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'[EE]: 15, -15V @~3A PSU.'
2002\11\28@000003 by A.J. Tufgar

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Hey all,
       I'm trying to design a switching or non-switching PSU that will
output -15V and 15V @ 3A.  I tried national semi's online PSU designer
and had limited success.  Same with a google search.  Can anyone point
me in the right direction as to where I could find such a design?

Thanks in Advance,
Aaron

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2002\11\28@003516 by Chris Hunter

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Presumably with an AC mains input....  my choice would be a 18 - 0 - 18
secondary transformer, a bridge rectifier, smoothing caps, and a 7815 & 7915
regulator pair with "wraparound" transistors to increase the current
handling (the ICs are only good for about an Amp).  You can get 3 Amp
versions of the 15 volt regulators, but they tend to be quite expensive.

Chris

{Original Message removed}

2002\11\28@003524 by Russell McMahon

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> Hey all,
>         I'm trying to design a switching or non-switching PSU that will
> output -15V and 15V @ 3A.  I tried national semi's online PSU designer
> and had limited success.  Same with a google search.  Can anyone point
> me in the right direction as to where I could find such a design?

There are a zillion ways of doing this (most very similar)
Useful info would include:

- Variable or fixed.
- If variable, tracking or independent.
- Do you have existing input supply or transformer etc? If so what?
- What is the load?
- Noise important.
- Efficiency important?

The better people understand your need the more likely it is that they can
help.


       RM

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2002\11\28@005804 by A.J. Tufgar

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Sorry I'll clarify a little.  :)

I'm assuming AC mains in, so I need a step-down, etc yes.
I don't need variable but I also will be using 12V, -12V .1A regs on it
as well, so I prefer the 18V, -18V transformer Chris suggested.
I would prefer to keep noise down to <5 mV.
Load is varying anything from 3ma to 3A.
Efficiency isn't really important but I'd like to get away with smaller
heat sinks, but oh well if not.  :)

I don't mind describing the application but I wanted to keep the post
as short as possible.

Could I use a hexfet as a pass-transistor so it won't get as hot or is
this a bad idea for some reason?

Thanks,
Aaron

{Quote hidden}

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2002\11\28@032512 by Russell McMahon

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> I'm assuming AC mains in, so I need a step-down, etc yes.
> I don't need variable but I also will be using 12V, -12V .1A regs on it
> as well, so I prefer the 18V, -18V transformer Chris suggested.
> I would prefer to keep noise down to <5 mV.
> Load is varying anything from 3ma to 3A.
> Efficiency isn't really important but I'd like to get away with smaller
> heat sinks, but oh well if not.  :)

Chris's suggestion is typical of conventional solutions.
If you are getting a transformer custom wound (can be as cheap sometimes
depending on what's available) you could consider getting as close to
optimum as possible at full load. But 18-0-18 OK.
That peak rectifies to say 18*1.414 = 25 volts say or somewhere over 20 on
load. So at 3A per side you'll dissipate 3A x (20-15) = 15 watts per
regulator. Add another 1A x (20-12) = 8w for the each of the 1A supplies. So
if these are all on the same heatsink you have about 50 watts of dissipation
and 110 watts odd of load power. That's a moderately large heatsink at
reasonable temperatures. Depending on ripple and dropout of the regulators
you can probably halve that with a lower DC voltage. One way which is a
little brute force is to have a multitapped mains winding. A 15-0-15
transformer will give you the better part of 20 volts DC and may be enough
for this application depending again on ripple (which depends on filter
caps). .

5mV noise is much easier with a linear regulator than a switcher. One trick
I have seen used which is quite effective is to use an SCR pre-regulator-
two of the 4 diodes in the main rectifier are SCRs and these are phase angle
controlled to keep the DC bus level as low as reasonably possible. Not  a
totally noise free solution but worth a thought.

Anything used as a linear regulator will dissipate the same power, whether
hexfet or IC or bipolar transistor. A switcher(which the SCR pre-regulator
is arguable a version of) avoids this by duty cycle modulation the input in
some manner.




       Russell McMahon

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2002\11\28@104405 by Olin Lathrop

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> Could I use a hexfet as a pass-transistor so it won't get as hot or is
> this a bad idea for some reason?

In some applications FETs don't get as hot as bipolar transistors would
because they have a very low on resistance (10 - 20 mOhms is readily
available) and therefore have less voltage drop accross them.  FETs don't
just run cooler by some magic.

However, the pass element in a linear power supply will have the same
voltage accross it and current thru it at a particular setting regardless
of how it is implemented.  Voltage x current = power, so whatever pass
element you use will need to dissipate the same power.  That usually
favors bipolar transistors because they tend to be cheaper at the same
power levels.  Also, many FETs are intended for switching use where they
don't dissipate much power, and therefore aren't available in high power
packages compared to their current ratings.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\11\28@120711 by Chris Hunter

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You're absolutely right about the dissipation, but I was just suggesting a
simple solution that could be built with parts available anywhere.  He could
probably get away with a 15 - 0 - 15 TX, but ripple may suffer, and most
small transformers have very poor regulation - as he loads it, the TX
secondary voltage will probably droop.....

A switcher might be a viable solution, but these tend to be more expensive,
less reliable, and almost always have some amount of ripple!

Chris

{Original Message removed}

2002\11\28@121335 by Chris Hunter

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The dissipation will remain the same irrespective of the type of series pass
device used (to all intents and purposes) - I'd probably choose  plastic
power bipolars, as they're cheap, have plenty of current handling
capability, and don't need to be isolated from their heatsink.

Chris

{Original Message removed}

2002\11\28@124325 by A.J. Tufgar

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Allright I think this is my plan, I'm going to modify the supply here
from an LM7812 to LM7815 and only use one transistor.
http://www.mitedu.freeserve.co.uk/Circuits/Power/1230psu.htm

I'm going to use a VCT trans as suggested and hook up another circuit
in the same configuration (with some obviuous modifications i.e.
7915)to the neg terminal of the rectifier.

For the negative side though I assume I use an NPN pass trans. instead.
Is this okay any 'gotchas' on doing this?  Just want to make sure I
hook the negative pass transistor up correctly.

Thanks for all your help,
Aaron

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2002\11\28@151300 by Chris Hunter

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That's correct - obviously, the negative regulator requires a transistor in
the "opposite" sense - ie: NPN.  Can't see the reason for having the 1 Amp
fuse on the output of the regulator, however, because if the fuse blew, the
output would rise to the unregulated input voltage, which could result in
the destruction of whatever you're powering....

If you're concerned about overvoltage, you should employ a "crowbar"
circuit - look it up on the 'net - which will blow the supply fuse before
anything dramatic happens to the gear attached to the output.

You can use a  single 2N2955 and a single 2N3055 for the series pass
transistors - at 3 Amps, they'll be coasting if they're on a reasonable
heatsink.  These devices are cheap, but the cases are NOT isolated - you'll
need heatsink isolation kits for them.

The only real "gotcha" is that the pinouts of the 7815 and 7915 are
different!

Chris

{Original Message removed}

2002\11\28@153956 by A.J. Tufgar

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Thanks Chris,
            Yeah I plan on leaving out the fuse and putting one on the
mains and probably the secondary.

Anyone know where I can get the 2N2955, I can find the 2N3055 but no
luck with 2N2955, from digikey, arrow, newark or mypioneer.

Thanks again for all your help,
Aaron

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2002\11\28@183945 by Olin Lathrop

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> Anyone know where I can get the 2N2955, I can find the 2N3055 but no
> luck with 2N2955, from digikey, arrow, newark or mypioneer.

These are both large brick-outhouse type bipolar transistors in a TO-3 case.
The 2N3055 is NPN and the 2N2955 is PNP.  It doesn't have to be exactly a
2N2955, just a big fat PNP transistor that can handle a lot of current and
dissipate a lot of power.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\11\29@001549 by Chris Hunter

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You can definately get them from Farnell (or Newark).  You don't actually
need anything quite as big as the 3055 / 2955 - you could easily use a pair
of the smaller, cheaper plastic devices - I used a BD 185 / 186 in a similar
PSU recently - these are 4 Amp rated at 30 Volts CE.

Chris

{Original Message removed}

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