EMI Noise Sources for SMPS

Thanks for joining us in our special blog series on filter design for switched-mode power supply (SMPS)!

In this third installment of our four-part series (read the first two posts here), Lorandt Fölkel M.Eng will cover EMI noise sources for SMPS.

Representative Noise Sources

Once you have selected the proper filter topology for SMPS, you will need to consider the representative noise sources for your design. But where exactly are those noise sources?

The representative noise source will depend on the type of current in the power supply:

  • Input current caused by voltage ripple: conducted emission
  • Power traces and choke radiating EMI: radiated emission
  • Output current caused by voltage ripple: conducted emission

If you are using a DC/DC converter, you must first consider that on input, you will not have a continuous current. Instead, you will have a switch that you must connect and disconnect regularly to generate a wave on input, which will then automatically generate conducted emission.

The same thing goes for output. You will not have a battery or other continuously discharging current; rather, you’ll have a switch, which is charging and discharging the conductance in the capacitor.

The most important (and dangerous!) thing to note is the ground path between the input capacity and the output capacity. This is the path where radiated emission happens.

Try to keep the radiated emission track (and thus the impedance) as short as possible. Don’t make an antenna from it when making your design.

Conducted Noise at Converter Input vs. Output

So, how can you know which noise source to expect in your design?

As long as you know the parasitic from your device, you should be able to predict the conducted noise of your converter. This is called your input capacity. There is a minimum input capacity you’ll want to be aware of when crafting your design; generally, you’ll want low ESL and low ESR.

If you are experiencing ringing or overshooting, this high-frequency resonance circuit is caused by your output capacity. The best way to avoid this is by using an ideal diode.

For converter input, conducted emission is generated by the voltage drop across RSup and ESRL. For converter output, conducted emission is generated by the voltage drop at ESRC.

In order to filter the conducted noise, you’ll want to select the proper input or output filter. Below, we’ll cover the different types of filters used for SMPS.

“L” Input Filter

The “L” input filter is the minimal recommended filter for input SMPS. Please note that this filter is not efficient at reducing common mode noise on input lines.

That said, the simple L-filter is good for:

  • Reducing current ripple on input line
  • Reducing differential mode noise on input line
  • Reducing radiated emission via input traces

When calculating the input inductance with the L-filter, choose the next-highest standard inductance value. You will enjoy better filter performance.

There are a few things to keep in mind when calculating the rated current, too:

  • To avoid overload considerations, choose a choke with a higher rated current.
  • To avoid losses in efficiency, choose a choke with low DCR.

“T” Wideband Input Filter

The “T” wideband input filter is our recommended filter solution for input SMPS. Again, this filter is not efficient at reducing common mode noise on input lines.

The T-filter is, however, recommended for wideband filtering. Here are a few parameters to keep in mind:

  • Lin for low-frequency filtering (DC/DC converter switching frequency)
  • Ferrite for high-frequency filtering
  • Cfilter shorting AC noise to GND (220pF < Cfilter < 1nF, low ESR)

“L/C” Output Filter

The “L/C” input filter is the minimal recommended filter for output SMPS. Please note that this filter is not efficient at reducing common mode noise on output lines.

There are a few other things to keep in mind with simple L/C-filters:

  • Output filter reduces voltage ripple on output traces (conducted emission)
  • Output filter reduces radiated emission via output traces (radiated emission)
  • There is no optimal solution for radio power devices

“T” Output Filter

The “T” output filter is our recommended filter solution for output SMPS. Again, this filter is not efficient at reducing common mode noise on output lines.

The T-filter is, however, recommended for wideband filtering. Here are some things to note:

  • Lfilter for low-frequency filtering (DC/DC converter switching frequency)
  • Ferrite for high-frequency filtering
  • This kind of output filter is highly recommended for powering radio devices

Decoupling Common Mode Noise

If you have common mode rejection problems, or if you need to supply power over long distances, use common mode chokes.

We recommend using sector-winded common mode chokes, rather than data-line chokes, because the sector-winding chokes have a nice leakage inductance (1-3% maximum of initial value). You can actually place another capacitor in front of it to get a better output value. You will now have a component that makes nice filtering in differential mode as well.

Additional capacitors can reduce differential mode noise:

These can be used for both input and output lines.

After the common mode choke, you will declare your ground. If you connect the inside ground and outside ground together, you will actually make a short circuit in your common mode choke. From this time on, your common mode choke will become a heavy product for your device. So pay attention, and make your ground connect!

Stay Tuned to Our SMPS Blog Series!

For more information on filter design for SMPS, read the first two posts in this series, and stay tuned to our blog for the fourth and final post!