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 -----
From: "Russell McMahon" <apptech@PARADISE.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, November 28, 2002 8:18 AM
Subject: Re: [EE]: 15, -15V @~3A PSU.


> > 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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