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'[EE] UF diodes as rectifiers'
2011\05\28@061038 by IVP

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Hi all,

I'm making a variable linear PSU out of recyclables. I have all
the parts for a 0 - 35V 5A. One thing I'm not sure about are the
diodes I could use

Amongst those to hand are MUR3020 and BYV52-200

http://www.onsemi.com/pub_link/Collateral/MUR3020PT-D.PDF

Both are ultrafast, with the pdf suggesting use in SMPS, but
no mention of linear supplies

Would either be suitable in the attached schematic ? If not
I have a few MR750 (6A, 600V)

http://www.onsemi.com/pub_link/Collateral/MR750-D.PDF

I note that the lead length is very important

TIA

Joe

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2011\05\28@062835 by Michael Watterson

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On 28/05/2011 11:06, IVP wrote:
> I'm making a variable linear PSU out of recyclables. I have all
> the parts for a 0 - 35V 5A. One thing I'm not sure about are the
> diodes I could use
>
> Amongst those to hand are MUR3020 and BYV52-200
As long as you are within peak current and peak voltage they should be fine. Swap you some 1N4007s :-)


It's just that 1N4001 ... 1N4007, BY127  etc are too slow for SMPSU.  The diodes/Rectifiers suitable for SMPSU are more expensive.

Higher voltages tend to use "Fast" parts as Schottky don't usually be available in the past for higher voltages.

The IN4007 are so slow due to higher PIV that with small DC forward bais you can vary current to attenuate RF at tens of volts without the diode switching.

MUR120 seems to have interesting varicap range, but due to high speed, the minimum varicap volts has to be more than the oscillator peak to peak. Most semiconductor junctions reverse biased act as varicaps, hence FETs have much lower Gate/Drain capacitance and higher RF gain at high voltage

2011\05\28@082107 by Olin Lathrop

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IVP wrote:
> Both are ultrafast, with the pdf suggesting use in SMPS, but
> no mention of linear supplies
>
> Would either be suitable in the attached schematic ?

The speed issue of a diode is how fast it stops conducting after a reverse
voltage is applied.  This is more accuratelly referred to as "reverse
recovery time".

This is important in switching power supplies since they can be operating at
a high frequency.  For example, consider a basic boost regulator run in
continuous mode.  The diode is forward biased and therefore conducting when
the switch turns on.  Before it turns off, the input supply is basically
shorted thru the diode and the switch.  This not only wastes power, but can
really beat up the switch.  I've had a switching FET fail in this case when
the wrong diode got installed.

Let's say the supply is switching at 100kHz, which is 10uS period.  The
switch on time duration must therefore be less.  A slow diode that might
conduct for 1us or even a few 100 ns would be doing that for a significant
fraction of the switch on time.

This is one reason Schottkey diodes are used in these applications when the
voltages allow it.  Ordinary diodes are a P-N semiconductor junction.
Remember that conduction is all about availability of minority carries in
the depletion region.  The reverse recovery time is the time during which
the remaining minority carriers are swept out of the depletion region.
Schottkey diodes are a metal-semiconductor junction, so have half the
depletion region to start with, and can also flush out the minority carriers
in the depletion region that is there faster.  For most purposes Schottkey
diodes have infinitely fast recovery.

Back to your situation.  You are switching the diodes at the 50Hz line
frequency.  Even slow diodes will be no problem since the voltage is
changing slowly at the crossover point.  The diodes being faster does no
harm since even slow ones are plenty fast enough.  However, specifically
fast diodes may have a larger forward drop at the same current due to
tradeoffs made in the design to get the faster speed.  However, this looks
like it will be a minor issue, if at all, at your voltage.

This application is just asking for the common 1N400x diode series.  These
are meant for line power rectification.

another thing to watch out for is that labels like "fast" and "ultrafast"
are pretty meaningless.  If speed matters, check the datasheet for the real
numbers.  I have seen "ultrafast" diodes that are significantly slower than
"fast" diodes.  Beware of marketing math in use.


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Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\05\28@091417 by IVP

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Thanks Michael, Olin

Most of the larger diodes I have from disassemblies are TO220
Schottkys, which I often re-use in SMPS or for steering. AIUI
they're not suitable for bridges because of their poorer reverse
characteristics. Just checking that fast/ultrafast didn't have any
limitations I wasn't aware of and have more in common with
standard silicon rectifiers

Jo

2011\05\28@122356 by Bob Blick

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Hi Joe,

You can use them but one of the characteristics of slow rectifiers is
that they are "quieter" and you'll get less RF emission than with the
fast ones.

Cheers,

Bob

On Sat, 28 May 2011 22:06 +1200, "IVP" <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz> wrote:
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