Author Topic: How to measure rippel of small DC/DC converter.  (Read 823 times)

Offline PekkaNF

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How to measure rippel of small DC/DC converter.
« on: February 26, 2018, 04:54:41 PM »
Planning to use a small DC/DC convertter to power up an instrumentation amplifier and maybe later an isolated amplifier. Power supply rippel (and RF-componenet) could screw up an amplifier signal.

Don't remember much how to measure rippel of the power supply. Luckily I rememeber some termonology.

Searched some  "how to measure power supply ripple" videos


And this is the first victim (because I happen to have it):
https://www.tme.eu/en/Document/a29dd13f72ddce9b78226ed02d82913d/qdc2wsil5_15-15.pdf

I could conform the switching frequency and it does seem to have rippel - which is actually harder to measure than I though. I can separate swiching spikes (and see swiching frequency and see if it has any effect on amplifier).

Found out easily that voltage and rippel depends a lot on how you load amplifier.

Two scrneenshots, neither of them even close to trutfull. First shows the effect of gounding clip. Second is measured with a "spring clip" (shortest probe length/grounding loop), 70% of nominal loading - purely resistance and keeping all wires as short as possible.

The scope is RIGOL DS1052E and it is 50 MHZ 2GS/s model. It has very little control over samplig rate/filtering, set it 20MHz and can't verify DC/DC converter ripple one way or another.

I should see characteristic ripple, should I?


Offline PK

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Re: How to measure rippel of small DC/DC converter.
« Reply #1 on: February 26, 2018, 05:02:30 PM »
Scope settings look right, ripple period should be either 8uS or 4uS depending on what rectification is used. Maybe it's a good power supply?


PK

Offline PekkaNF

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Re: How to measure rippel of small DC/DC converter.
« Reply #2 on: February 28, 2018, 02:56:23 PM »
It does looks good enough. But it was not a simple aim&shoot probe hitting.

Turned out that ground clip picks up all signals there is in the universe. First picture: probe grounding with springgy thinggy.

Second picture: feeder PSU, DC/DC converter not connected.

Third picture: same spot than #1 (primary  of  DC/DC) converter connected and loaded 70%. Looks like could use capacitor on input side.

Fourth picture: Output of DC/DC converter. Looks ok.

Need to wait op amps to arrive and then hook them together and see if the power supply ripple created problems.

Is there any easy rules on ferrite beads and miniature chokes on power line?

eskoilola

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Re: How to measure rippel of small DC/DC converter.
« Reply #3 on: March 01, 2018, 10:53:57 AM »
Rules with interference tolerant circuits:
  • Keep the impedances suitable - not too high and not too low either
  • The Op-Amp inputs should be grounded at he same physical point. This is to avoid Your ground-clip effect  :zap:
  • The supply traces should be as short as possible and be shorted wit a low ESR capacitor. The capacitor does net need to be large - just low ESR. Maybe a 0u1 ceramic surface mount
  • If you can filter out high frequency components out of the ripple then the Op-Amp can take care of the rest if there is no differential interference in the inputs. The common mode rejection usually takes care of common mode interference.

Offline PekkaNF

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Re: How to measure rippel of small DC/DC converter.
« Reply #4 on: March 01, 2018, 12:50:40 PM »
Yup. Thank you, sounds vaguely familiar. There is ofcourse a small problem....

Planning to use this DC/DC converter to feed:

1: isolation amplifier, low voltage(immediate need is <60V, really would not mind to build final version to survive three phase 400 VAC), low frequency (isolation amplifiers are not fast) AMC1200 + some pre amplifier - which I haven't fiqured out yet.

2: small signal (TTL and such) differential amplifier. Nothing too fancy, my scope is 50 MHz. Video amplifier should do. There are many designs, some very ambitious and very involved.

There are some hurdles to overcome...some are very trivial to those who are in the know, but I'm fighting to understand.

Right now I am in PSU rippel measurement/definition phase.

Next thing is to figure out how to transfer that signal to scope. For low frequency righ now shielded and twisted pair cable, maybe even ethernet cat5e cable looks pretty good. Traditional 50 ohm transmission line feels safe, but my scope is that basic - it does not even have internal 50 ohm termination.

If I need passive probe, I'll buy it. I wish there were cheap 1:5 or 1:10 ready made compact scope probe for 0,1" square test pins - maybe I need to buy cheap one and bucher one to fit a pinheader.

Pekka

Offline PekkaNF

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Re: How to measure rippel of small DC/DC converter.
« Reply #5 on: March 03, 2018, 08:01:11 AM »
I have a silly question:

If I feed on one line cable a remote probe (1-2 metres away) have first linear regulator feeding an  non isolated part of the circuit and then nearby (shielded) DC/DC converter providing an isolated power for the rest of circuit.

1: Any obivous problem feeding an linear regulator and DC/DC converter on same feed line?

2: have used before 317 type regulators, but there seems to be new low drop regulators available. Any suggestions to check data sheets and application notes on +/- 5 to 15 V range?

Thank you,
Pekka

eskoilola

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Re: How to measure rippel of small DC/DC converter.
« Reply #6 on: March 03, 2018, 10:10:33 AM »
317 series regulators are actually power amplifiers. As such they suffer from same symptoms like limited bandwidth. It has difficulties in regulating very fast changes in input voltage or fast changes in load. In Your case some filtering would be a good idea. A small inductor and a good quality capacitor in the input side.

Further more, it would not be a bad idea to not let the DC/DC converter to yell into the feed line. Again, a small inductor and now maybe two good guality capacitors.

Offline PekkaNF

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Re: How to measure rippel of small DC/DC converter.
« Reply #7 on: March 03, 2018, 01:15:19 PM »
Thank you, that is very valuable. Used them on good old times, but had to make sure they had enenough voltage drop, bulk capacitors, some extra diodes (in automotive or such environment) and almost got comfortable with them.

I don't know the new kids on the block.

There are some new breed low drop stuff like LT1761, but I don't know how those behave with op.amps and converters.

« Last Edit: March 04, 2018, 03:09:52 AM by PekkaNF »

eskoilola

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Re: How to measure rippel of small DC/DC converter.
« Reply #8 on: March 03, 2018, 04:17:00 PM »
The LDO does not defy physics. The same rules apply - the to220 case WILL became hot unless cooled, the same voltage drop at a current still produces the same amount of heat.

I would suspect the new devices behave a tad better than the old (LM317 for example) with transients and stuff. They might also be less noisy and accurate.

The LDO is there just telling that the device is capable of working very near the input voltage (hence the Low Drop). The newer types also consume less current.

Offline WeldingRod

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Re: How to measure rippel of small DC/DC converter.
« Reply #9 on: August 12, 2018, 07:38:38 PM »
I suggest a tiny ac transformer.   Speaking  from experience, high frequency is hard to get rid of.
If you have some $$, call up AAK Corporation; quietest DC power supply I've ever found, and I was measuring 1 part in 15,000 signals on top of a 7000 out of 15,000 signal.  Isolation is your friend!

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