Technical Notes

These notes address the technical questions we get at Surgetech in more depth than is usually available on datasheets
or catalogues. They will be updated regularly. We have a wealth of material available already, but would welcome any
particularly difficult or interesting problems which we would be delighted to respond to.

SUR22 Ultrafast Hybrid Protectors
These products are called Hybrids because they utilise the best features of both gas discharge tubes and semiconductors.
Unlike traditional and expensive 5-point hybrid circuits, typically built up on a circuit board and housed in a big expensive box,
SUR22 is only marginally bigger than a standard 3-electrode gas discharge tube.(These traditional circuits will be the subject
of a later technical note !). The best way of looking at how they work is to examine some oscilloscope pictures:

A surge of 1kV/us is applied to a SUR22/230 (Fig A). If this were just a simple gas discharge tube a Sparkover Voltage of
500 - 600 volts would be expected - i.e. considerably higher than the nominal DC Sparkover Voltage of 230 volts, an overshoot
caused by the delay in the plasma being ionised whilst the voltage rises (Fig D). A remarkable thing can be seen, though: the
semiconductors have clamped the voltage to approx. 270 volts for around 1 microsecond before the gas discharge tube takes
over and the voltage drops to the arc voltage of around 30 volts. An idealised graph is shown in Fig B which shows clearly the
"spike" which is removed.
 



What we have succeeded in doing, then, is effectively reducing the Impulse Sparkover Voltage of the gas discharge tube, improving
its effectiveness as a protector against fast transients enormously, whilst retaining its high Impulse Discharge Current rating.

A little thought will show that this is not straightforward. On the one hand the semiconductor clamping voltage must be chosen
at the correct level above the DC Sparkover Voltage, or the gas discharge tube will not operate at all, or will take too long to operate,
and the semiconductor will be destroyed. Fig C shows an actual trace where the gas tube has been vented so we see the action of
the semiconductors only, which are destroyed after 15 microseconds. Two points to observe about the transient voltage protectors
which are often overlooked: firstly there is a rising V/I characteristic - many assume it is flat, and secondly the device does not go
completely short circuit when destroyed - it has gone down to 10 volts or so. If a lot of current is being carried this could be a fire
hazard on its own. Fortunately our SUR22 can use the failsafe which is a typical feature of gas tube design.

On the other hand, if the semiconductor voltage chosen is too high, the energy dissipated in the period before the gas tube takes
over may well also destroy it. So there is a balancing act to perform.

As the Impulse Sparkover Voltage of gas discharge tubes goes higher and higher as dv/dt increases, the improvement in
performance is even more marked. At 1kV/ns SUR22 operates at only slightly higher than at 1kV/us, whereas an ordinary
gas discharge tube will be well over 1kV. So for those of you who are interested in pulses of kV/ns risetimes, this is one
product you ought to be looking at.