Requirements in EMC for Switched Mode Power Supply (SMPS)

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In this new four-part blog series, Lorandt Fölkel M.Eng, our acclaimed Field Application Engineer and Business Development Manager, will walk you through everything you need to know about filter design for SMPS.

When you are building a DC/DC power supply, there are some requirements in electromagnetic compatibility (EMC) that you need to comply with. This post will explore those EMC requirements as they relate to SMPS.

CE Marking for EMC Compliance in Europe

In order to ensure that electrical and electronic devices operate properly in an electromagnetic environment, standards have to be put in place.

When the single European market was formed, it needed to enact one universal standard to remove technical barriers to trade. There are currently 20 new approach directives in all European countries, including electromagnetic compatibility (EMC).

The European Union uses the CE marking to denote EMC compliance. Under the EMC Directive, the CE marking is required on any electrical or electronic product to ensure that any intentional RF signals are not interfered with.

There are different EMC laws in other continents. However, if you want to sell to the European market, you have to put the CE logo on your device and have a Declaration of Conformity.

You could simply buy a CE sticker and stick it on your device, but beware — failure to comply with the law can be a criminal or civil offense (or both). In the German market, there’s an “EMC police” that can charge you up to 500,000€ if your device is found to be incompliant with EMC standards. In other countries, failure to comply can mean jail time.

So, EMC compliance for SMPS is pretty important, wouldn’t you say?

In order to (legally) obtain the CE logo for your device, you must submit the EC Declaration of Authority to the EMC Lab. There are several restrictions, so if you have questions about how to fill out your form, contact the EMC Lab directly.

Other EMC Approval Marks

Additional EMC approval marks exist in other markets, such as the following:

  • United States: Federal Communications Commission (FCC)
  • Japan: Voluntary Control Council for Interference (VCCI)
  • Australia: Australian Communications and Media Authority (ACMA)

The bottom line is that no matter where you market your product, you need to comply with the national EMC regulations. Don’t think you can get away without them!

Conducted Emission

When you go to the EMC Lab, they will check your device for conducted emission, which is measured from 150KHz up to 30MHz according to European law.

The conducted emission is caused by the ripple current at input lines for common mode or differential mode noise. It is measured over a wide band of frequencies.

Radiated Emission

The EMC Lab will also check the radiated emission in the anechoic chamber of your device.

Radiated emission can be measured to a variety of GHz, from 1GHz all the way to 400GHz, depending on the type and frequency of your device. For a standard power supply, 1GHz is more than enough.

There are two different limits for both conducted and radiated emission: home and industrial. Industrial environments are allowed to make a little more noise because they aren’t so, shall we say, sensible. In a home environment especially, you’ll want to make sure your emission levels do not exceed the legal limits, or you could have a big problem on your hands.

The Magnetic Field

Magnetic fields are lazy. They try to take the easiest way.

If you apply the magnetic field to air, the magnetic field will go from the north side to the south side without a hitch. If you apply the magnetic field to a ferro-magnetic material, on the other hand, you will find an easier medium to couple through it. This applies to both rod core and ring core ferrites.

This means the formula differs for induction in air vs. induction in a ferrite. You can see the two formulas illustrated in this image.

The Permeability of Core Materials

Permeability is very temperature-dependent in ferrites.

In the core material of your device, the magnetization will fluctuate based on the temperature. So if you qualify your product as “ambient room temperature,” but the temperature drops, the permeability will drop as well.

If you have a very hot ambient temperature (especially for automotive or heating applications), you may reach what’s called the “curie-temperature,” at which point your device probably won’t be working.

That said, as soon as you cool the inductor down, the ferrite will go back to normal, provided the wire didn’t burn. This means you actually can’t damage the core material just based on temperature alone.

Calculating Complex Permeability

So how do you measure the permeability of your device? You can see the formula for measuring impedance in this image.

When calculating permeability, it’s important to know not only the impedance itself, but also which type of impedance you have: reactance or losses?

The type of impedance will all depend on your intention for your device. If you’re using the inductor for power supply, you’ll need to focus on impedance reactance, because they will give you a good idea of whether or not the core has a good ability to store energy. If you’re using the inductor to filter something, you’ll need to focus on impedance losses.

Core Materials of Inductors (Energy Storage)

Conducting the impedance for inductors (energy storage) will depend on the type of switching frequency you use.

The three main type of inductance materials are iron (1-200kHz), manganese-zinc (1-5MHz), or nickel-zinc (1-20MHz). Iron core is used primarily for toroids, manganese-zinc is used primarily for transformers, and nickel-zinc is used primarily for power inductors.

The world of inductor materials is improving greatly in recent years. Believe it or not, you can actually build a DC/DC converter up to 10MHz, thanks to new developments from Texas Instruments and Linear Technology. Ten years ago, that capability would have been unheard of!

Core Losses

So, what is causing the noise in your device? It all has to do with the laws of energy.

Electromagnetic energy cannot just disappear. It will simply be transformed into another energy form, according to the energy conservation law.

This means that electrical energy would be transformed into thermal energy. In that case, the core losses from the ferrite will transform the noise energy into heat. Even if you used a very sensitive camera, though, you wouldn’t be able to measure this temperature rise.

If you ever build a device with noise so loud that your ferrite starts getting hot, you don’t have an EMC problem — you have a safety problem. Run away as fast as you can!

Stay Tuned to Our SMPS Blog Series!

For more information on filter design for SMPS, stay tuned to our blog for the next post in this series!