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'[EE]:Schottky rectifiers'
2003\03\28@182118 by Jinx

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part 1 1003 bytes content-type:text/plain; (decoded 7bit)

A friend is revamping a variable output SMPS. The original used
a single 24VAC winding and a 35A conventional bridge. He found
that this bridge became very hot when the full 28V / 8A was being
drawn and so changed to this set-up, which uses "push-pull" windings,
as he calls them. He says that now there's very little heating in the
diodes at full output. He wonders why Schottky diodes aren't used
more often as power rectifiers. Maybe he's not looking around
enough, but a quick glance through RS's catalogue shows that most
if not all of their bridges aren't Schottky (I think). The other question
he has is whether there's something about Schottkys he doesn't
know that makes them unsuitable as power rectifiers, especially
for long-term use, and should he go back (reluctantly) to standard
diodes

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2003\03\28@203955 by Sean H. Breheny

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One aspect where Schottky's are not a good as regular junction diodes is
peak inverse voltage rating. I believe that Schottky's are usually rated
only to about 50V or so (often less) although there are some with higher
ratings, whereas run of the mill diodes with PIVs of more than 1000 volts
are common.

Sean

At 11:18 AM 3/29/2003 +1200, you wrote:
{Quote hidden}

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2003\03\29@083759 by Olin Lathrop

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> The other
> question he has is whether there's something about Schottkys he doesn't
> know that makes them unsuitable as power rectifiers, especially
> for long-term use, and should he go back (reluctantly) to standard
> diodes

Schottky diodes can be appropriate in power supplies.  The biggest
impediment to more widespread use is their large reverse leakage current.
Even relatively "little" 1A diodes can have 20mA reverse leakage.  This
tends to be highly temperature dependent, so thing may appear to work fine
in the lab but fail in a less controlled environment.


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2003\03\29@205943 by Brendan Moran

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>Schottky diodes can be appropriate in power supplies.  The biggest
>impediment to more widespread use is their large reverse leakage current.
>Even relatively "little" 1A diodes can have 20mA reverse leakage.  This
>tends to be highly temperature dependent, so thing may appear to work fine
>in the lab but fail in a less controlled environment.

Interesting... That's one of those "little" things that wasn't mentioned in
a power supply basics course I took recently.  Virtually every problem
relating to SMPSs assumed the use of a Schottky diode, but no mention was
made of reverse leakage current.

--Brendan

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2003\03\30@002034 by David Duffy

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At 11:58 AM 30/03/2003, you wrote:
>>Schottky diodes can be appropriate in power supplies.  The biggest
>>impediment to more widespread use is their large reverse leakage current.
>>Even relatively "little" 1A diodes can have 20mA reverse leakage.  This
>>tends to be highly temperature dependent, so thing may appear to work fine
>>in the lab but fail in a less controlled environment.
>
>Interesting... That's one of those "little" things that wasn't mentioned in
>a power supply basics course I took recently.  Virtually every problem
>relating to SMPSs assumed the use of a Schottky diode, but no mention was
>made of reverse leakage current.

SMPS's use the Schottky's for the high switching speed, not so much
the lower forward voltage drop. Try using normal "slow" diodes in there
and you'll find it either won't work or will heat up & die really quick!
David...

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U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
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2003\03\30@005151 by Brendan Moran

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>SMPS's use the Schottky's for the high switching speed, not so much
>the lower forward voltage drop. Try using normal "slow" diodes in there
>and you'll find it either won't work or will heat up & die really quick!
>David...

Actually, that was a small portion of one of the labs.  But aren't there
ultrafast diodes without the PIV and leakage limitations, but with a higher
forward drop?

--Brendan

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2003\03\30@052526 by David Duffy

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David:
>>SMPS's use the Schottky's for the high switching speed, not so much
>>the lower forward voltage drop. Try using normal "slow" diodes in there
>>and you'll find it either won't work or will heat up & die really quick!

Brendan:
>Actually, that was a small portion of one of the labs.  But aren't there
>ultrafast diodes without the PIV and leakage limitations, but with a higher
>forward drop?

We use the MR852's in a switching power supply which are 200V (PIV)
and 3 Amps. They have a forward drop of around 1 Volt and a reverse
current of <100uA. They're much cheaper than the Schottky versions.
David...

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U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: http://www.audiovisualdevices.com.au
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2003\03\30@072446 by Oliver Broad

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You're right about the diodes, and for what it's worth "fast" diode
performance varies enourmously. Recovery time is a major limitation in
higher voltage circuits with the result that there are diodes available made
from GaAs and SiC in order to combine the ultra low recovery time of a
Schottky diode with a 400v or 800v PIV.

I think APT make GaAs rectifiers and Cree make SiC

Oliver.

{Original Message removed}

2003\03\30@073754 by Oliver Broad

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Mainly the expense. If you can get away with it fast recovery types are
cheaper than schottky and standard recovery are cheaper still.

I made this decision on a stepper motor controller. Eight 3A rated schottkys
was just too much. Since then cheap multi-diode packs have become available,
effectively two schottky bridge rectifiers in DIP8.

{Original Message removed}

2003\03\30@080614 by Olin Lathrop

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> You're right about the diodes, and for what it's worth "fast" diode
> performance varies enourmously. Recovery time is a major limitation in
> higher voltage circuits with the result that there are diodes available
> made from GaAs and SiC in order to combine the ultra low recovery time of
> a Schottky diode with a 400v or 800v PIV.

Another approach is to ensure that the diode is no longer conducting when
the next pulse starts.  This is not so hard to guarantee in a buck
converter, but a little more tricky in a boost converter.


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2003\03\30@091226 by Oliver Broad

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Must (not all) of the buck designs I've seen run in continuous mode.

----- Original Message -----
From: "Olin Lathrop" <spam_OUTolin_piclistTakeThisOuTspamEMBEDINC.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: 30 March 2003 14:05
Subject: Re: [EE]:Schottky rectifiers


> > You're right about the diodes, and for what it's worth "fast" diode
> > performance varies enourmously. Recovery time is a major limitation in
> > higher voltage circuits with the result that there are diodes available
> > made from GaAs and SiC in order to combine the ultra low recovery time
of
{Quote hidden}

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2003\03\30@120137 by Olin Lathrop

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>> Another approach is to ensure that the diode is no longer conducting
>> when the next pulse starts.  This is not so hard to guarantee in a buck
>> converter, but a little more tricky in a boost converter.
>
> Must (not all) of the buck designs I've seen run in continuous mode.

I don't know what you mean by "continuous mode", or how that relates to
the point you responded to.  Switching mode power supplies transfer
individual packets of energy to from the input to the output.  This means
they sequence thru phases of operation.

A simple buck voltage regulator, for example, has three phases.  First,
the pass element is enabled.  This starts the output current ramping up
and also charges the inductor.  Second, the pass element is disabled,
forcing the inductor current to come from ground thru a diode until it
ramps back down to zero again.  This phase is how the buck converter
achieves higher output current than input current.  The input current is
zero (pass element off), but there is still output current.  The third
phase is the quiescent state.  It is waiting for the output voltage to
drop below the threshold to start the next pulse.

The reverse recovery time of the diode is immaterial as long as a new
pulse isn't started in the second phase.  As I said above, this isn't hard
to guarantee in a buck converter.


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2003\03\30@132848 by Scott Dattalo

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On Sun, 30 Mar 2003, Olin Lathrop wrote:

> >> Another approach is to ensure that the diode is no longer conducting
> >> when the next pulse starts.  This is not so hard to guarantee in a buck
> >> converter, but a little more tricky in a boost converter.
> >
> > Must (not all) of the buck designs I've seen run in continuous mode.
>
> I don't know what you mean by "continuous mode", or how that relates to
> the point you responded to.  Switching mode power supplies transfer

<snip>

In Continuous mode, the current in the inductor does not drop to zero
inbetween consecutive switching cycles. In discontinuous mode, the current
does drop to zero. If you look at the voltage waveform between the
inductor and the switch, you'll (typically) see high frequency
oscillations for discontinuous mode operation. This is caused by the
inductor resonating with the (relatively small) capacitance of the switch
and/or catch diode.

Depending on your PCB layout, this parasitic oscillation could cause
excessive radiation. However, the current is typically much smaller than
the current that is being switched and hence has less energy for
radiating. I've seen problems emission problems with discontinuous
switchers - but I've also seen others that oscillated wildly and but
hardly radiated.

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2003\03\30@141307 by Herbert Graf

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{Quote hidden}

       Continuous mode is exactly that, the next phase starts in the second phase.
Discontinuous conduction mode is what you are speaking of Olin. There are
advantages to either scheme. Continuous mode is generally easier to get
stable, is more linear in it's behaviour, and is MUCH easier to model.
Discontinuous mode is generally harder to model and requires more filtering.
However there are circumstances (such as what you describe) where
discontinuous mode has it's advantages.
       Which mode is chosen is up to designer. Generally a converter uses either
continuous or discontinuous conduction mode, very rarely will you see a
converter designed for both. TTYL

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2003\03\30@154401 by Oliver Broad

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----- Original Message -----
From: "Olin Lathrop" <olin_piclistspamKILLspamEMBEDINC.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: 30 March 2003 17:20
Subject: Re: [EE]:Schottky rectifiers


{Quote hidden}

A continuous mode converter is one that's designed to run without the
quiescent part of the cycle. This confers substantial advantages at high
power density as the output ripple is reduced to the point that
non-electrolytic bypass capacitors are practical, the switch and diode pass
near-square current pulses, the inductor is not subject to significant
hysteresis loss and the control characteristic is very predictable to the
point that output regulation is possible even without negative feedback.

You are describing discontinuous mode. Discontinuous mode works well when
combined with PFM (Pulse Frequency Modulation) in a fixed on-time and
variable off time mode for minimum quiescent current.

Continuous mode causes problems in boost applications because power is
delivered to the inductor during transistor on-time but is only delivered to
the load during off time. It can be seen that an increase in on-time will
cause a decrease in delivered power in the short term, followed by an
increase as the inductor current rises. This is severely detrimental to
control loop stability. The loop is stabilised by ugly dominant pole
compensation rather than the elegant phase lead schemes employed elsewhere.

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2003\03\30@193316 by Russell McMahon

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> > Must (not all) of the buck designs I've seen run in continuous mode.
>
> I don't know what you mean by "continuous mode", or how that relates to
> the point you responded to.

"Continuous mode" is when the inductor current does not fall to zero before
the cycle repeats.

       RM

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2003\03\31@034652 by Jinx

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part 1 554 bytes content-type:text/plain; (decoded 7bit)

I'm using this circuit to protect the PIC's input against voltage
spikes from relays and a motor. With a 1N5818 the input is
successfully stopped from going too far below 0V wrt the PIC's
internal diodes. Is the 1N5818 a bit OTT and could a smaller,
cheaper diode like a BAT be used ?

1N5818, 1A, Vf 0.55V
BAT81, 30mA, Vf 0.40V
BAT46, 150mA, Vf 0.45V
BAT43/BAT86, 200mA, Vf 0.33V

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2003\03\31@060810 by Russell McMahon

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> I'm using this circuit to protect the PIC's input against voltage
> spikes from relays and a motor. With a 1N5818 the input is
> successfully stopped from going too far below 0V wrt the PIC's
> internal diodes. Is the 1N5818 a bit OTT and could a smaller,
> cheaper diode like a BAT be used ?


Such wisdom is seldom seen!
(And I'm serious)(and I know that could be taken as a negative comment about
wisdom being lacking but it isn't meant that way :-) ).

The Schottky makes an otherwise dangerous circuit safe.

The smaller diodes would be fine as long as their rated currents weren't
exceeded.

I am certain that people will question the need for the Schottky diode
there.
Because of the ongoing comments from some re using body diodes for
protection I recently duplicated this circuit using an actual PIC body diode
in parallel with a forward conducting zener. The object was  to see how much
current flowed into the PIC when a forward biased zener was used for
"protection". Results varied somewhat with current but AFAIR typically about
80% of the current went through the PIC! This is because the zener is not a
normal silicon diode junction and has a forward diode drop substantially
above that of most diodes.

In the circuit shown WITHOUT the Schottky, if -12 volts was applied to the
input,  about

       0.8 x (12-0.7)/470 = ~19 mA

would flow in the PIC body diode. This would be nearly certain to disrupt
PIC operation.
With the Schottky diode present, ~0 mA will flow in the PIC body diode.

One could ask why such a small input resistor should be used.
Using a MUCH larger resistor reduces potential (no pun intended) current .

10 k= ~1 mA
100k = 0.1 mA.

A proportionately smaller filter capacitor would be used if response speed
was to be maintained.

The 470r/10 nF gives about 5uS time constant or say 20uS response time.
100k may start to get a bit large for absolute max response with stray
capacitance but it looks like you are not looking for that here. At 100k and
5 uS Tc you need ~100 F which is probably still OK.



   Russell McMahon

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2003\03\31@061336 by erholm (QAC)

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Russell McMahon wrote:

> ...you need ~100 F which is probably still OK.

Any pointers/URL to those caps ??
I'd like to see one...

Jan-Erik Soderholm

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2003\03\31@064731 by Russell McMahon

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> Russell McMahon wrote:
>
> > ...you need ~100 F which is probably still OK.

> Any pointers/URL to those caps ??
> I'd like to see one...

Ummm - the p seems to have vanished :-)
That was, of course, 100 pF.

I do have some 10F caps but that's not quite what I had in mind.


However -

2700F !!!            http://www.eetimes.com/news/98/991news/ultra.html

100F
www-nrd.nhtsa.dot.gov/edr-site/uploads/PowerCache-PC100_capacitor.pdf

100F   300J at 2.5V, 10 x power density of typical batteries (they claim)


http://www.maxwell.com/ultracapacitors/news/press_releases/2000/pc100.html

Google has 60+ hits for "100 farad"! :-)





           RM

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2003\03\31@115157 by William Chops Westfield

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   > ...you need ~100 F which is probably still OK.

   Any pointers/URL to those caps ??
   I'd like to see one...


http://www.powerstor.com/pdfs_html/PowerStorB_Specs.pdf

50F caps...

BillW

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2003\03\31@133442 by Ned Konz

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On Monday 31 March 2003 12:46 am, Jinx wrote:
> I'm using this circuit to protect the PIC's input against voltage
> spikes from relays and a motor. With a 1N5818 the input is
> successfully stopped from going too far below 0V wrt the PIC's
> internal diodes. Is the 1N5818 a bit OTT and could a smaller,
> cheaper diode like a BAT be used ?
>
> 1N5818, 1A, Vf 0.55V
> BAT81, 30mA, Vf 0.40V
> BAT46, 150mA, Vf 0.45V
> BAT43/BAT86, 200mA, Vf 0.33V

Depends on the voltages and currents you're likely to be seeing.

What I do is even simpler: I use a series-connected dual Schottky in a
SOT-23 case (like the Fairchild BAT54S
http://rocky.digikey.com/WebLib/Fairchild/Web Data/BAT54_A_C_S.pdf),
running the cathode to V+, anode to GND, and common to the input pin.
Then I use a resistor/capacitor to limit the current and fast pulses.

The BAT54 is a 200mA 30V part (though of course the voltage rating
doesn't matter here, as you're never going to see more than 5V
reverse). You could use a much larger resistor than a 4.7K. If you're
using this as an analog input, your external capacitor, if large
enough, can provide the current to charge the sampling cap.

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2003\03\31@161103 by Brent Brown

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Russell McMahon wrote:

> I am certain that people will question the need for the Schottky diode
> there. Because of the ongoing comments from some re using body diodes
> for protection I recently duplicated this circuit using an actual PIC
> body diode in parallel with a forward conducting zener. The object was
>  to see how much current flowed into the PIC when a forward biased
> zener was used for "protection". Results varied somewhat with current
> but AFAIR typically about 80% of the current went through the PIC!
> This is because the zener is not a normal silicon diode junction and
> has a forward diode drop substantially above that of most diodes.
>
> In the circuit shown WITHOUT the Schottky, if -12 volts was applied to
> the input,  about
>
>         0.8 x (12-0.7)/470 = ~19 mA
>
> would flow in the PIC body diode. This would be nearly certain to
> disrupt PIC operation. With the Schottky diode present, ~0 mA will
> flow in the PIC body diode.

I was just wondering about a slight variation in the circuit. Omit
the schottky diode and add a resistor (Rin2) between the zener and
the PIC input pin. The difference in zener forward voltage drop and
PIC body diode forward voltage drop is small, so consequently the
voltage drop across the resistor is small and doesn't increase the
same as the input voltage increases. For an example similar to above:

Vin = 12V, Vdiode = 0.7V and Rin = 470R => Iin = 24mA.

If Vpicdiode = 0.6V and Rin2 = 10k, => Inpic = 10uA.

Back to the old argument of using body diode for anything, but at
least keeping it in the microamp range is somewhat safer. Circuit is
cheaper because the schottky is replaced with just a resistor. Input
impedance to the real world is the same, so peaks are clamped the
same, and input impedance to PIC is set by Rin2 and does not need to
be rediculously high (where input can become suceptible to noise).
Also if the zener fails (in such a fault condition where the schottky
diode would blow as well) the 10k resistor (Rin2) will still provide
current limiting and save the PIC.

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