Power Supply Basics

How do I select an external EMI filter for use with a power supply?

Posted by Dulcie on July 9, 2015
Posted in: Power Supply Basics

Although most power supplies have an internal EMI filter, there are situations where an additional system filter is needed to comply with regulations. Power supplies are tested for the manufacturer by certification laboratories in optimum, repeatable conditions. Input and output cabling is well separated, and resistors used to load the product.

In reality, system space constraints for cabling, loads with fast switching microprocessors and/or the use of more than one power supply (EMI noise is additive) can generate additional noise. In this case an additional EMI filter may be needed.

Let us consider an application using TDK-Lambda’s SWS1000L-24 1000W power supply, and that the system will be operated on world-wide AC inputs of 115 or 230V.

The first parameter to consider is the input voltage. Common input voltage ratings are 250Vac or dc, 500Vac or dc (three phase) or 75Vdc. Note – a 250Vac filter can be used with 115Vac inputs. For the SWS1000L power supply, then we will need a filter rated at 250VAC.

The current rating should then be determined. The power supply datasheet should state maximum input current, or alternatively on the rating label. The rating label for the SWS1000L-24 power supply is shown below. Note that the input current shown is for a worse case of 100Vac input and accounts for the power supply inefficiency and power factor. Never assume that a 1000W power supply will only draw 10A at 100Vac input!

SWS1000L rating labelSWS1000L-24 Rating Label

If the power supply is to be used only with a 230Vac input, then the filter rating can be reduced to approximately half. That information will provided on the datasheet.

Continuing with our SWS1000L-24 example, we will need a filter rated at a minimum of 16A to give us a 20% safety margin, as it is not good practice to operate parts at their maximum rating. Running a filter above its rated value, even for short periods, can cause the internal inductors to saturate and become ineffective.

From TDK-Lambda’s website selector guide the RSEN2016 looks suitable. Looking at the derating curve for 50oC ambient, we are within the specification.

Fig 2RSEN2016 Derating Curve

To see how effective the filter is, we then need to look at the attenuation characteristics on the datasheet.

Fig 3RSEN2016 Attenuation vs Frequency Characteristics

From the evaluation data on the SWS1000L-24 we can see what the EMI performance of the unit is. We can then check if the filter has a good insertion loss at any of the frequencies we want to reduce further, in this case 1-10MHz.

Fig 4SWS1000L-24 conducted EMI plot

As the SWS1000L series has medical safety certifications, earth leakage current may be a consideration. The low leakage RSEN2016L should be used, as the RSEN2016 has 1mA leakage, compared to the L version’s 0.01mA.

Attenuation of the RSEN2016L is still good for the application.

Fig 5RSEN2016L Attenuation vs Frequency Characteristics

Further options are available for the TDK-Lambda R series if more attenuation is required. The RSHN2016L is a two stage filter with better performance (0.1mA leakage current).

Fig 6RSHN2016L Attenuation vs Frequency Characteristics

If input transients are a concern, the RSMN2016 or RSMN2016L filters feature a two stage filter design with an amorphous core to clamp any high voltages surges.

If it is desired to mount the filter on a DIN rail, then any of the above models can be ordered with a DIN rail clip, for example RSHN2016LD.

When taking a system to an external test house, TDK-Lambda advises bringing a selection of different performance filters. This avoids having to reschedule with the test house (at extra cost), and also minimise the filter cost. TDK-Lambda Technical Support can also assist with your design and give advice on cable routing and noise suppression

For more visit For more visit www.emea.lambda.tdk.com/uk/filter_select