transient
I'm trying to understand the meaning of
8 x 20-µs and 10 x 1000-µs
standardized transient impulses (or sometimes it's called a waveform). But what does it actually mean. Why is there a multiplier in front of the time unit?
The quoted line above is constantly mentioned in surge testing document. But no where in the document does it mention what is the meaning of it.
Most Google search results point to Datasheet values. Maybe I'm searching for the wrong search term.
The formula is:
$$V_{avg}=duty\_cycle \cdot (V_{high}-V_{low})$$
You can find the exact mathematical derivation on the web. I'll try to provide some intuition.
So, if the average output voltage of the RC filter driven from a PWM signal is not dependent on RC values, then what is? Mainly two factors:
#1 is easy to explain - just recall that high time constant means longer raise/fall time of the voltage in a simple RC circuit. The same applies to this RC filter.
#2 follows from the same argument - the higher the raise/fall time of the RC filter, the lower the change in voltage that will be seen on the output during the high/low phases of the input PWM signal.
Please note that the above two factors are conflicting - you will (usually) want a fast turn-on/turn-off time and a low output ripple. The former requires low RC while the latter requires high RC. You will choose the RC values according to the constraints of your application, and if they can't be satisfied with a simple RC network - you'll search for a more sophisticated filters.
Nooo... I would definitely not allow the gate voltage to approach that limit with 150 milliseconds duration.
Here is an ON Semiconductor application note where they include a data sheet spec of the conditions.
5 microseconds and duty cycle (0.1%?).
Note also the failure mechanism, which is partly thermal, so (as the lawyers say) time is of the essence.
The voltage across the dielectric, between the gate and emitter can cause tunneling of carriers through the dielectric if it exceeds the leakage limit, especially where imperfections (traps) exist. This tunneling creates heat, which if allowed to continue for a sufficient period of time, can cause damage to the oxide which in turn creates more traps. This process can quickly increase to the point where significant damage occurs in the gate oxide. It should be apparent that at higher gate voltages it is necessary to limit the duration of the event to assure that the temperature rise due to the tunneling electrons does not exceed a safe level.
If the oxide becomes hot enough to cause damage, that damage will be cumulative. This is why both the time and duty ratio of the transient event are included on the data sheet. Over a period of time this damage can cause the threshold to shift lower which may cause improper operation of the circuit or in extreme cases can cause a failure of the gate oxide.
ON Semiconductor IGBTs are tested in qualification testing, at levels well above the transient voltage rating to assure that this is a safe transient level for the gate-to-emitter voltage.
The DC rating is a very conservative gate voltage level and no tunneling or other degradation will occur at or below that operational level.
Best Answer
"8 x 20us" means a transient that rises in 8us and decays to a 50% value in 20us: -
There are many specs for transient testing and 10 - 1000us is another. They are usually referenced as tests done on electrical equipment to prove said equipment is capable of withstanding surges associated with lightning strikes or similar phenomenon.
Here is a good article from Wiki EIG (electrical installation guide)