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.