netgear smpsu update

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Ross Herbert kindly drew up a schematic of his unit.
I've posted it at:
http://members.ozemail.com.au/~pebarhug/images/smpsu.jpg
please ad a 10nf cap from pin 8 of the 3843 IC to primary ground and  the
zener on pin 7 is 20Volt.

Peter



Re: netgear smpsu update



"Peter"
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** Some of the component values are kinda hard to read   -   is the word
"Schotty" next to the diode on the primary side of the transformer  ??





......  Phil




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yeah, looks like it.
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Re: netgear smpsu update



"Peter"
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**  When you find an 800 volt Schottky diode  -  let us know.





.......   Phil





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hahahahahahaha

reel him in!!!!!
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Silicon Carbide:

http://www.sensitron.com/sic.htm

600 and 1200V schottky diodes :)


of course it'd be overkill there, thats what god invented BYV26E for.

Cheers
Terry

Re: netgear smpsu update



"Terry Given"  = criminal  KIWI  FUCKWHIT



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**  1.6 to 1.8 volts drop  at rated current !

  ROTFLMAO

  What a schizo,  sheep shagging cretyn !!





.........   Phil



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Phil, whats the voltage drop of a silicon diode at 10-20A? what about
Qrr? Now you see why these exist.

And they do exist, hence my "letting you know"

Cheers
Terry

Re: netgear smpsu update


On Tue, 23 May 2006 17:56:45 +1000, "Peter"

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Note that there should also be a 10nF from pin 8 to gnd (error of
omission)and the zener at pin 7 is 20V.

Ross H

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what possible use is the 750k? by itself, it wont trip the current
comparator until the DC bus reaches 2800V or so.

It'd be interesting to look at the turn-on overshoot, i'd expect it to
be quite bad. Likewise the response to a step load removal will be
pretty shitty. I presume the output choke is a bobbin core?

Cheers
Terry

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That one has me stumped as well. I had to triple check to make sure I
was seeing right. I haven't seen this arrangement on any SMPS before.

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Re: netgear smpsu update


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I can think of no valid reason to do this.

its often done (albeit at lower voltages) to reduce the current sense
threshold by adding an offset to the comparator, but here the offset is
only 137mV, 1/8th of full-scale. If that was the intention, the smps
supply could do that with a much smaller resistor (40k or so).

it is also common to add in some slope compensation at the current sense
comparator, with a resistor to the Rt/Ct pin (actually, a buffered
version thereof), but thats NOT whats going on here.

I'd be a bit concerned about the feedback loop, although a pcmc flyback
looks like a single pole, the output filter adds another pair, the opto
adds a 4th pole, and the 10nF cap across the opto adds a 5th pole. thats
probably why the feedback network is a 1uF cap. yuk.

I once worked on a 400W flyback (1000Vdc in, 24Vdc out) whose
closed-loop bandwidth was 1Hz - you could see the step response on a
moving-coil meter.

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Cheers
Terry

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"Ross Herbert"


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**  The 750 kohms simple pre sensitises the current sense input.

    Likely saves the mosfet in some overload conditions.





.......  Phil





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what, by an extra 137mV from 1V? thats not very sensitive. And a 45k
resistor to UC384x supply would do the same job.

Cheers
Terry

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"Terry Given"

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 **  WRONG.




.........   Phil



Re: netgear smpsu update


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not if the 20V zener is actually clamping.

if the zener is not clamping and the chip supply is less, then yes, a
smaller resistor will do. baby maths:

137mV = Vcc*(301.8/(Rvcc + 301.8))

if Vcc = 340V, Rvcc = 750k, Pr = (340-0.137)^2/750k = 154mW

this needs to be a 1/4W 400V rated resistor


if Vcc = 20V, Rvcc = 43.8k, Pr = (20-0.137)^2/43.8k = 9mW

an 0201, 0402, 0603, 0805 or 1206 smt resistor would work fine here.


if Vcc = 12V, Rvcc = 26k,   Pr = (12-0.137)^2/26k = 5mW

and here.

750K to Vdc is a pretty daft thing to do.


Cheers
Terry

Re: netgear smpsu update



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You mean mosfet will save resistor in some overload conditions.

Q. What is the fastest thingie on three legs?
A. FET

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