A.6 Technical rationale for the use of the contact discharge method
In general the reproducibility of the previous test method (air discharge) was influenced by,
for example, the speed of approach of the discharge tip, humidity, and construction of the test
equipment, leading to variations in pulse rise time and magnitude of the discharge current.
In previous designs of ESD testers, the ESD event was simulated by discharging a charged
capacitor through a discharge tip onto the EUT, the discharge tip forming a spark gap at the
surface of the EUT.
The spark is a very complicated physical phenomenon. It has been shown that with a moving
spark gap the resulting rise time (or rising slope) of the discharge current can vary from less
than 1 ns and more than 20 ns, as the approach speed is varied.
Keeping the approach speed constant does not result in constant rise time. For some
voltage/speed combinations, the rise time still fluctuates by a factor of up to 30.
One proposed way to stabilize the rise time is to use a mechanically fixed spark gap. Although
the rise time is stabilized with this method, it cannot be recommended because the resulting
rise time is much slower than the rise time of the natural event to be simulated.
Using various types of triggering devices (e.g. gas tubes or thyratrons) instead of the open
spark, is another possibility, but such kinds of triggering devices produce rise times which are
still too low compared to the rise times of the real ESD event.
The only triggering device known today which is able to produce repeatable and fast rising
discharge currents is the relay. The relay should have sufficient voltage capability and a
single contact (to avoid double discharges in the rising part). For higher voltages, vacuum
relays prove to be useful. Experience shows that by using a relay as the triggering device, not
only the measured discharge pulse shape is much more repeatable in its rising part, but also
the test results with real EUTs are more reproducible.
|小黑屋|电磁兼容网 电磁兼容小小家 EMC工程师家园 电磁兼容(EMC)小小家学习园地
发表于 2006-12-31 22:25:19
发表于 2007-5-9 22:14:30
