OT: Diode snubbers

[Rick Shindley]

What kills solid state PN junctions is excessive power dissipation (heat).
You can abuse the breakdown voltage of a PN junction as long as the power
dissipated there is within the power dissipation range of the PN junction.
That's how Zener diodes work and it is why they have a power dissipation
rating. All diodes will function as low-current Zener diodes if you manage
to provide a high enough reverse voltage across them to break them down and
limit the current so the power they must dissipate won't destroy the PN
junction. An ordinary 2N2222 can be used as a low-current 7V Zener if you
reverse bias its base-emitter junction.
 
Relay "kickback" energy is a function of the relay coil size, applied
voltage, coil resistance, and relay coil core permeability (determines how
fast the field can collapse thus the max voltage the thing can make with no
coil current flowing).   A very large coil will have a lot of energy stored
and all of that must be dissipated when the coil is de-energized.  A
forward-biased diode must burn off that energy that is not dissipated in the
coil winding resistance (and core losses).  If there is too much power
applied to the diode for too long, the diode junction will melt and then it
electrically shorts or opens, depending on luck.
 
The "schematic" of a diode is more than what we all see in application
schematics.  It has a tiny amount of resistance in series with it a
capacitance in parallel with what's left.  When conducting the tiny
resistance is what allows the forward voltage drop to be greater than the
otherwise fixed forward drop we see at DC.  The junction capacitance is also
drawing current until it's charged so that makes the small voltage drop
across that unwanted tiny resistance a bit greater, as you can see in the
scope shots in the Clifton Labs pics.
 
The bottom line is that a clunky power diode is fine as a snubber for small
12 and 24VDC relay applications found in amateur radio gear.  The coil
energy is not large enough to melt the diode PN junction and the tiny series
resistance of the diode is generally inconsequential.  Placing a 1N4148
across a large relay coil is not wise since the diode is "fast" and that
implies a tiny junction area that is easily melted.
 
The PIV rating of the diode need only be greater than the DC voltage applied
to the relay coil when it is energized since that is all the diode will see
when it is reversed biased.  The small differences in max forward voltages
seen between various diodes is due to the tiny series resistance each diode
has.  
 
Rick
KC0OV (for now-just upgraded!)
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[David Gilbert]

I worked in the discrete semiconductor industry for 30 years
(engineering, manufacturing, management) and that statement is simply
not true at all.

Zener diodes are designed to break down uniformly across the entire
junction, but most other semiconductors are not.  A reverse voltage
breakdown of the collector-base junction will in most cases force
current through an extremely small region of the junction (often
referred to as a "puncture") and fuse the silicon at that point,
rendering the device useless.  The current required to do so is much
less than would be required to heat the device even a few degrees, and
the time required to destroy the device is short.  Junctions with
steeper diffusion gradients (RF devices, switching transistors, etc)
will fail more easily (sometimes in microseconds), while sloppier
junctions (power devices, etc) will take considerably more abuse.  It is
not possible to predict the current at which catastrophic failure will
occur.

Emitter-base junctions are typically more graded and won't fail
catastrophically as quickly, but repeated reverse bias conditions will
degrade the transfer gain of the device by creating defect centers that
kill the carrier lifetimes in the base region.

The inductive kick from even a small relay is sufficient to puncture the
junction of many commonly used transistors or driver ICs, and there are
reams of failure analysis reports documenting that fact.  There isn't a
manufacturer on this earth that will honor the warranty for a device
used as you describe unless the device was specifically designed to
survive it.

Dave   AB7E



Rick Shindley wrote:
> What kills solid state PN junctions is excessive power dissipation (heat).
> You can abuse the breakdown voltage of a PN junction as long as the power
> dissipated there is within the power dissipation range of the PN junction.
> That's how Zener diodes work and it is why they have a power dissipation
> rating. All diodes will function as low-current Zener diodes if you manage
> to provide a high enough reverse voltage across them to break them down and
> limit the current so the power they must dissipate won't destroy the PN
> junction.
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