Re: Need low cost ±15 VDC power supply design

On Sat, 20 May 2006 22:57:12 GMT, "Harry Dellamano"
<harryd@xxxxxxxxxxxxx> wrote:


"Tom" <twp@xxxxxxxxxxxxxx> wrote in message
news:126v0ju3bu8qc6d@xxxxxxxxxxxxxxxxxxxxx
I am looking for a low cost dual output DC power supply design with the
following requirements:

-input voltage 4.75 vdc to 30 vdc (must be able to work throughout the
range)
-output voltage +15 vdc and -15 vdc
-output current requirement 25 mA maximum (anything over this is overkill)
-relatively noise free output (ie. ideally less than 10 mV

To date we have looked at using a DC to DC converter for this application
but the startup current, although brief, is way beyond the 25 mA maximum
which makes it unacceptable.

We are also looking at a design that uses a Linear Technology LTC1871
chip. This device shows promise but we are experiencing severe noise
problems.

A complete design or any suggestions you might have on using the LTC1871
chip to do this job would be most appreciated.

If the converter must start at 4.75VDC then the starting current must be
greater than:
Is= (2*15*0.025)/(0.85*4.75) = 186 mA which is greater than 25mA.
Does this design require any laws of physics to be compromised?
A flyback topology can completely control it's start up current profile.

Voltage controlled circuits can also be configured to limit inrush
through tailoring of the output rise-time.

As the LTC1871 has only a crude peak input limit, and no slow start,
then using the voltage control loop might be the only way to do this.

The OP seems a little confused about why an unspecified DC-DC
converter was 'unacceptable'.

He also didn't really specify what the 'noise problems' were with the
LTC1871 - emissions, interference, unpredictable regulation or
whatever.

Still no word on isolation requirements.

The 4.75-30VDC input requirement suggests that he doesn't have a
practical application in mind, as this doesn't describe any typical
industrial source. He may simply be reading control chip specs -
always a bad sign - because it means you likely can't do it with the
chip described in the data *** he's reading. The 1871 is ruled out -
5V6 typical start-up.

RL
.

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