Friends Don't Let Friends Use Un-Shielded Inductors

Yikes – I saw another one of these power supplies in a high sensitivity receiver. You've seen them too, those sweet little problem solver DC/DC converter modules (figure 1).

 

Figure 1 – These sweet little DC/DC modules sure solve power problems, but...

Yes, they solve power problems but they cause all sorts of havoc with the sensitive Analog Signals, especially in wideband systems. I have seen many issues over the years with unshielded inductors and in fact my standard design catalog of parts only lists shielded inductors for this very reason.

An unshielded inductor does not enclose the magnetic field (or the electric field for that matter) and ‘will’, notice I didn't say 'may', because it 'will' couple switching noise into other parts of the system.

Into PLL Synthesizers, RF front ends and IF circuits these little beasts spew their evil. In analog radios the noise level was high enough that you would probably never notice the issue, but with wideband digital radios and other measuring circuits that have dynamic ranges of 90 dB and more the problems are hard to miss.

There are a few alternatives,

1) Only buy a DC/DC module with shielded inductors. Downside: That's hard to do because cost is everything in these designs and shields cost money, so shielded designs are not easy to find.

2) Build your own DC/DC using good shielding. Downside: Your design will almost certainly be bigger and probably cost more too.

3) Add secondary shielding to the power modules: Downside: Size and cost.

4) Don’t do anything and hope for the best. Downside: Your product is cheap, but doesn't work very well, if at all.

To demonstrate the problem, I setup a magnetic / electric field probe [1] a few inches from the operating DC/DC module as shown in figure 2.

 

Figure 2 – A measurement of the ‘spew’ was made with a small probe placed a few inches from the operating DC/DC converter module.

Measuring the resulting field produced with an oscilloscope trace as shown in figure 3.

 

Figure 3 – The probe measurement for the unshielded inductor of the DC/DC converter.

To demonstrate how simply shielding the inductor works, I located a Ferrite ring of the proper size in my junk box of parts. The Ferrite ring was a bit tall but otherwise it was a perfect fit diameter wise as can be seen in figure 4.

 

Figure 4 - I found a Ferrite ring in my junk box of parts that fit this inductor early perfectly.

With the probe in the same location as it was when the measurement of figure 3 was made, the Ferrite ring was added to the DC/DC operating on the PCB and another measurement was made as shown in figure 5.

 

Figure 5 – The probe measurement is vastly improved simply by adding a Ferrite shield to the inductor of the DC/DC converter.

The reduction in measured field strength is obvious, it’s is nearly a 5:1 peak-peak voltage reduction, just by adding a Ferrite ring around the switching inductor!

As noted above it may be difficult to find commercial DC/DC modules that use any shielding, especially in the low cost open frame market segment. In these cases enclosing the power section in a steel can type of shield on your PCB will help. A small steel can PCB shield like is shown in figure 6 is effective at shielding electric and magnetic emissions over a broad frequency range. The use of shields especially when combined with proper analog grounding techniques [2], noise reductions of 25 to 50 dB are typically obtained.

 

Figure 6 – These low cost, semi-custom shields are a rel lifesaver when designing with high noise switching (or digital) circuits around sensitive analog. Place these shields both the analog sections and the noise generating circuits for maximum effectiveness.

Remember: Be a good friend and don't let your friends do the wrong things, if they do anyway, be sure to hand them a shielding can!

Notes:

[1] This home made probe consists of a cut in half torrid wound with 10 turns of magnet wire. The signal is fed through a 50 Ohm coax line to a 20 MHz bandwidth limited oscilloscope input that is terminated in 50 Ohms at the oscilloscope.

[2] Proper analog grounding is defined as: Adequate separation between analog traces AND maximum, unbroken ground planes everywhere else.


 
Article By: Steve Hageman www.AnalogHome.com    

We design custom: Analog, RF and Embedded systems for a wide variety of industrial and commercial clients. Please feel free to contact us if we can help on your next project. 

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