Thursday, 13 June 2013

Design for life - Part 5

Last time we looked at optocoupler ageing and saw how this innocent little component constitutes such a high failure risk for power supplies.
This week we look at spike and surge - a major cause of non-age related failure in the field.
Spike & Surge
The majority of engineers are aware of the catastrophic effects of high transient energies on the input and output lines of a power supply.
Indeed, voltage fluctuations on the local grid are commonplace and the variance in the quality of the
AC mains from location to location can be surprisingly large. However, a typical power supply which meets EN 61000-4-5 (basic immunity test for surge) does not guarantee low susceptibility in the field. The financial rewards of producing reliable products over and above the basic EMC standard are usually very worthwhile.

 A certain UK manufacturer saw their warranty costs fall by £2.7million per year a=er spending less than £100K on improved immunity. In the UK at least, a relatively small proportion of energy fluctuations on the grid originate from lightning strikes. Contrary to popular belief it is not a direct strike which causes the most problems, but the voltage induced on overhead lines from the magnetic field of indirect strikes. Some of the largest discharges have been confirmed at >200,000A and there are o=en several discharges per strike. You could probably even measure some transient voltage induced in a paper clip lying on your desk within 500m of a storm if you were quick enough with the scope!

Buildings in Europe whose AC power is carried by overhead wires can reckon on having 80-120 surges every year due to lightning. These are typically limited to around 6kV because the standard domestic style mains socket flashes over at the rear connectors at around this voltage and acts like a spark gap suppressor! Industrial premises with only 3ph supplies can see much more. A modest strike of 15,000A would induce around 10kV on a transmission line 150m away (even when buried in the ground).

Heavy industrial switchgear, large photocopiers & laser printers, HVAC systems, electric motors and thyristor devices are all notorious for imposing spike and surge on transmission lines, and not always at lower energies than lightning strikes. It is not widely appreciated that even if such transient energies do not cause instant catastrophic failure, repeated exposure has a proven degenerative effect, particularly with highly integrated silicon devices. Call it transient ageing if you will. It has a significant impact on long term reliability.

In all cases, a well considered surge protection stage is essential but is o=en overlooked or poorly optimised. Indeed, there are a great many variables to consider and not every engineer appreciates the subtleties of the various protection devices available.
Looking specifically at the input of an AC-DC power supply, it is desirable to place surge protection devices in both the line-to-line and line to earth positions, giving both common and differential mode protection. Metal oxide varistors (MOV’s) or VDR’s, are the most commonly used device in low-cost applications.

However, a MOV may not be able to successfully limit a very large surge from an event such as a lightning strike where the energy involved is many orders of magnitude greater than it can handle. We have seen many designs where the power supply has a scattering of varistors on the input with no sacrificial protection (e.g. a dedicated thermal fuse). The result is that the first high energy surge to arrive either causes the varistors to explode, o=en accompanied by a large plasma discharge which destroys everything else in the vicinity, or the main input fuse to blow. Either way the power supply fails and has to be returned for service the same way as if there were no protection fitted at all!

An important characteristic to consider with MOV’s is that they degrade when exposed to a few large transients, or many smaller ones. As they degrade, their trigger voltage falls lower and lower, ultimately leading to thermal runaway of that particular device. Therefore to ensure good long term reliability, correct voltage rating is essential. It is also worth noting that selecting a device with a higher energy (joule) rating typically increases the life expectancy exponentially.
It is common to see multiple MOV’s in parallel to increase the overall joule rating of the network, however unless specifically matched sets are used, each MOV will have a slightly different non-linear response when exposed to the same overvoltage. This invariably leads to current hogging and premature failure of the individual device. Thus the ‘effective’ surge energy of the network is dependent on the MOV with the lowest clamping voltage, and the additional parallel MOV’s do not provide any benefit.

Furthermore, because each MOV has a relatively high leakage current (typically around 0.5mA at working voltage for a 20mm device), using many devices in parallel can lead to unacceptably high earth leakage currents. The other two devices commonly used in protection networks are transient voltage suppressor diodes (commonly referred to as Transorbs and also sold under the name Transil) and gas filled discharge tubes (GDT’s)

Whereas the practical response times of MOV’s are in the 40-60ns range, suppressor diodes respond to spikes within 1 - 10 pico-seconds, mostly limited by the inductance of the connecting circuitry. This makes diodes ideal for suppressing sub-nanosecond spikes generated by the many thyristor controlled devices sat on the mains supply. Sub-nanosecond spikes show up, do their damage, and are gone before MOV’s even notice.

Diodes also have the added benefit that they do not degrade with repeated surges which means they can be selected with clamping voltages much nearer to the AC working voltage than with MOV’s. The disadvantage of suppressor diodes is that they offer a lower ‘cost/energy handling’ ratio in comparison to other devices and they tend to be physically larger for the same energy rating. However, if space and cost are not critical, they are one of the most effective devices available for suppressing fast energy transients.

Gas discharge tubes consist of two electrodes surrounded by a special gas mixture in a sealed glass or ceramic enclosure. The gas is ionized by a high voltage spike which causes an arc to form between the electrodes and current to flow. GDT’s can conduct more current for their size compared to diodes and MOV’s but are crucially different in that they continue to conduct until the source voltage has dropped close to zero.

This has huge implications for DC and indeed has to be considered carefully for AC whereby it is quite possible to have a full half cycle of mains energy to absorb in addition to the initial spike or surge energy. It is critical therefore that this follow-on current is controlled. Like MOV’s, gas discharge tubes have a finite life and can only handle a few very large transients. The typical failure mode is a modified trigger voltage or, if subject to very high energies, a dead short. GDT’s take a relatively long 'me to trigger, 100nS pulses 500v above rated voltage will o=en be completely unsuppressed. However, gas discharge tubes offer the highest energy handling capabilities of all protection devices and have exceptionally low capacitance.

By far the most effective suppression networks utilise a combination of components to give high energy, high current capability with a very fast response 'me. Parallel devices are to be avoided unless using specifically matched sets and thermally vulnerable devices must be protected by dedicated components. Any design which neglects a well optimised surge and spike suppression network can expect substantially increased failures rates in the field.

Next _me we will look at ways to improve circuits from environmental factors such as moisture, electrolytes and contamination and why conformal coating is not as simple as it first appears

Advance Product Services Ltd

Paul Horner is Managing

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