Noise Countermeasures for DC-DC Converter Input Lines using 3-terminal Filters (Feed-through Filters for Power Lines)

In recent years, switching speed of DC-DC converters has increased as the switching frequency has become higher. As a result, high-frequency noise is generated when inductance and stray capacitance of the wiring of board or inside IC resonate due to a sudden change in input current. The high-frequency noise can conduct to external circuits and cause abnormal operation of the set. This Solution Guide introduces examples of noise countermeasures using a three-terminal filter (feed-through filter for power lines) with low ESL, which is effective for noise countermeasures against noise generated on the input side of DC-DC converters.

Basic Configuration of an Input Filter
Normal (differential) Mode Noise and Common Mode Noise
Evaluation Conditions for Checking the Effect of Noise Suppression for Each Filter Configuration
Results After Checking the Effect of Noise Suppression for Each Filter Configuration
Conclusion

Basic Configuration of an Input Filter

In input line of a DC-DC converter, large noise is generated by the input voltage fluctuations due to the switching noise or Nth harmonics consisting of the fundamental frequency during switching. The noise has two modes: common mode and normal (differential) mode, and so appropriate components must be selected depending on the noise mode.
As shown in Figure 1, during the initial design stage, a π-type filter consisting of capacitors and an inductor (C+L+C) is a desirable pattern configuration for normal mode noise countermeasures. Also, a common mode filter (CMF) is effective for common mode noise countermeasures.

Figure 1: Example of a Filter Configuration

Normal (differential) Mode Noise and Common Mode Noise

There are two types of conducted noise: normal (differential) mode noise and common mode noise. Normal mode noise is generated between circuit lines and flows in reverse phase, while common mode noise is generated between circuit lines and GND and flows in the same phase. For noise suppression measures, use of appropriate countermeasure components according to the noise mode is necessary.
Inductors and capacitors are typically used for normal mode noise, while a common mode filter is effective for common mode noise.

Figure 2: Conducted Noise Transmission - Normal Mode

Figure 3: Conducted Noise Transmission - Common Mode

Evaluation Conditions for Checking the Effect of Noise Suppression for Each Filter Configuration

As the switching frequency of DC-DC converters increases, the noise level in the FM band also increases. Normally, 2-terminal capacitors are used for π-type filters, but further noise suppression can be achieved by using 3-terminal feed-through filters, which feature low ESL. In this guide, we compared and verified the effects of three configurations of π-type filters with the following evaluation contents.
For the conducted noise voltage method, we separated normal mode noise and common mode noise and compared the filter effects. All three configurations were measured using our common mode filter as a common mode countermeasure.

■DC-DC converter conditions
Input voltage: 12V
Output voltage: 5V
Switching frequency: 2MHz
Output current: 2A
■Evaluation Items
1) Transfer characteristics of π-type filter (S21)
2) Conducted noise voltage method

Evaluation Circuit

Figure 4: Transfer Characteristics of π-type Filter (S21)

Figure 5: Conducted Noise Voltage Method

Evaluation Configuration

ACM7060-701-2PL-TL00

π-type filter configuration	Configuration 1: No filter	Configuration 2: Capacitors (2-terminals)	Configuration 3: 3-terminal filters (feed-through filter)
Wiring image
C1	-	3.2mm×1.6mm, 4.7μF,35V	YFF series 3.2mm x 1.6mm,1μF,100V,6A Feed-Thru connection*
L1	-	2.0mm×1.2mm,220Ω at 100MHz,3A	2.0mm×1.2mm,220Ω at 100MHz,3A
C2	-	3.2mm×1.6mm,4.7μF,35V	YFF series 3.2mm x 1.6mm,1μF,100V,6A Feed-Thru connection*

*For more information on feed-through connection,
please refer to the following link.

Solution Guide : Key Points when Mounting 3-Terminal Feedthrough Filters

Results After Checking the Effects of Noise Suppression for Each Filter Configuration

The verification results of each configuration show the 3-terminal filter (feed-through filter) has a very high attenuation effect in the high-frequency band.

Figure 6: Evaluation Result 1) Transfer characteristics of input line with π-type filter (S21)

Typically, the normal mode noise level is greater than the common mode noise level.
For normal mode noise countermeasures, a π-type filter with a 3-terminal feed-through filter is very effective in noise suppression in a wide range of frequency bands, including the FM band.

Figure 7: Evaluation Result 2) Conducted Noise Voltage Method

Conclusion

Generally, DC-DC converters generate large high-frequency noise in the input line due to high-speed switching. Therefore, countermeasures against high-frequency noise including FM band are particularly important. In this case, a distinction between normal mode and common mode is important, and appropriate components should be selected according to the noise mode.
Usually, on the input line of a DC-DC converter, the normal mode noise level is greater than the common mode noise level. Therefore, we recommend the configuration of a π-type filter, which is effective for normal mode noise.
Furthermore, the use of 3-terminal feed-through filters for the capacitors of the π-type filter is more effective in high-frequency noise suppression.