Design Article
Understand and reduce DC/DC switching-converter ground noise
Jeff Barrow, Senior Director of Analog IC Design, Integrated Device Technology, Inc.
11/21/2011 10:14 AM EST
The second, major ground-noise problem, shown in Figure 5, is a result of parasitic-inductor capacitance.
Figure 5: Changing LX node voltage pumps charge through the parasitic buck-inductor capacitance, CL, and into the parasitic ground-path inductors, Lp1 and Lp2 , causing ground noise. (Click here for enlarged image.)
Voltage cannot change instantaneously across a capacitor, nor can current instantaneously change through an inductor. So, voltage changes on the LX node couple directly across both the parasitic buck-inductor capacitance, CL, and the buck-filter capacitor, Cbuck, to appear across the parasitic ground inductors, Lp1 and Lp2.
Initially, no charge flows, but in the next moment, current builds in all of those components until the energy stored in the parasitic buck inductor capacitor,
ECL = ½ CLVLX2,
transfers to the wiring’s parasitic magnetic field,
ELp = ½ Lpi2changing_max
(where Lp = the sum of all parasitic loop inductors). Then like a swing, that unwanted energy passes back-and-forth from the electric to the magnetic field until it radiates or dissipates in resistive elements not modeled in Figure 5.
Both the peak voltage and the duration of a ground-noise oscillation are a problem. The peak voltage, measured at node Vgb, is a function of the LX node’s voltage change, the parasitic buck inductor capacitance, CL, and additional parasitic trace capacitance (not shown). A large CL stores more energy, so smaller is better. After selecting the buck inductor’s inductance and current rating, choose an inductor with the highest self-resonate frequency to limit the capacity of CL.
An inductor’s self-resonate frequency is:
fself_resonates = 1/[2π√(LbuckCL)].
Notice that a doubling of the self-resonate frequency reduces the parasitic inductor capacitance, and therefore the ground-noise energy, by a factor of four!
In the case where performance takes priority over cost, maintain the same value of inductance by replacing the single Lbuck inductor in Figure 5 with two series-connected inductors, each having ½Lbuck (Figure 6). For a manufacturer’s series of inductors, the parasitic capacitance is typically proportional to the rated inductance, so one-half the inductance results in one-half the parasitic capacitance.
When inductors are series connected, their values add to increase inductance, but parasitic capacitors add as the inverse sum of inverse values, to decrease total parasitic capacitance. In the case of two series-connected one-half Lbuck inductors, total inductance will be Lbuck_new and total parasitic capacitance will drop by a factor of four to one-quarter CL.
This reduction is parasitic inductor capacitance will, in turn, reduce ground bounce, Figure 6.
Figure 6: Two series-connected inductors have the same inductance but with one-quarter the parasitic capacitance; charge-pumping is reduced and, therefore, so is ground bounce. (Click here for enlarged image.)
By exploring the models and understanding the two sources and mechanisms of ground noise as induced by the ubiquitous DC/DC switching converter, engineers can minimize the effects in the early stages of design, component selection, and layout, and the subsequent product headaches and re-spins.
About the author
Jeff Barrow is a Senior Director of Analog IC Design at Integrated Device Technology, Inc. in Tucson, AZ. He works on the development and usability of power integrated circuits and is an active analog-IC designer. He received a bachelor's degree in electrical engineering from the University of Arizona (Tucson), and his personal interests include geology, astronomy, physics, and electronics.
Editor's note: Liked this? Want more?
If you are interested in "power" issues such as components; efficiency; thermal concerns; AC/DC and DC/DC supply topologies; batteries; supply ICs; complete supplies; single- and multi-rail management; and supply monitoring: then go to the Power Management Designline home page here for the latest in design, technology, trends, products, and news. Also, sign up for our weekly Power Management Designline Newsletter here.
|
|
|
|
|
Sanjib.Acharya
11/21/2011 1:17 PM EST
Very good article. I am going to apply the learnings into the power supply design I got involved in. I would love to see a continuation of this series for other power supply topologies as well.
Sign in to Reply
zeeglen
11/21/2011 2:56 PM EST
Good article, and also would like to see more.
Years ago I ran into a problem with noise from an isolated 48V to 5V converter into a fiber optic receiver, but only when the card was placed into a metal card cage. On the bench alone it worked fine. The local ground noise was common mode in the optical receiver PIN diode to the external transimpedance amplifier connection on the bench, but the metal optical receiver housing was physically installed on the card front faceplate. Placing the front faceplate at earth (chassis) ground capacitively quieted the local ground, which then caused a 3dB worse bit error rate. We ended up having to mount the optical receiver diode on the pcb and run an optical pigtail to the front faceplate connector.
On a subsequent optical receiver I anticipated the problem and used a pigtail in the initial design as well as including mounting holes for a shield for the the optical receiver diode. The shield was needed, the card was stand-alone in a metal box and installing into the box caused similar converter noise problems. All that was needed was a square inch of metal on a couple standoffs at pcb ground to hide the photodiode and transimpedance input from the box.
Sign in to Reply
hm
11/21/2011 7:36 PM EST
Interesting and informative article. Can there be webinar from Author and link available on eetimes?
Sign in to Reply
abraxalito
11/21/2011 11:41 PM EST
Very interesting - but the enlarged drawing links appear to be broken on this page. Second the request for more similar articles, this is first-principle stuff so has wide applicability beyond DC-DC converters.
Sign in to Reply
agk
11/22/2011 5:36 AM EST
The inductor can be made with minimum parasitic capacitance. The bobin of the inductor to have corrugated sections and the wire is wound in these sections will reduce the parasitic capacitance very much. In turn can minimize the ground noises.
Sign in to Reply
PierreBTOL
11/23/2011 7:10 AM EST
Concise & very interesting !
If we could have the same level of explications about the "i/o CommonMode Noise Isolated" DCDC Converter, it will be great !
Sign in to Reply
studleylee
11/23/2011 12:36 PM EST
@PierreBTOL Loved this article! This other one was also useful in tips for common mode noise:
http://www.interpoint.com/product_documents/DC_DC_Converters_Output_Noise.pdf
Measurement and Filtering of
Output Noise of DC/DC
Converters
Sign in to Reply
Fabio007
11/24/2011 3:27 AM EST
Great article, I would like to get a PDF copy of it, but the print link does not seem to work.
To add to comment of agk re: winding inductor with low parasitic C, another trick is to have the turns at the ends of the winding (near the connection terminals) spaced further apart compared to turns in the middle of the winding. This is an old trick used by ham-radio builders, and can result is a smaller inductor than would result from spacing all turns the same distance apart, but of course the parasitic C is a bit higher.
Sign in to Reply
Sail Mike
11/24/2011 3:58 AM EST
Missing links for larger images & cant print
Sign in to Reply
BicycleBill
11/24/2011 7:48 AM EST
There is some system bug which is preventing the "enlarge" of Figures 5 and 6 from appearing properly. Here are the direct links for those figures:
http://www.eetimes.com/ContentEETimes/Documents/Schweber/C0893/C0893-Figure5.pdf AND http://www.eetimes.com/ContentEETimes/Documents/Schweber/C0893/C0893-Figure6.pdf
Sign in to Reply
huanben
11/25/2011 1:12 AM EST
GOOOOOOOOOOOOOOOOOD
Sign in to Reply
PierreBTOL
11/26/2011 3:14 AM EST
@ studleylee:
Very useful, thank you for that link !
Sign in to Reply
Jerry.Deng
11/29/2011 2:31 PM EST
good article. However it assumes constant current on load, most IC actually works at constant voltage with varing current dynamically, any idea?
Sign in to Reply
Swamp
12/1/2011 8:53 AM EST
As a slight improvement, it might have been informative to include the specific physical location of the input capacitor described in your article (between the Vin pin of the converter IC/switch transistor and the anode of the switching diode).
Great article, Jeff -thanks!
Sign in to Reply
SLN MURTHY
5/15/2012 1:43 AM EDT
It is an excellent article. Jerry Deng's point is valid and impact of varying current in a constant voltage environment would add more punch to this basic understanding.
Sign in to Reply
silker
4/27/2013 5:28 PM EDT
Can we say that RC inductor current sense method has the drawback of ground line pollution? We intentionally put a capacitor (along a resitor resistor) between inductor legs.
Sign in to Reply