.........-----------Cut lots of good stuff by Win and
Genome______..........................
turns
the
Are you saying a diode (any diode) will store charge based on the peak
forward current and not the instantaneous current just prior to turnoff?
Let's say we have a diode with 10A forward current that is ramped down to
1.0A in 10uS. We then reverse bias it in 50nS, the charge is proportional
to
10A and not 1.0A??
Regards
Harry
Greetings Mr. Dellamano. My name is not Genome but I thought I might try to
answer your very good questions anyway.
From my understanging of diodes there are two competing processes that
ultimately achieve the same end result (removal of stored charge so the
diode can become fully blocking) that need to be considered separately. In
order to remove the excess diode minority carriers you can either:
1. Apply an external electric field (forcefully reverse bias the diode) to
force the excess carriers out.
or
2. Let recombination naturally remove them from the diode.
Item number 2 is always occurring within the diode to remove excess minority
carriers, and once the diode is manufactured you no longer have the ability
to change how fast this will occur. At the circuit level you typically do
have some control over how fast and when you apply external electric fields.
In your example the current very slowly ramps down from 10A forward to 1A
forward in 10us. This is a rather slow dI/dt of 0.9A/us. If the diode is a
fast recovery diode (say with datasheet published trr in the range of ~50ns)
the minority carrier lifetime will be much smaller than 10us (in the range
of the trr value). As a result by the time the current reaches 1A there
will be very few excess minority carriers (above and beyond the amount
needed to conduct a forward current of 1A) in the diode. So in this case
when you rapidly reverse bias the diode in 50ns the total excess charge
carriers that need to be removed will be from the 1A forward conduction
rather than the 10A.
That is to say, the reverse recovery stress/heating of the diode will be the
same as if the diode had simply always had a forward current of 1A and then
a reverse bias was quickly applied in 50ns.
Suppose now instead of a fast recovery diode we had a very slow diode. A
normal very slow "standard recovery" rectifier (such as the 1N400X for
instance) will probably have a minority carrier lifetime of something in the
low microseconds range (maybe around 3us IIRC). The diode reverse recovery
time specified in the datasheet (under their very specific given test
conditions) isn't quite the same number as the minority carrier lifetime,
but they are approximately the same and certainly very much related. Okay,
so lets suppose in this scenario the minority carrier lifetime is in the
vicinity of 3us.
Now as the 10A ramps down to 1A in 10us and then the diode becomes reverse
biased in 50ns the power loss will be higher than if the same diode had
instead been conducting 1A all along and then a reverse bias was applied in
the same 50ns. The total loss was however less than if the diode had been
conducting 10A all along and then a reverse bias was applied in 50ns. As
the current ramped down from 10A to 1A the minority carriers present by the
time the current reached 1A was more than the minimum normally required to
conduct a forward current of 1A.
Okay now suppose another scenario. Suppose we have a diode with extremely
slow reverse recovery characteristics and correspondingly very long minority
carrier lifetime. Suppose the minority carrier lifetime is in the vicinity
of say 200us.
Now as the forward current ramps down from 10A to 1A in 10us the number of
minority carriers present does not decrease correspondingly at all. After
the 10us are up almost all of the minority carriers needed to conduct a
forward current of 10A will still be present. Now when the diode becomes
reverse biased in 50ns the losses caused by the diode will be about the same
as if the diode had always been conducting the full 10A just prior to
becoming reverse biased.
So as you can see the minority carrier lifetime is a very important figure.
The situation is rather complicated and it [the amount of charge stored that
must be forcefully removed by external electric fields at the time of when
the diode becomes reverse biased] depends on relative time frames being
considered. Further complicating the matter is also how fast the reverse
bias gets applied.