Magnetic saturation v electronic saturation

[Bohdan L Bodnar]

Original letter attached at end (for reference)...


An inductor is "saturated" when there is no more flux change in it with
increased stimuli.  Theoretically, this takes forever.  Practically, 15 time
constants is sufficient to determine when saturation occurs (I used to use 5
time constants in power electronics work I did some 15 years ago).  A
transistor is saturated when its base/collector junction is forward biased;  I
don't know what driving "...a transistor really hard..." means.  Vce(sat) is a
function of collector current, temperature, and a whole bunch of other
esoteric parameters (e.g., doping profile, added impurities -- such as gold --
for decreased electron/hole recombination time, etc.).

The "saturated" term comes into play because the driver transistor is used as
a class-C amplifier:  in steady-state, it's either fully on or fully off.
Stated differently, in steady-state it is operated as a binary device.  For
the purists, I've neglected the (non-linear) transitions from "on" to "off."

For those who are REALLY interested, you may consider going to your local
technical library and pulling books on magnetic amplifier design.  Mag Amps
were "hot" research items in the 1950s and 1960s.  You'll learn more about
inductor saturation than necessary.  As for transistor operation in pulse
mode, just about any reasonable undergrad textbook on pulse or power
electronics will cover this topic.

Cordially,

Bohdan Bodnar
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I suggested that a coil would be come saturated when the voltage on the
collector emitter junction showed a rise to the Vce sat level. Thinking
about it a little more, I may be wrong, or I may not be.

If you drive a transistor really hard, then it goes into saturation. If
there is no voltage on the collector, there will be no voltage across the
emitter - collector junction. 

If there is no current drawn through the collector then will there be a Vce
sat voltage?

If a lot of current is drawn, will the Vce sat voltage be the same? 

My original contention was that the transistor goes into saturation
immediately (sic) on application of the drive current, but the significant
Vce sat voltage will not occur until the coil is fully saturated and there
is no change in current flow. This, because the current and voltage are out
of phase until the coil is saturated.

What do you think?

Peter