If you have a hefty power transistor and heat sink, you could build a
"Ralph regulator" (if that is the correct spelling) to filter the noise. It
is done with a bipolar transistor. The collector goes to the power source,
and the emitter goes to the load. A large capacitor is tied from the base
to ground. A resistor is wired from base to collector. The resistor is
selected so that the transistor is operating out of saturation, but with a
small voltage from emitter to collector to keep the power dissipation down.
A voltage drop of 1 to 1.5 volts (giving about 50 to 75 watts dissipation)
might be all that is needed. Adjust the power source accordingly to make up
the voltage lost across the regulator.
Alternatively, you could still use a similar set-up, but eliminate the
resistor (but keep the capacitor). Use a known quiet power supply capable
of an amp or two to supply voltage to the base. Adjust the base voltage as
needed (about 0.7 to 1 volt above the desired output voltage) to get the
desired output voltage at the emitter. Keep the main power source 1 to 1.5
volts above the output voltage.
Neither of these solutions is 100% stable over temperature, but once they
achieve final operating temperature, you should have no trouble maintaining
the desired ±0.5 volt if the current draw is constant.
There are also other "boosted regulator" solutions that use regulator ICs
and power transistors. You might be able to keep dissipation in the power
transistor low by powering the input of the regulator IC from its own power
supply set (say) 5 volts above the desired output voltage, but source the
current passing through the power transistor (to supply the device under
test) from your noisy supply, set 1 to 1.5 volts above the desired output
voltage.
Donald Borowski
Schweitzer Engineering Labs
Pullman, WA, USA
Derek Walton
<lfresearch@aol.c
om> To
Sent by: Ken Javor
emc-p...@ieee.org <ken.ja...@emccompliance.com>
cc
EMC-PSTC@LISTSERV.IEEE.ORG
08/21/2009 10:05 Subject
AM Re: 28 volt Linear Power Supply
Hi Everyone,
thanks for the constructive comments so far.
I'm not really in a position to use batteries, my voltage spec is very
tight at +- 0.5 volts.
I have tried running through the facility filters, they are 100 amp
rated, but I think a combination of that impedance and the LISN
impedance is too much for the EUT starting current. It seems I can start
the EUT with just the LISN's, but then I have too much noise from my
power source.
I swapped the room filters for some 10 uF caps, same ones that are
called out in MIL std testing, and I almost get quiet enough.
Ideally, I'd like to find a quieter source rather than try to filter.....
Sincerely,
Derek Walton
Ken Javor wrote:
> That would be on the power input. Derek is looking at cleaning up the
> output. For his load, the LISNs will dominate the input impedance above
> about 10 kHz. If those batteries are in place in the test chamber the
> facility filters have been completely bypassed.
>
> Ken Javor
>
> Phone: (256) 650-5261
>
>
>
>> From: Ralph McDiarmid <ralph.mcdiar...@xantrex.com>
>> Date: Fri, 21 Aug 2009 09:35:54 -0700
>> To: <EMC-PSTC@LISTSERV.IEEE.ORG>
>> Conversation: 28 volt Linear Power Supply
>> Subject: RE: 28 volt Linear Power Supply
>>
>> I've seen facility filters make some power supplies oscillate too.
>>
>>
>> Ralph McDiarmid, AScT
>> Compliance Engineering Group
>> Xantrex Technology Inc
>>
>>
>> -----Original Message-----
>> From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Ken
>> Javor
>> Sent: Friday, August 21, 2009 9:12 AM
>> To: EMC-PSTC@LISTSERV.IEEE.ORG
>> Subject: Re: 28 volt Linear Power Supply
>>
>> Your facility EMI filters should provide 100 dB of insertion loss
>> (MIL-STD-220, 50 Ohm system).
>>
>> If that isn't enough, you can bypass the 28 Volt output with a couple
>> lead
>> acid car batteries. Then you can use a pair of 10 uF feedthrough caps
>> you
>> have laying around, and finally your LISNs.
>>
>> If all that doesn't do the job, you need to do some troubleshooting, see
>> if
>> some of the noise isn't common mode (not spec'd by manufacturer) and
>> build
>> your own cm choke.
>>
>> Ken Javor
>>
>> Phone: (256) 650-5261
>>
>>
>>
>>> From: Derek Walton <lfresea...@aol.com>
>>> Date: Fri, 21 Aug 2009 10:13:33 -0500
>>> To: "EMC-PSTC@LISTSERV.IEEE.ORG" <EMC-PSTC@LISTSERV.IEEE.ORG>
>>> Subject: 28 volt Linear Power Supply
>>>
>>> Good morning folks,
>>>
>>> I have been requested to perform a conducted emissions test on a unit
>>> that draws up to 50 amps from a 28 volt source.
>>>
>>> I have a 300 amp supply and its a little too noisy being a switcher.
>>>
>>> Can anyone suggest a make/model that can do 28 volts at up to 50 amps
>>> thats either linear or super quiet please. Its very hard researching
>>>
>> how
>>
>>> quiet supplies are.
>>>
>>> Many thanks,
>>>
>>> Derek Walton
>>> L F Research
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