Capacitor self resonance and DC-DC converter - Page 1
Forum section: electronics / projects

author: ivan747
author_rank: Super Contributor
timestamp: April 25, 2014, 12:48:02 am
content: Alright, the short story is in bold fonts, but I'd like input on the whole storyI've been designing a pi filter for a DC-DC converter output that has to work form least 20KHz (-3dB point) to 3.5MHz (maximum bandwidth of my single op amp analog circuit). The DC-DC converter works at 80KHz and is one of those self-contained units. It outputs +/-15V at 33mA.I calculated that with a 100uH inductor and a 2.2uF capacitor I had a -3dB point at little over 10kHz.The bandwidth of the filter (10kHz to at least 3.5MHz) meant I had to care about the impedance change in both capacitors and inductors over the frequency range. I found a small inductor with a self-resonant frequency of 6MHz, that works great for me. Now, I needed to check the self resonant frequency of the capacitor I was going to choose to make sure its impedance won't start to rise below 3.5MHz.The problem is that I cannot find many capacitors that have their self resonance frequency specified.Is this normal?How am I supposed to know?I don't want much noise in this circuit. I want to take the noise form 75mV down to 2mV or 5mV from DC to 3.5MHz. Even above 3.5MHz if possible. So I added a ferrite bead in series with each rail of my op amp, and 1nF and 0.1uF capacitors close to the chip. The circuit is supposed to plug into an oscilloscope.Finally, am I theorizing too much on this? Maybe I need to get my hands on it.Thanks guys for clearing that upBy the way, I am trying to make a better version of this, using the same chip, but with a nicely routed PCB and enclosed:http://www.dorkbotpdx.org/blog/paul/measuring_microamps_milliamps_at_3_mhz_bandwidth

author: nickm
author_rank: Regular Contributor
timestamp: April 25, 2014, 02:20:29 am
content: The inductance of the capacitor is basically determined by the package size. 1210 have pretty low inductance and you can get larger capacitance in that size. You will probably incur more stray inductance in your routing than the cap itself. You might have more trouble with common mode noise so you might consider a common mode choke too. I recommend you find out a little more how much filtering you actually need because 100uH is very big and most opamps will have decent psrr in the 10kHz range.

author: ivan747
author_rank: Super Contributor
timestamp: April 25, 2014, 03:10:10 am
content: I'm using 0805 for most passives, and relatively thick traces on power rails.This is the filtering I currently have. That's using the capacitors and inductors I mentioned. 10KHz is quite low, but the app note I used as guide says to pick a frequency well below the switching frequency.http://datasheet.octopart.com/DCP010505BP-U-Texas-Instruments-datasheet-10253576.pdfI am using a high performance differential amplifier with a fixed gain of 2000, that's mostly why I wanted so little noise. Below is the power supply ripple rejection. One of the rails starts rejecting less at exactly 10kHz, although the ripple rejection is excellent, being 140dB at 10kHz and 80dB at 1MHz.I also attached the full schematic and my current PCB layout, it's missing most of the power supply section. It's got a ground plane on the bottom layer, no other traces run through.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 25, 2014, 04:32:40 am
content: Is the PSRR measured at Av = 1? Is that representative of your use?Don't worry about self resonant frequency, the equivalent element (ESL) matters more. ESL effectively acts as a voltage divider with your choke, limiting high frequency performance (or even getting some gain back, rather than trailing off to -infinity dB at very high frequencies). Squigglies of the choke itself obviously matter. If you need big attenuation, better to use multiple stages at various cutoff frequencies.And don't forget to use proper damping. A filter only filters at matched impedances. e.g.https://www.eevblog.com/forum/beginners/lc-filter-op-amp-supply-(ultrasonic-range-psrr)/Tim

author: itdontgo
author_rank: Regular Contributor
timestamp: April 25, 2014, 12:15:27 pm
content: In my experience the answer isn't what you want to hear. You might struggle to find that info. I don't know what anyone else does but I take an educated guess at the value and try various decoupling caps in my circuits to see if they get rid of the frequency I'm looking at especially at high frequency. And also over a few hundred MHz your better with 0603 or smaller for lower ESL.

author: ivan747
author_rank: Super Contributor
timestamp: April 25, 2014, 08:39:33 pm
content: That link proves very helpful, Tim, thanks. I can use that as a clear guideline.You're right about the ripple rejection. It actually gets amplified by 2000.Idontgo, my plan is to get 0805 components, maybe some 0805 jumpers too and a variety of caps just in case.

author: G0HZU
author_rank: Super Contributor
timestamp: April 25, 2014, 10:22:28 pm
content: QuoteDo you have a reasonable 2 channel scope?You can measure the capacitor's series resonant frequency with a signal source (sine wave from a FG or sig gen) and a series sense resistor and the scope.At the SRF the voltages each side of the series sense resistor will be aligned in phase.You can measure the capacitance, and the ESR at various test frequencies this way (i.e. measuring both sides of the sense resistor and the relative phase) as well as finding the SRF.note: a decent 10uF SMD tant should have an ESR of about 0.1R typical from 20kHz through several hundred kHz and the SRF will be about 1MHz if the package inductance is a few nH.

author: peufeu
author_rank: Contributor
timestamp: April 27, 2014, 05:27:27 pm
content: Murata provides data about all their ceramic caps, ferrite beads etc, you can download spice models... very handy.http://ds.murata.co.jp/software/simsurfing/en-us/index.htmlMost SMD caps of the same physical size have roughly the same inductance.A multistage filter is preferable to get rid of the DC-DC converter spikes which are the most annoying.Usually you don't use inductors but ferrite beads, which are lossy at HF and pose less LC resonance problems.Check Linear Technology AN101.

author: ivan747
author_rank: Super Contributor
timestamp: April 28, 2014, 02:00:05 am
content: Quote from: peufeu on April 27, 2014, 05:27:27 pmThe spice models at leat give me some idea of how's it going to look.Thumbs up to Murata and to you. I was kinda looking at another application note by Jim Williams, as I knew he had written about this, but it wasn't as relevant as I thought. It was written in the mid-80s and DC-DC converters were very different back then. But this one is relatively recent. Thanks.Quote from: G0HZU on April 25, 2014, 10:22:28 pmA signal generetor is already in my list of things to buy, so I don't have one right now. :/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 28, 2014, 04:57:42 am
content: Quote from: peufeu on April 27, 2014, 05:27:27 pmI wouldn't suggest using FBs on anything but signal lines. They saturate at the drop of a hat... and have fun finding DC bias characteristics for most of them.The small monolithic "inductors" (say 0.5-2uH and 0.1-2A) are essentially ferrite beads made of actually correct material, which does not saturate in the specified range. They usually have higher Q too, so double check your circuit -- put a resistor across, or R+C to ground (if the lossy bulk cap isn't enough) as needed.Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 29, 2014, 07:19:23 am
content: I have measured the self resonant frequency of quite a few 0805 capacitors with my spectrum analyser/tracking generator and found that the lead inductance works out to about 1.4nH, regardless of capacitor value.Obviously the self resonant frequency changes with capacitance value but using the 1.4nH rule you can predict the SRF of the capacitor reasonably accurately.The SRF is essential information if you are designing RF circuits.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 29, 2014, 07:28:00 am
content: ^ Which proves that paralleling different value caps is silly; use the biggest value you like, and the impedance will be just as low at, you know, 1MHz as 100MHz, versus a different value. Staggered sizes can help, but for most purposes, the impedance magnitude is low enough not to need that (who cares if it's inductive or capacitive if it's 20 miliohms). At RF of course, it's good to know all these things. Also worth noting that a via is 1-3nH, so if you need serious low nH, use multiple in parallel, either side of the cap and such.Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 29, 2014, 07:52:58 am
content: T3sl4co1lGood advice from you. I should actually try to measure via inductance.

author: peufeu
author_rank: Contributor
timestamp: April 29, 2014, 07:19:10 pm
content: Quote from: T3sl4co1l on April 29, 2014, 07:28:00 amActually the only reason for spreading values is to tame antiresonance peaks...But if all caps have the same value, there is no peak...But if you have too many caps of the same value, at the SRF the resistive part of the impedance is very low and the regulator may bitch about it...etc...