Transmission Modes & Filter Topologies for SMPS

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

In this second installment of our four-part series (read the first post here), Lorandt Fölkel M.Eng will explain transmission modes and filter topologies for SMPS.

Calculating Insertion Loss

Which kind of transmission modes or filter topologies are possible to use for SMPS?

First off, when you go to the EMC Lab with your power supply, they won’t tell you which part number you ought to use — they’ll tell you what insertion loss decibel (dB) you need to have.

To calculate your insertion loss, you’ll need to use that math you learned back in college. If you need a refresher, you can see the formulas for system attenuation and impedance in this image.

How to Troubleshoot Problems with Insertion Loss

As you can imagine, though, your insertion loss probably isn’t as clear-cut as your university led you to believe. So what do you do if your device doesn’t measure up at the EMC Lab? Well, you have some EMC troubleshooting to do!

Some people, recent graduates especially, will think to describe their noise source and load by creating a simulation of the equivalent circuit and putting an LC filter in between the two. However, we think this is a waste of time.

Instead, we’d recommend using some practical values (Za & Zb) for source and load impedance, such as:

  • Ground planes (<1 … 2 Ω)
  • Vcc distribution (10 … 20 Ω)
  • Video, clock, or data lines (50 … 90 Ω)
  • Long data lines (90 … >150 Ω)

Recommended Filter Topology for Insertion Loss

Depending on the source and load impedance, the recommended filter topology for your device will be different.

For example, if you have low source impedance but high load impedance, we would recommend the LC capacity. If you’re unsure, we suggest starting with LC, then adding a second L if the impedance is too low. One of those filters should work, offering you the right differential noise filtering.

You can see our recommended filter topology chart in this image.

Three Best Practices for Selecting Filter Topology

There are a few key things to keep in mind when selecting the filter topology for your design.

First, use small values for capacity. The smaller the capacity, the better the influence and frequency range.

Second, use inductance according to the appropriate Q factor for switching frequencies. Use a low Q factor for filtering, and use a high Q factor for storage. Remember the Q factor formula: Q= XL/R

Finally, pay attention to the self-resonant frequency (SRF) of used components. It’s always important to keep SRF away from the operating frequency.

Stay Tuned to Our SMPS Blog Series!

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