MIL-STD-461 Conducted Emissions

Hi, I'm Steve Butler. Today, we'll talk about MIL-STD-461 —conducted emissions. Drawn here, we show a basic switching topology. Switching topology used in any switching power supply. And a few things— this is the MOSFET, which will be driven in some kind of on-off controlled manner and a transformer. And this current will be a pulsing current. That will be drawn by the switching topology.

VPT's converters include some sort of LC or even two-stage LC input filter, which will reduce this large pulsating square-wave current into this current here — will be a small sinusoidal-type ripple. But this will appear on the input line and go back into the system. So, this is what we refer to as conducted emissions.

Additional filtering will be needed to make this meet a MIL-STD-461 limit for conducted emissions.

This is the setup for measuring conducted emissions according to MIL-STD-461 revision C.

So, this is an older method and it's similar to what is used for DO-160 conducted emissions.

Additional filtering will be needed to make this meet a MIL-STD-461 limit for conducted emissions. This is the setup for measuring conducted emissions according to MIL-STD-461 revision C. So this is an older method and it's similar to what is used for DO-160 conducted emissions.

So, in revision C we use a 10 microfarad feedthrough capacitor and a clamp-on current probe to measure the conductive emissions on the input line to the UUT, or unit under test, which is DC-DC converter and associated EMI filter. So, the current probe will feed into the EMI analyzer. There are two conducted emissions requirements in 461 C which are relevant to power converters.

The first is CEO1, which is in the frequency band 30Hz to 20kHz. VPT's DC-DC converters do not emit any noise in this range. The second is CEO3, which is in the range of 20kHz to 50MHz this is where we will primarily be concerned.

And this is the setup for MIL-STD-461 revisions D, E, and F.

This setup uses a LISN (or line impedance stabilization network), and the LISN is placed between the 28-volt supply and the unit under test or the DC-DC converter. A schematic of the LISN is shown here, but it's basically a large inductor, and on the converter side or the unit under test side, there is a capacitor wherein we plug into the EMI analyzer and will measure the noise in this manner.

So, the output of the LISN goes directly to the EMI analyzer. The other LISN on the other line is terminated into 15 ohms. In 461 D, E, and F there are two specs for conducted emissions which concern us again. The first is CE101 which is from 30Hz to 10kHz. Again VPT's products do not emit any noise in the spectrum.

The second is CE102 which is from 10kHz to 10MHz and this is where we will be concerned again.

This is the typical schematic of one of our EMI filters. It includes both common mode and differential mode filtering to filter both common mode and differential mode noise, which is generated by the DC-DC converter.

28-volt source, the filtered side is here, and the DC-DC converter is over here. So, first, common mode filtering is taken care of with a common mode choke up front and a set of common mode capacitors on this side. That's a single-stage LC filter for common mode with a resonance around 40kHz.

For differential mode filtering, we have a two-stage LC: the leakage inductance of the common mode choke, this capacitor, differential inductor, and this capacitor. That forms a two-stage LC filter — again core frequency also around 40kHz. For the differential mode we also have this RC. This is important — this is a damping branch.

It will eliminate any resonance, hi-Q resonance, in the EMI filter which can cause interactions with the DC-DC converter. Okay, so this filter placed in front of the DC-DC converter will filter conducted emissions on this line such that, back at the source, the conducted emissions will be low enough that you will meet requirements for MIL-STD-461. VPT's filters meet both 461 C as well as 461 D, E, and F.

This is a typical system with one EMI filter and several DC-DC converters. One EMI filter can filter multiple DC-DC converters up to its rated input current, the rated current of the filter, or rated power of the filter. So, in this diagram, this is the 28-volt input. It is quiet. It's a quiet filtered input and these are the outputs, which are noisy.

All of our DC-DC converters and EMI filters are contained in six-sided metal cases, which does minimize both radiated noise and radiated noise pickup. Okay, in a system like this, what you want to do is be careful not to conduct or radiate any noise back onto this filtered input. So, ideally your input, 28 volts, may come in this side of the board and you might have, if your load circuitry is on board, you might have your load circuitry over here, Just away from the input. If this is purely a power supply card, you need to run your outputs back off the card. Maybe they go off this side — that's ideal. That may not be practical in a real-world situation. So you can run them back off the same side — just be careful. What you don't want to do is overlap the input to the EMI filter and some of the DC outputs. Okay, this will result in a lot of noise coupling from this output back to the input, and you'll have an EMI problem if you do something like this.

You want to keep the inputs and outputs separate. Okay, you can get them on the same input connector. That's usually not a problem. Just don't overlap and you'll be okay. Just maintain some separation, and that's the best way to avoid EMI problems when you get into your board layout.

Okay, now let's go to the lab and take a look at some of this on the bench.

Now we'll take a look at the MIL-STD-461 setup in the lab. This is the DC-DC converter and EMI filter. We have the DVTR2805S, so 30-watt part. And the filter is the DVMC28 and those are mounted here along with the load.

This is the conductive ground plane for the 461 setup. You can see the 28-volt input wire is here. Over here we have the 10 microfarad capacitor, for MIL-STD-461 C. On the back is the LISNs for 461 D, E, and F. The power supply is here and EMI analyzer is here. Now we'll move closer and take a look at the actual measurement.

Now we'll take a look at the actual 461 C measurement. So, first we'll turn the unit on. We have loaded on the EMI analyzer the low-frequency band up to 1MHz. So we'll go ahead now and take a sweep. And you can see it sweep across the bottom of the screen and the limit lines on top. And the emissions of the DVTR are well under the limit. Now we will load the higher frequency.

And just one sec — this is the higher frequency portion of C, 461 C, and it will go ahead and take a sweep. So, again, you can see the measurement sweeping across the bottom of the analyzer screen there — well below the limit line again. 

Okay, now we will change our setup over to MIL-STD-461 D, E, F setup using the LISNs and we'll take a sweep of that.

Now we'll take a look at the measurement for 461 D, E, and F using the LISN. So, we've moved the DC-DC converter and EMI filter over to the LISN side. We've connected the EMI analyzer to the LISN and we’ve also moved the power source. Now loaded on the screen, we have the low-frequency portion of 461 D, E, or F, and we'll go ahead and take a sweep. So, you can see the measurement sweeping across the bottom of the screen well below the limit line.

And now, we'll go ahead and load the high-frequency portion. So while it's loading — again — this is the measurement for the DVTR DC-DC converter with the DVMC EMI filter. We're measuring the conducted emissions. Here is the sweep. You can see a few harmonics show up but they are well below the limit line, so that's the high-frequency sweep of MIL-STD-461 CE102 and the measurement is up to 10MHz.

And that concludes our video for MIL-STD-461 EMI conducted emissions. Thank you for watching.