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'[EE]: Diode reverse failure, was [PIC] Help in PIC'
2003\02\25@055701 by Alan B. Pearce

face picon face
>         IIRC it deals with minority carrier storage effects. It takes
>         a while for
> the junction to "get rid of" the stored charge, during which time the
> diode is conducting current, usually at many times the normal forward
> drop. For more info pretty much any power electronics book covers it,
> I can't find the text I often refer to right now. TTYL

Changed to EE, as has got of PIC's themselves

The problem that arises is when the diode is conducting, and the voltage
across the diode reverses. The diode keeps conducting for a short period of
time as the carriers get rid of the stored charge. what seems to happen
while these keep conducting in the reverse direction is the voltage drop
across the diode increases, so you do get a massive increase in the
dissipated power for the period of the reverse current flow. If this happens
too often the diode power dissipation spec is exceeded resulting in failure.

This should not normally be a problem with a catch diode on a relay coil, as
the current in the relay coil should have fallen to nothing before the relay
is energised again anyway, so the reverse charge carriers will have
dissipated anyway. When the coil is next energised the diode will then have
reverse voltage across it, with no carriers to carry current.

I have seen oscilloscope pictures of the reverse turnoff of early 1n400x
family diodes being a visible portion of a 50/60hz sinewave when used as
ordinary rectifiers. I am not sure if the technology used in these diodes
has improved so they no longer have a significant turn off time at mains
frequency, or it may have been that the ones I observed were generic diodes
labelled as such, without any great attempt at checking some of these
characteristics. It was a good few years ago now.

If one finds that 1n400x diodes are just not quite hacking the pace in this
area, companies like Unitrode (now part of TI?) used to make "fast" 1n400x
families, called UN400x as I recall. These were guaranteed turn off times
that were faster than 1n400x, but I am not sure if they were fast enough for
the likes of TV Horizontal rate rectifiers. These days there are probably
enough other fast rectifiers readily available, driven by the switch mode
power supply market that these have probably gone by the wayside.

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2003\02\25@062258 by Russell McMahon

face
flavicon
face
> If one finds that 1n400x diodes are just not quite hacking the pace in
this
> area, companies like Unitrode (now part of TI?) used to make "fast" 1n400x
> families, called UN400x as I recall. These were guaranteed turn off times
> that were faster than 1n400x, but I am not sure if they were fast enough
for
> the likes of TV Horizontal rate rectifiers. These days there are probably
> enough other fast rectifiers readily available, driven by the switch mode
> power supply market that these have probably gone by the wayside.


I use BYV26C for this. 1 amp rated. Nice diode.
BYV28-xxx (xxx = voltage ratting) for 3A needs.


       RM

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2003\02\25@083341 by Spehro Pefhany

picon face
At 10:56 AM 2/25/2003 +0000, you wrote:

>I have seen oscilloscope pictures of the reverse turnoff of early 1n400x
>family diodes being a visible portion of a 50/60hz sinewave when used as
>ordinary rectifiers. I am not sure if the technology used in these diodes
>has improved so they no longer have a significant turn off time at mains
>frequency, or it may have been that the ones I observed were generic diodes
>labelled as such, without any great attempt at checking some of these
>characteristics. It was a good few years ago now.

trr of a 1N400x is around 1.5usec if that what you are asking. It's not on
the datasheet.

>If one finds that 1n400x diodes are just not quite hacking the pace in this
>area, companies like Unitrode (now part of TI?) used to make "fast" 1n400x
>families, called UN400x as I recall.

I often use UF400x series when required. They are quite a bit more
expensive, so it is generally not a good idea to use them at low frequencies.
trr of 50/75ns depending on the voltage. Fairchild has them.

>  These were guaranteed turn off times
>that were faster than 1n400x, but I am not sure if they were fast enough for
>the likes of TV Horizontal rate rectifiers. These days there are probably
>enough other fast rectifiers readily available, driven by the switch mode
>power supply market that these have probably gone by the wayside.

Schottkys (for low voltage only) are fast and low drop, if you don't mind
the cost, low breakdown voltage, and how leaky they are.

***Note that the device doing the switching also sees excessive heating when
trr is too long.***

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2003\02\25@155354 by Brent Brown

picon face
> >         IIRC it deals with minority carrier storage effects. It
> >         takes a while for
> > the junction to "get rid of" the stored charge, during which time
> > the diode is conducting current, usually at many times the normal
> > forward drop. For more info pretty much any power electronics book
> > covers it, I can't find the text I often refer to right now. TTYL
>
> Changed to EE, as has got of PIC's themselves
>
> The problem that arises is when the diode is conducting, and the
> voltage across the diode reverses. The diode keeps conducting for a
> short period of time as the carriers get rid of the stored charge.
> what seems to happen while these keep conducting in the reverse
> direction is the voltage drop across the diode increases, so you do
> get a massive increase in the dissipated power for the period of the
> reverse current flow. If this happens too often the diode power
> dissipation spec is exceeded resulting in failure.

OK, I'm still trying to learn a little bit more here. Using the same
circuit, perhaps a relay coil which we wish to control by PWM,
then...

When the transistor or other switching device turns on for a
subsequent cycle and the diode remains on for a short period, say
1.5us, what exactly determines the current through the diode in this
period? I think...

The diode would create a low impedance path from V+ to GND through
the transistor with nothing much to limit the current. So the current
is momentarily in the opposite direction (to when it was
"freewheeling"), the voltage drop across the diode is increasing
because it is turning off, and the current is higher than previous
because there is now a short circuit across the power supply, so all
these contribute to a large peak power. In practice I guess the
transistor will have a considerable turn on time which will help to
reduce this peak current, but like Spehro says it will get a hard
time too.

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2003\02\26@051019 by Alan B. Pearce
face picon face
>When the transistor or other switching device turns on for a
>subsequent cycle and the diode remains on for a short period, say
>1.5us, what exactly determines the current through the diode in this
>period? I think...
>
>The diode would create a low impedance path from V+ to GND through
>the transistor with nothing much to limit the current. So the current
>is momentarily in the opposite direction (to when it was
>"freewheeling"), the voltage drop across the diode is increasing
>because it is turning off, and the current is higher than previous
>because there is now a short circuit across the power supply, so all
>these contribute to a large peak power. In practice I guess the
>transistor will have a considerable turn on time which will help to
>reduce this peak current, but like Spehro says it will get a hard
>time too.

This sounds like a reasonable probability to me. Do not forget to include
the hash that is now going to be seen on the supply line feeding the relays
due to the momentary near short circuit. If your diodes are that slow in
turn off time that this becomes a problem, then it is probably going to be
good practice to have a resistor in series with the diode to limit the
current. The resistor should be sized to a value determined by maximum safe
back emf voltage, and required decay rate of the current. OTOH the cost of a
faster diode may be a better trade off due to less component count.

However you may also find that a better way to deal with the same problem of
reducing the coil current is to have a resistor from the supply to the relay
coil, and a capacitor (say 1uF) from the resistor/coil junction to ground.
Then you do not use PWM to reduce the relay current, but size the resistor
to an appropriate value. When the relay is off, the capacitor is charged to
full rail voltage, and supplies the initial peak pull in current to the
relay when the transistor turns on. As the relay draws current from the
capacitor, and the voltage drops, then the current will decrease to a value
set by the resistor. You will still need a diode across the relay to clip
the back emf on turn off though.

Which of these becomes the best solution will be determined by the
particular application.

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2003\02\26@095030 by Roman Black

flavicon
face
Alan B. Pearce wrote:

> However you may also find that a better way to deal with the same problem of
> reducing the coil current is to have a resistor from the supply to the relay
> coil, and a capacitor (say 1uF) from the resistor/coil junction to ground.
> Then you do not use PWM to reduce the relay current, but size the resistor
> to an appropriate value. When the relay is off, the capacitor is charged to
> full rail voltage, and supplies the initial peak pull in current to the
> relay when the transistor turns on. As the relay draws current from the
> capacitor, and the voltage drops, then the current will decrease to a value
> set by the resistor. You will still need a diode across the relay to clip
> the back emf on turn off though.


1uF?? I've been using this trick for about 20 years,
in most cases 470uF is a minimum and 1000 or 2200
much better.
-Roman

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2003\02\26@153201 by Brent Brown

picon face
> > However you may also find that a better way to deal with the same
> > problem of reducing the coil current is to have a resistor from the
> > supply to the relay coil, and a capacitor (say 1uF) from the
> > resistor/coil junction to ground. Then you do not use PWM to reduce
> > the relay current, but size the resistor to an appropriate value.
> > When the relay is off, the capacitor is charged to full rail
> > voltage, and supplies the initial peak pull in current to the relay
> > when the transistor turns on. As the relay draws current from the
> > capacitor, and the voltage drops, then the current will decrease to
> > a value set by the resistor. You will still need a diode across the
> > relay to clip the back emf on turn off though.
>
> 1uF?? I've been using this trick for about 20 years,
> in most cases 470uF is a minimum and 1000 or 2200
> much better.

Well thanks guys but it's all academic really. I just wanted a better
understanding of what was happening during diode turn off. In
practice I would now know how to do proper PWM control of relay if
required, with a suitably fast diode (probably 1N4148 for small
relays). The RC idea is good for controlling on/hold current, PWM
with voltage or current feedback even better if the supply was not a
fixed value. Even then a small RC circuit to isolate current pulses
from the main supply might be a good idea.

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Ph/fax: +64 7 849 0069
Mobile/txt: 025 334 069
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2003\02\27@072511 by Alan B. Pearce

face picon face
>1uF?? I've been using this trick for about 20 years,
>in most cases 470uF is a minimum and 1000 or 2200
>much better.

Well OK, maybe it needs to be a bit bigger, so long as it has enough
capacity to ensure the relay pulls in. Don't forget that it is also
dependant on relay coil voltage (maybe I have been playing with 28V space
specc'd relays for too long) and the current limit resistor. Higher relay
coil voltages should get away with smaller value capacitors.

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2003\02\27@083155 by Roman Black

flavicon
face
Alan B. Pearce wrote:
>
> >1uF?? I've been using this trick for about 20 years,
> >in most cases 470uF is a minimum and 1000 or 2200
> >much better.
>
> Well OK, maybe it needs to be a bit bigger, so long as it has enough
> capacity to ensure the relay pulls in. Don't forget that it is also
> dependant on relay coil voltage (maybe I have been playing with 28V space
> specc'd relays for too long) and the current limit resistor. Higher relay
> coil voltages should get away with smaller value capacitors.


Sorry Alan, I didn't mean to sound bitchy, heck there's
enough of that on the list lately. :o)
Relay pull-in for *most* relays is within a typical
energy (coulomb) range, and 470uF is about typical
for most voltages and energy levels for the relays
i've worked with. Many require more. I just couldn't
imagine 1uF doing much for any relay i've seen! ;o)
-Roman

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