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
