Noise terminal voltage noise suppression using the BNX series

03/31/2023

One requirement under the noise standard (CISPR 11^*) that applies to medical and healthcare equipment deals with conductive noise (noise terminal voltage). The level of such conductive noise in the frequency band from 150 kHz to 30 MHz must comply with the standard.

This article introduces an example of the use of a BNX series block type EMI filter to suppress conductive noise.

* CISPR 11 is a noise standard that applies to medical and healthcare equipment.

Verification using DC-DC converter

For verification, we prepared a measuring environment with a DC-DC converter as the EUT in order to evaluate conductive noise as specified in CISPR 11.

(Note)
The measurement system and measurement method stipulated by CISPR 11 are used to evaluate leakage of conductive noise into the DC power line.

Conductive noise measurement results

We observed the noise spectrum of frequencies that are integral multiples of 500 kHz, the switching frequency. In addition, noise exceeding the allowable level for CISPR 11 Class A Group 1 (industrial settings) was measured.

It seems likely that noise due to switching by the DC-DC converter (switching noise) was transmitted onto the DC line.

(Reference)
Allowable noise values for Class A (industrial settings) and Class B (residential settings)

Graph of Noise measurement results — Noise measurement results

Countermeasure using noise filter

We inserted a BNX029-01 block type filter on the input side to suppress the conductive noise. The BNX029-01 is a filter product that comprises an LC π-type filter as a single component. This filter provides noise suppression over a wide frequency band from 15 kHz to 1 GHz. It can tolerate large currents of 20 A and is a suitable noise countermeasure for DC power lines.

Part Number	Size	Rated Current	Insertion Loss
BNX029-01	12.1 × 9.1 mm	20 A

Countermeasure results

Inserting the BNX029-01 on the input side brought noise into compliance with the allowable value over the entire frequency band.

Conclusion

Noise due to switching by the DC-DC converter can be transmitted onto the DC line. This conductive noise can be suppressed by using a BNX series filter with a wide frequency band. The BNX series is a filter product that comprises an LC π-type filter as a single component, allowing noise countermeasures to be implemented quickly and using very limited space.

The following SMD type versions of BNX series products are available. Choose the one that best matches the usage environment and noise conditions.