Re: Low noise DC/DC converter

From: Fred Bloggs (nospam_at_nospam.com)
Date: 12/27/04 

Date: Mon, 27 Dec 2004 23:46:06 GMT

Jeroen wrote:
> "Fritz Schlunder" <me@privacy.net> wrote in message
> news:33b8saF3usut1U1@individual.net...
>
>>"Jeroen" <jayjay.1974@xs4all.nl> wrote in message
>>news:41cf366a$0$6219$e4fe514c@news.xs4all.nl...
>>
>>
>>>I'm currently exploring the interesting details of building and
>>
> designing
>
>>>transformers. I need an isolated 5V to -15V/+15V/3V3 convertor, that
>>
>>should
>>
>>>be very low noise, as it supplies a 12 bits ADC with additional signal
>>>conditioning circuits. Currently I'm using two C&D Tech. 1W DC/DC bricks
>>>followed by low noise LDOs. The noise (or better, the switching residue)
>>
>>can
>>
>>>generate errors up to 48 ADC counts. Actually these little bricks seem
>>
> to
>
>>>generate more noise then three switchers in the digital part of the
>>
> board
>
>>(a
>>
>>>LT1940 and a LT3431 to create a 1V5/1.4A, 3V3/1.4A and 5V/3A supply).
>>
>>Hi there.
>>
>>Have you tried hanging one or more extra LC low pass filter(s) on the
>
> output
>
>>of the DC/DC converter modules? In my experience playing with DC/DC
>>converters this extra LC filter can sometimes greatly improve at least the
>>aesthetic appearance of the output voltage as viewed on an oscilloscope.
>
> On
>
>>the other hand, I have been significantly disappointed by the noise
>
> reducing
>
>>capability of hanging a linear regulator on the output of the switching
>>supply. A typical DC/DC converter runs at 100kHz+, so presumably most of
>>the noise produced by it will be at that frequency and higher.
>>Unfortunately the supply voltage rejection of normal linear regulators
>
> seems
>
>>to be fairly low at 100kHz and even lower in the MHz range which (at least
>>for me) is the most aesthetically unpleasing noise to look at on the
>>oscilloscope.
>>
>
>
> I did include LC filters on the inputs and outputs and I used low noise
> linear post regulators. It's a six layer board with two ground planes. With
> very close grounding measuring techniques I still have nice 100mVp-p spikes
> on my scope reading, nicely coincident with the switching of the regulators
> (there are a few test pins on them). Even with the 512x averaging mode on
> the scope these spikes stand out in an otherwise rock stable supply.
>
> These spikes contains a lot of higher harmonics that simply shoot through
> every parasatic component of each part, including the filter inductors (the
> winding-to-window capacitance). I think the operating frequency of the DC/DC
> is of no point here; you either end up with 200.000 spikes per second or
> 2.000.000 spikes per second. The problem is even worsened by having two of
> these DC/DC that are not synced, they both run at approx. 100KHz depending
> on input voltage. The sampling rate of the ADC is around 37-40 ksamples/sec.
> Digital noise is of no concern in this design; all digital parts are
> non-operative while it's converting (it's a serial DAC).
>
> I think no amount of mu shielding, bypassing, inductors, beads and ground
> planes will kill these spikes; it's simply easier not to generate these
> spikes to begin with.
>
>

Are you using shielded inductors and star-grounding? It sounds like you
have a serious magnetic induction problem. Then again oversized
switching inductors can be used to kill the current ripple. Your scope
reading spikes are almost certainly magnetically induced to get 100mVpp
on close GND readings- this can't happen unless your switching inductors
have some hellacious parasite shunt capacitance. It looks like shielded
components, physical separation, differential signaling in critical
paths, and optimal pickup loop minimization everywhere, are in order.

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