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James Meyer

Jan 1, 1970
0
James Meyer wrote...

The diode has a traditional logarithmic component, which
is both current and temperature sensitive, and a series
resistive component, which follows ohms law. At high
currents the resistive component by far dominates. If
two high-current power diodes are paralleled, the current
sharing will be largely determined by the relative values
of their intrinsic series resistance. While for a given
type of diode these resistance values will likely be the
same, they are not guaranteed to be the same, and may in
fact not be, especially if they have different date codes.
The series-resistance component of diodes I have measured
have varied by up to 50%.

Thanks,
- Win

whill_at_picovolt-dot-com

And what does that 50% in Rs variation together with the diode's other
characteristics translate to in terms of current sharing? If one of the diodes
would otherwise tend to hog current, its temperature and junction potential
would rise and begin to force current into the loafing diode.

A 1N4148 in parallel with a 1N4004 would not share current very well,
but two of either type should share nicely.

Jim
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that James Meyer <[email protected]>
Are you aware that a diode's forward voltage drop is proportional to the
current it's passing?

For sufficiently vague values of 'proportional'! Actually, there are two
parts to the voltage drop, one IS due to real resistance and IS
proportional to the current. The other is the junction voltage, and that
is proportional to the logarithm of the current.
 
S

Spehro Pefhany

Jan 1, 1970
0
And what does that 50% in Rs variation together with the diode's other
characteristics translate to in terms of current sharing? If one of the diodes
would otherwise tend to hog current, its temperature and junction potential
would rise and begin to force current into the loafing diode.

A 1N4148 in parallel with a 1N4004 would not share current very well,
but two of either type should share nicely.

They may share the surge current better than more normal currents.

Best regards,
Spehro Pefhany
 
W

Winfield Hill

Jan 1, 1970
0
James Meyer wrote...
Winfield Hill wrote...1


And what does that 50% in Rs variation together with the diode's
other characteristics translate to in terms of current sharing?
If one of the diodes would otherwise tend to hog current, its
temperature and junction potential would rise and begin to force
current into the loafing diode.

This can serve to help ameliorate the problem at ordinary current
levels, but at currents in the region of the diode's peak-current
maximums the voltage drop across the resistive component can exceed
the "diode drop" component, greatly reducing the effects you
mentioned to a minor role. Having made pulsed measurements
of power diodes I'm used to seeing this phenomenon. But talk is
cheap - let's find some evidence in a manufacturer's data sheet.

The op hasn't stated his diode-bridge part, but for exploration,
we'll try the 1n5400 series. Although rated at only 3A continuous,
these have an impressive 200A peak rating. Actually, that's a
"Non-repetitive Peak Forward Surge Current, 8.3 ms single half-
sine-wave" rating, to be more precise. These single-diode parts
are often used within in diode bridges. We can look at ON Semi's
data sheet, see http://www.onsemi.com/pub/Collateral/1N5400-D.PDF

On page three figure 1, we see the classic logarithmic portion of
the instantaneous forward voltage from 0.2A to roughly 5A. With
a ruler we can draw a straight line showing a slope of about 90mV
per decade (other manufacturer's data sheets show higher slopes,
e.g. Codi Semi, at 120mV/decade). If we extend this straight line
up to 200A, we get a forward voltage of about 1.10 volts. But the
fig 1 curve shows 2.70V at this current. The extra 1.6V drop is
due to the diode's internal 8-milli-ohm resistance. Here the 1.6V
resistive voltage-drop component exceeds the 1.1V diode component.

This significant 8-milli-ohm series resistance isn't specified in
the diode specs, and it may vary considerably from part-to-part,
from run-to-run, and between manufacturers.

Thanks,
- Win

whill_at_picovolt-dot-com
 
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