3AD) to be caused by the earth-lift (ground-lift) resulting from follow-on currents in line-to-ground connected GDTs.

GDT trigger voltage is imprecise, so special three-terminal types are used for protecting differential signal lines (e.g. telephone lines). If two ordinary two-terminal GDTs were used, one would trigger before the other, causing the CM surge on the line to be converted into a DM surge - potentially more damaging to the following circuitry than the CM surge would have been. The three-terminal GDTs are designed so that both sides of the device trigger at the same instant.

Spark gaps

These are simply a point where two conductors come close together, spaced apart by a suitable distance so that a surge voltage above the gap’s breakdown voltage will cause it to spark over, thereby creating a low-impedance (the arc channel) that diverts the surge energy. The breakdown voltage of the air at sea level and 50% humidity is approximately 1kV/mm, but variations in humidity and pressure have a significant effect so it is impossible to design spark gaps so that they operate with any precision.

Spark gaps created between two traces on PCBs, or two conductors on a plastic or ceramic substrate, suffer additional variations in trigger voltage due to contaminants on the surfaces of the PCB or substrate. Another problem with spark gaps is that their electrodes tend to get further apart due to melting when they operate, increasing their trigger voltage over time; they can splash molten metal onto other conductors causing short-circuits; and if mounted on organic substrates (PCBs, plastics, etc.) their arcs can cause carbonisation, and they can become resistive and start to dissipate power, possibly leading to malfunctions, even overheating and fire.

Some parts of the world suffer from very intense thunderstorms, and the author knows of some Australian manufacturers who, having had rapid wear-out problems with GDTs, construct their own spark gap protection devices out of plates of metal.

A GDT is essentially a spark-gap in a controlled atmosphere, so that it has a repeatable trigger voltage. A spark gap behaves just like a GDT, except that it has an unreliable and unpredictable trigger voltage.

Silicon Controlled Rectifier (SCR)

These are available as uni-directional (based on thyristors) or bi-directional (based on triacs), so it is important to choose the correct type. They tend to have high current and energy ratings for their package size and cost, and are available as arrays of devices in an IC package as well as discrete items such as the type shown in Figure 3AL. Like all semiconductors they can fail short or open, and like all SPDs they do wear out eventually. Proprietary devices include ’Surgector’, ’Sidac’, ’Sibar’ and ’Trisil’.

SCR types have to reach a trigger voltage before they start to conduct significantly, but once they start to conduct they switch to a low resistance state which persists until the current through the device drops below some ’holding’ value.

3.5.8 Characteristics and comparisons of SPD types

Figure 3AM shows the voltage/current characteristics of MOV and Avalanche Diode devices, which are often called ’voltage-limiting’ types, because their terminal voltages increase much more slowly as current increases, once they pass their threshold voltage.

The figure is drawn assuming the threshold voltage for the MOV is the same as that of the Avalanche device, to show that the Avalanche type generally has a lower on-resistance, so for a given surge current it generally has a lower terminal voltage and provides better protection (in this regard, at least).

Keith Slide 36

Figure 3AM MOV and Avalanche Diode characteristics

Figure 3AN shows the voltage/current characteristics of GDT and SCR devices, which are called ’voltage-switching’, ’foldback’ or ’crowbar’ devices, because once they reach their triggering voltage their terminal voltage suddenly decreases, and stays very low even with very high currents in the device.

An important issue for these types of SPDs is the minimum system DC voltage. If the clamping voltage of the device is the same or lower than the system’s DC voltage, the SPDs will never switch off - a condition that should generally be avoided, not least because without appropriate protection the SPD could overheat, be damaged, emit toxic fumes or even catch fire.

Keith Slide 37

Figure 3AN Spark-gap / GDT and SCR characteristics

Figure 3AP compares the generic performance of the various types of SPDs against a time axis, as they are activated by an example surge voltage. MOV and Avalanche devices act like zener diodes, with a threshold or knee voltage where they start to draw current. As their current increases their clamping voltage rises, sometimes to more than double their knee voltage. The figure also shows that GDT and SCR devices have to reach a trigger voltage and during this time they could let through surge voltages that could be high enough to cause damage. Then they foldback (or ’crowbar’) and the voltage across them drops to well below the voltage they previously would have blocked. The figure also shows the minimum system DC voltage that was discussed uinder Figure 3AN.

Keith Slide 38

Figure 3AP Comparing the time-domain performance of SPD types

3.5.9 Minimising the inductance in series with SPDs

When surge currents flow in the inductance inherent in any conductors that are in series with an SPD, they increase the ’let-through’ voltage. It is best to connect the incoming power (or signal) directly to the terminals of its SPD, and then connect the protected circuitry to the SPD terminals too.

Just as for filter capacitors, it is important that SPDs have wires or PCBs trace layouts that add just the minimum inductance in series with the device. It is best to take the conductors (wires or PCB traces) for the circuit to be protected directly to the SPD’s terminals, and then route the conductors from the SPD’s terminals to the protected circuit.

Where wires are used to connect to a GDT, and it is not possible to avoid using a long ’spur’ connection to the protected circuit, the wires should be as short as possible and twisted as shown in Figure 3AQ.

Keith Slide 39

Figure 3AQ Reducing SPD let-through voltage by minimising series inductance

3.5.10 Rating SPDs

The lightning protection systems fitted to most sites are not intended to protect electronics. On such sites, and on those without any lightning protection at all (e.g. most residential properties that are not high-rise apartments), real-life AC mains supply surges can be expected to reach at least ñ6kV several times each year. At sites fed by overhead mains cables, the number of such events can be as high as several hundred a year, according to figures collected in the UK (which is hardly a high lightning incidence area).

Figures 3AR and 3AS show some statistics that have been collected for peak voltages on AC supplies in the USA. Similar statistics apply in most of the developed countries, but in less well-developed countries the incidence of surges can be higher.

Keith Slide 40

Figure 3AR Power line surge exposure chart (from IEEE C62-41:1991)

Keith Slide 41

Figure 3AS "How many times will the mains attack your data?"

Where equipment could be operated on a private mains supply (e.g. hospital equipment expected to work on local generators during power cuts, and tested every week), it might have a quite different surge exposure. For example, there are standards, reports and conference papers describing the types of surges that should be expected in the AC and DC power supplies used in land, sea, rail, air and space vehicles (these are not listed under the EMC Directive or tested by the IEC/EN 61000-4 series of standards).

6kV "or higher" surges are permitted by EN 50160, the European standard for the power quality of public mains supplies, and the author’s experience is that such high voltages reliably occur over most of Europe. In fact, a peak surge voltage of 6kV is typical of single-phase public mains supplies worldwide, and is this value because it is the typical spark-over voltage at the terminals of typical single-phase mains sockets. The single-phase mains sockets are acting as accidental spark gaps, protecting the equipment connected to the mains supply from overvoltages higher than approximately 6kV.

In dedicated three-phase mains distribution systems, where there are no single-phase plugs or sockets, the clearance between terminals is greater (e.g. between the terminals in IEC 309 style mains plugs ands sockets) and peak voltages of at least 12kV should be expected according to certain manufacturers who have had to design to this level to solve real problems in the field. Maybe as much as 20kV could be possible.

The venerable lightning protection standard BS6651 Appendix C (which will be replaced by EN 62305-4:2006 in August 2008) deals with this issue, and specifies the SPD ratings for equipment fitted in different parts of a site. This standard, or others which deal with the lightning protection of electronic equipment (e.g. IEEE C62.41-1991, IEC 61312-1) should be used where the EMC test standards applied are lacking in surge requirements, or where their surge requirements are incomplete, or where there is concern that the use of these ordinary EMC test standards might not give sufficient protection for the desired level of reliability in the intended environment.

When a product is adequately protected against lightning surges, it is generally protected well enough against common surges generated by other means, such as switchgear. But some industrial or medical environments can suffer from significantly high levels of mains surges that are not caused by lightning, and these could possibly also have a higher rate of occurrence. Such sites include superconducting magnet or power generation applications, and high-power switched reactive loads such as very large motors or transformers.

To achieve adequate reliability of equipment, hence low warranty costs, satisfied and loyal customers, increased levels of repeat sales, and a virtuous circle leading to greater profitability, it is important to rate SPDs to handle the current and energy related to the likely surge exposure at the intended operational sites. It will not generally be sufficient for reliability to rely on testing at the 1 or 2kV levels specified by the EMC test standards listed under the EMC Directive.

Taking the above into account, the maximum ratings of SPDs should be carefully chosen for:

  • The maximum clamping voltage when activated, when carrying the maximum current
  • Surge energy handling, in Joules
  • Peak current handling, in kA
  • The number of peak current or peak energy events that can be handled before the device degrades too much
  • Continuous power rating in Watts, especially where surges are frequent, or where power-frequency overvoltages or power-cross incidents can occur (see 3.4 and 3.5.12)

3.5.11 Combining SPDs

It is sometimes hard to obtain all the necessary characteristics in one device, and to maximise surge protection performance and optimise cost, it might be necessary to combine different types of SPD. In the example shown in Figure 3AT the high power handling of a GDT is combined with the fast clamping action of an MOV. The GDT absorbs most of the surge energy - but only after it has triggered. The series inductor allows the GDT to be triggered by overvoltage, while the MOV protects the load against the GDT’s trigger voltage.

Keith Slide 42

Figure 3AT Example of combining a GDT with an MOV

Simply paralleling a voltage-switching (crowbar) type of SPD with a voltage-limiting type such as an MOV usually prevents the crowbar device from triggering. There are a wide range of surge protection units available from lightning protection companies, and their proprietary designs often include series inductors where they combine voltage-limiting with voltage-switching SPDs.

Devices are available (e.g. TISP from Texas Instruments) that behave in Avalanche Diode mode up to the point where the voltage reaches a trigger level, at which point they change to SCR mode and crowbar the voltage to a lower level.

3.5.12 A hierarchy of surge protection

Full protection from surges when using SPDs usually requires a hierarchy of devices. High-energy SPDs such as large GDTs or very large MOVs are fitted at the incoming mains supply to the building or other structure. Medium-energy SPDs, such as MOVs, are installed in local mains distribution cabinets or at the mains inputs of equipment enclosures. Finally, low-energy (but fast) SPDs are fitted where needed to the PCBs inside equipment to protect electronic devices that interface with external cables. The PCB-mounted SPDs are often designed to protect against ESD as well, during (unpowered) assembly, as well as during operation. See Part 6 of this original series [1] for more on designing this type of protection. Part 6 of this revised series will be published during 2007.

For more on using a hierarchy of SPDs on a site, refer to Chapter 9 of "EMC for Systems and Installations" [13].

3.5.13 Protecting SPDs

Some faults in power networks can cause the supply voltage to increase significantly (even nearly double) for a few seconds. In some cable applications mechanical damage can short cables together, applying mains power to signal circuits, which is why telecommunications equipment has to withstand ’power cross’ tests that apply 230V AC for several minutes onto their signal cables. Where SPDs are used, and the increased voltages are sufficient to trigger them, the very long durations of these overvoltages will cause them to dissipate large amounts of power, causing overheating, damage, possibly even toxic fumes, fire or explosion.

Although SPDs can handle kA and kW, they can only do so for very short periods of time. Their total energy ratings are quite small - for example a surface mounted MOV in an 0603 package might be rated at 0.1J; a 7mm diameter radial wire-leaded MOV might be 3J, and a 60mm square block-type MOV with spade terminals might be 500J. But these ratings are achieved with test pulses that last no more than 1ms, when even a 3J device can handle 3kW. But even a 500J device can handle no more than 10W for 50 seconds, so it is clear that SPDs need to be protected from overheating.

Figure 3AU shows the principle of using a (surge-rated, high-voltage, high-wattage) resistor, PTC thermistor, fuse or circuit-breaker in series with the input to an SPD, to prevent unreliability or damage caused by overheating.

Surge-rated resistors are special types that can handle very high levels of overcurrent (for a short time). They are manufactured by several companies, and may also be known as pulse or transient rated resistors. Some types, known as fusible resistors, are designed to open-circuit to prevent their thermal rating from being exceeded, helping to avoid smoke and fire hazards.

Keith Slide 43

Figure 3AU Protecting SPDs from overheating due to power-frequency overvoltages

As already mentioned, SPDs are highly stressed components and wear out eventually (quite quickly, if not rated adequately for the surges in their EM environment). So another reason for using resistors, PTCs, fuses or circuit breakers in series as shown in Figure 3AU, is to prevent fire hazards arising if the SPD fails low-resistance (as MOVs always do, unless they are removed from the circuit by ’explosive disassembly’). Some manufacturers offer MOVs that include thermal protection in their packages (e.g. TMOV and TPMOV products, see [18]).

SPDs can carry very large currents whilst suppressing surges, even kA, and it might seem that passing this current through a PTC, fuse or circuit-breaker is bound to cause it to operate. Of course, it would not be good if an equipment’s mains fuse (for example) opened every time its SPDs suppressed a surge. But it is not current alone that operates PTCs, fuses or circuit breakers - it is current x time, and co-ordinating of the current/time ratings of the fuses (etc.) with the SPD ratings (for example, using time-delay fuses) allows SPDs to suppress surges and be protected by the fuses, without suffering fuse reliability problems. More detail on how to coordinate fuses and SPDs is given in [18], which also describes a type of fuse specifically designed for use with SPDs.

Figure 3AV shows the normal method of fusing an SPD, and the principle applies equally well to SPDs protected by PTCs or circuit-breakers, see Figure 3AU. The fuse that protects the SPD is in series with the conductor that provides power or signal to the protected circuit. SPD failure or overheating protection will open the fuse, disconnecting the equipment from the input, making it easy for the user to discover that the fuse has opened and needs replacement. Replacing the fuse when an SPD has failed just makes the fuse open again, so the user knows to have the SPD replaced.

Keith Slide 44

Figure 3AV Examples of fusing SPDs

Figure 3AV also shows another method, in which the SPD is fused separately from the equipment. This method is sometimes used for critical equipment, on the (mainly erroneous) assumption that if SPD fuse opens the equipment will keep operating, although it will not be protected against another surge. But this is not a generally recommended method for ensuring that critical equipment keeps functioning.

An SPD is most likely to fail whilst it is carrying a heavy surge current, and when the fuse opens this will cause a large flyback voltage that could well damage the equipment, especially if it adds to the surge voltage. Also, during a thunderstorm it is quite likely that several surges will occur, so if one of them has caused the SPD to fail there is quite a good chance that another surge will be along in a few seconds to damage the equipment.

Instead, it is generally much better to use the normal method of fusing, and if the operation of the equipment is critical, to use diverse power sources or add a UPS or other power back-up facility (such as a battery) to keep the equipment running when a mains fuse has opened.

Some proprietary surge-protection units use many SPDs of the same type, connected in parallel and individually fused, so that any that fail short-circuit do not prevent the unit from continuing to protect the equipment, and do not cause the mains fuse that powers the protected equipment to open. This is an application of the second method of fusing that is generally quite acceptable, providing the failed SPDs are replaced before the unit ceases to provide surge protection. Some units of this type incorporate condition monitors and indicators, to encourage users to get them refurbished before all protection is lost.

3.5.14 Equipment reliability and maintenance issues

When protecting AC power inputs (e.g. to off-line DC power converters) from surges using voltage-limiting SPDs, if a resistor is used in series with the input and a long-duration power-frequency overvoltage occurs - the voltage peaks will be clamped but the protected circuit will still have an AC power input (although very distorted) and will still function. However, using a crowbar SPD or PTCs, fuses or circuit breakers will remove the AC power and so the protect circuit will no longer function.

As long as power-frequency overvoltages that activate the SPDs are rare enough, losing the functionality of the circuit is generally not a problem (assuming this does not increase safety risks). But some AC supplies suffer from quite common ’swells’ or voltage fluctuations, and it would generally be unacceptable for equipment to frequently stop working, especially if it meant replacing a fuse or resetting a circuit breaker to make it work again.

When designing a surge protection it is tempting to choose SPDs that have as low a threshold/trigger voltage as possible, to provide maximum protection to the following circuit. But this increases the likelihood that commonplace swells and fluctuations will activate the SPD overheating protection devices and cause the circuit to stop working. >/p>

A particular problem is equipment designed in countries where they are used to a 220V or 230V mains supply, but used in the UK where the nominal mains voltage is still 240V despite being called 230V. In rural parts of the UK it is not uncommon to have 245V as the normal mains voltage. An overheat protection circuit for an SPD that has quite a low threshold/trigger voltage might have an acceptable rate of activation in mainland Europe, but cause reliability problems in the UK.

Designers should also be aware that where local generation is used, in many third-world countries, and even in some areas in more developed countries (even parts of the USA and Australia, for example) the mains supply voltage and waveform can be very poorly controlled indeed. Designers who assume a mains supply of 230V +/-10% with a nice sine waveshape can find their products failing frequently when used in parts of the world that don’t have such well-controlled mains supplies.

So, for example, when designing an off-line switching power converter for use on 230V mains supply, instead of using 600V rated PowerFETs or IGBTs and trying to protect them with GDTs that trigger at 350V (the peak of a 240V pure sinewave is 339.4V) - a more reliable and robust design can be achieved, with less effort, using 900V transistors and GDTs that trigger at 600V.

SPDs eventually fail, and if they fail short-circuit they will generally be detected through their effect on the operation of the protected equipment, for example, by opening a mains fuse as discussed in 3.5.13. But they might also fail open-circuit, in which case the protected equipment might keep functioning as normal but no longer be protected from surges.

So maintenance is always important where SPDs are employed. Some types of proprietary surge protection units are available with condition indicators that inform the user if they are healthy, need repair but are still protecting, or are not providing surge protection any more. If using such units, it is important to have a reliable procedure that ensures the condition indicators are checked often enough, and that any units that need it are repaired.

But where ordinary SPDs are used, a procedure will be needed for checking the state of its surge protection at regular intervals, to be sure of replacing failing SPDs before their protection is lost completely. The designer of the equipment needs to design so that this can be easily done, and make sure that all corresponding user maintenance instructions are written and communicated to the users.

3.5.15 Surge protection products

Figure 3AL showed some examples of small SPD devices. There are a very large number of surge protection products available that use SPDs to provide protection for power, signals and data in every different application, from protecting the three-phase supplies entering large buildings from direct lightning strike, to protecting telephone, radio and Ethernet circuits.

Figure 3AW shows some examples from a very wide range of proprietary surge products offered by numerous manufacturers. Industrial cabinets often use DIN-rail mounted protection units for protection AC supplies, for their ease of wiring. But such units are unsuitable for protecting RF or high-speed signals, so a range of surge protection units is available in different styles for these applications, as shown in the figure.

Keith Slide 45

Figure 3AW Some examples of SPD products (from Phoenix Contact)

3.5.16 ’Earth lift’ problems in systems

So far, the above has discussed surge protection as it applies to an individual ’port’ (e.g. a connection to a cable) on an item of equipment. But additional issues arise in systems, especially the phenomenon of ’earth lift’ or ’ground lift’ - caused by surge currents flowing in an earth/ground structure that is shared between several items of equipment.

A related issue occurs when surge currents flow in the neutral lead of an AC supply, or the return lead of a DC supply, that is shared between several items of equipment.

Figure 3AX illustrates the problem - all conductors and conductive structures have an impedance, that is predominantly inductive above a few kHz. When kA surge currents with fast rise-times are allowed to flow in them (either from SPD operation, or a spark-over to a chassis due to inadequate insulation) significant potential drops arise. These potentials arising in the common earth/ground structure then expose the electronic devices associated with interconnecting cables to surge voltages.

In a typical building with wired earth/ground structures, the resulting differences in earth/ground potential between two items of equipment can even approach the voltage of the initial surge. These voltages are called ’earth-lift’ or ’ground-lift’, and values up to 10kV are not unknown in large buildings. They are CM voltages, and can cause damage or interference problems for circuits connected to any power, signal, control or data cables that interconnect different items of equipment.

Keith Slide 46

Figure 3AX ’Earth lift’ surge issues in systems

For example, a single straight wire in air (such as a green/yellow insulated earth/ground wire) has an inductance of about 1μH per metre, and a typical test that aims to simulate commonplace uni-directional surges has a peak current of 1000A with a current risetime of 10μs. Since V = -LdI/dt the earth/ground wire develops a longitudinal voltage (along the length of the wire) of 100V for every meter of its length.

Most systems and installations still use the outdated single-point earthing/grounding principle, that could have been designed to cause maximum surge voltage exposure to equipment [19], so lengths of earth/ground cable 10m long are not unusual, leading to 1kV earth-lifts with the above surge current waveform.

So a consequence of using SPDs in equipment - rather than relying on galvanic isolation (see 3.5.1 and 3.5.4) - is that when that equipment is used in systems and interconnected by signal, control or data cables to other equipment some distance away, it will often be necessary to provide surge protection at all of its ports.

This issue is often overlooked because of the tendency to assume earths and grounds are perfect conductors, with no impedance. Solutions include:

  • Not connecting any SPDs to the equipment chassis, frame or earth/ground. Instead, use galvanic isolation (e.g. isolating mains transformers, fibre-optics, etc.), plus adequate creepage and clearance distances between the incoming/primary circuits and the earth/chassis/frame/etc.
  • Insulating sufficiently to prevent spark-over to the chassis, frame or earth/ground
  • Reducing the impedance of the protective earthing/grounding system (sometimes known as the common bonding network: CBN), e.g. by connecting the chassis of the interconnected items of equipment together using short lengths of earth wire, or metal structures
  • Protecting signal data and control inputs and outputs from damage, using galvanic isolation, filters and/or SPDs

The ideal earthing/grounding system for controlling earth-lift is a mesh (’MESH-CBN’) as described in IEC 61000-5-2. Such structures are commonly used in large computer/telecom installations, and are recommended by their relevant IEC standards and ITU Recommendations. Ships and offshore oil/gas platforms are generally made completely of welded or riveted steel sheets, which can be used as an ideal earth/ground by bonding all the equipment directly to it.

3.5.17 Data needs error detection/correction

SPDs on data lines only protect the devices from damage, they do not prevent false data from occurring during a surge. So data lines exposed to surges also need to use a good error-correcting protocol, e.g. as used by Ethernet or CAN bus. The very best bus may well be the military ’1553’ bus, versions of which are now available in civilian guise.

It is not recommended that designers try to create their own error-correcting protocols. Surges and similar transient phenomena occurring in real life are usually not as ’clean’ as the waveforms used in the EMC tests supposed to simulate them, and typical protocols that are well proven to be robust in real life often have as much as 50 man-years of experience in robust communications behind them. The best advice is to purchase the devices and/or software that implement a proven robust communications protocol, rather than try to design one.

3.6 References

[1] Keith Armstrong, "Design Techniques for EMC", UK EMC Journal, a 6-part series published bimonthly over the period February - December 1999. An improved version of this original series is available via the "Publications & Downloads" page at http://www.cherryclough.com
[2] The Institution of Electrical Engineers (IEE), Professional Network on Functional Safety, "EMC and Functional Safety Resource List", via the "Publications & Downloads" page at http://www.cherryclough.com
[3] Arthur B Williams, "Electronic Filter Design Handbook", McGraw Hill, 1981, ISBN 0-07-070430-9
[4] John R Barnes, "Robust Electronic Design Reference Book, Volume I", Kluwer Academic Publishers, 2004, ISBN: 1-4020-7737-8
[5] Sokal, N. O., System Oscillations From Negative Input Resistance at Power Input Port of Switching-Mode Regulator, Amplifier, DC/DC Converter, or DC/AC Inverter, IEEE Power Electronics Specialists Conference (PESC) 1973 Record, pp. 138-140.
[6] Keith Armstrong, "Design Techniques for EMC, Part 2 - Cables and Connectors", The EMC Journal, May and July 2006, available from http://www.compliance-club.com.
[7] Keith Armstrong, "Design Techniques for EMC, Part 0 - Introduction and Part 1 - Circuit Design and Choice of Components", The EMC Journal, January 2006 pp 29-41, plus March 2006 pp 30-37, available from http://www.compliance-club.com.
[8] F Beck and J Sroka, "EMC Performance of Drive Application Under Real Load Condition", presented at the Industrial Forums in EMC Zurich 2001, and also a Schaffner EMV AG application note dated 11th March 1999. It was also presented by W L Klampfer at the 8th International Conference on Electromagnetic Interference and Compatibility, INCEMIC 2003, ISBN: 81-900652-1-1, publication date: 18-19 Dec. 2003.
[9] Keith Armstrong, "Advanced PCB Design and Layout Techniques for EMC", an 8-part series published in the EMC & Compliance Journal, March 2004 - November 2005. An improved version of this series is available via the "Publications & Downloads" page at http://www.cherryclough.com
[10]The United Kingdom Accreditation Service, http://www.ukas.com
[11]John R Barnes, Robust Electronic Design Reference Book, Volume II, Appendices, Kluwer Academic Publishers, 2004, ISBN 1-4020-7738-6
[12]RF Caf‚, Skin Depth, http://www.rfcafe.com/references/electrical/skin_depth.htm
[13]Tim Williams and Keith Armstrong, "EMC for Systems and Installations", Newnes 2000, ISBN 0 7506 4167 3, especially chapter 8, www.newnespress.com, RS Components Part No. 377-6463
[14]ITI (CBEMA) Curve and Application Note: http://www.itic.org/archives/iticurv.pdf
[15]"PCB Layout: The Impact of Lighting and Power-Cross Transients", Milton Hilliard, Compliance Engineering, January/February 2003 pp 25-30, available from the archives at http://www.ce-mag.com
[16]MIL-STD-275 "Printed Wiring for Electronic Equipment, Revision: E, Dated: 31 December 1984", available via: http://www.dscc.dla.mil/Programs/MilSpec/ListDocs.asp?BasicDoc=MIL-STD-275
[17]Akihiko Yagasaki, Characteristics of a Special-Isolation Transformer Capable of Protecting from High-Voltage Surges and Its Performance", IEEE Trans. EMC, Vol. 43, No. 3, August 2001, pp 340-347
[18]Daniel Dunlap, Protection of SPD Products, ITEM 2000, pp 148-157, visit www.interference-technology.com and search by ’