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As the safety and comfort of automobiles increases, more electronic devices are being used in vehicles. Electronic circuits have signal lines and power supply lines, and both require noise countermeasures. TDK’s common-mode filters are characterized by a diverse lineup of products for specific applications including product groups that pursue compact size or a low profile and product groups that adopt an independent terminal structure to meet the harsh environmental conditions of automotive applications. This chapter introduces common-mode filters for power supply lines.

Basic structure of DC input filter circuits

The filter configuration of a DC input filter circuit is greatly affected by the way the GND is arranged on the board pattern, and it is necessary to use appropriate components depending on the bandwidth to be attenuated and the noise propagation route. In the initial design stage of EMI countermeasures, it is desirable to have a pattern configuration that allows for the placement of inductors (L), capacitors (C), and common-mode filters (CMF).

Example of 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 occurs in the circuit line and flows in the opposite direction, while common-mode noise occurs between the circuit line and GND and flows in the same direction. When implementing noise countermeasures, it is necessary to confirm which mode of noise is occurring and to use appropriate countermeasure components.
For normal mode noise, inductors and capacitors are used, and for common-mode noise, common-mode filters are used.

How conducted noise is conducted

Normal Mode

Noise flowing in circuit lines (flowing in the opposite direction as the line)

Figure 4

Common Mode

Noise flowing in the frame ground (flowing in the same direction as the line)

Figure 5

Trends in DC-DC converters for automotive ECUs

As more electronic devices are used in automobiles, more ECUs are being installed, and common-mode noise countermeasures are necessary. Current automotive ECU DC-DC converters use 2 MHz as the DC-DC converter switching frequency in order to avoid the AM band, so high-frequency noise countermeasures are important.

Figure 6: Frequency Band Use by Application

Change in noise when the switching frequency is 2 MHz

Voltage method of conducted noise

Conditions: Input 5 V, Output 1.2 V/2 A, no input filter
If the switching frequency is 2 MHz, the fundamental wave will be in the AM band or higher, and noise in the FM band increases.

Figure 7: Switching Frequency 400 KHz

Figure 8: Switching Frequency 2 MHz

The CISPR 25 Level 5 (Peak) standard values are indicated for reference.

Factors that increase noise in the FM band

Factors that increase noise in the FM band are increasing the switching start up speed and increasing the high-frequency portion. The high-frequency portion is more susceptible to common current loss due to the parasitic capacitance of the frame ground.

1) When the switching start up speed is increased, and the high-frequency portion increases.

Figure 9

Figure 10: Trapezoidal Wave Spectrum

2) The high-frequency portion is more susceptible to common current loss from the parasitic capacitance of the frame ground.

Figure 11

It is difficult to predict common-mode noise because the stray capacitance varies greatly depending on the way the GND is connected, but common-mode filters are effective in preventing common-mode noise.

3) If countermeasures against common-mode noise are in adequate, there is a high probability of malfunction.

Assume that a noise current of 1 µA is flowing in a cable with a loop area of 20 cm2 at a frequency of 80 MHz. At a distance of 1 m, the electric field strength values are:

ED= 0.084μV/m

Even at the same current value, the effect of common current is larger
(about 100 times larger in the example above)

The magnitude of the radiated emissions in each mode can be determined using the following formula.
Differential current: ID
Common-mode current: IC
Line length: L
Line interval: s
Distance: d

Features of TDK's common-mode filters for automotive power supply lines and product lineup

TDK is expanding its lineup of filters compatible with various types of DC power lines up to a maximum current of 8 A. These products offer high impedance over a wide band width, and the operating temperature range Ta (environmental temperature) is -40° to 125°C.

Examples of supported applications
Product lineup
Item Common-mode impedance
(at 100 MHz)
DC resistance Rated current Rated voltage Insulation resistance
(Ω) min. (Ω) typ. (mΩ) max. (A) max. (V) max. (MΩ) max.
Common Mode Filters,Chokes:ACT1210G
ACT32P-102-2P-TL00 3.2x2.5x2.5 - 1000 250 0.65 80 10
Common Mode Filters,Chokes:ACM70v
ACM55V-701-2PL-TL00(under development) 5.5x5.5x3.5 - 700 - 3.0 80
ACM70V-701-2PL-TL00 7.0x6.0x3.5 500 700 15 4.0
ACM90V-701-2PL-TL00 9.0x7.0x4.5 500 700 10 5.0
ACM90V-152-2PL-TL00 1100 1500 16 3.6
ACM12V-701-2PL-TL00 12.0x11.0x6.0 500 700 6 8.0
ACM12V-172-2PL-TL00 1200 1700 12 4.8

Table 1

Product features

Wide Bandwidth and High Noise Suppression

Has high common-mode impedance in representative bands such as the FM band and the cellularband.

Low-Profile Design

The low-profile design is 3.5 mm max., making it ideal for low-profile design ECUs that require a shield.


In conjunction with higher DC-DC converter operating frequencies, noise suppression in the FM band is becoming more important. The key to EMI countermeasures is to separate the normal mode from the common mode, and noise caused by the common mode in particular is affected by the pattern and layer configuration and the way the GND is arranged. Common-mode filters are effective in preventing common-mode noise. As indicated in the figure below, a normal mode filter alone does not decrease noise at 50 MHz and higher, but when a common-mode filter is also used, noise at 50 MHz and higher can be reduced.
In cases where EMI is a problem due to common-mode noise, a common-mode filter is an effective countermeasure, but adding a common-mode filter in a subsequent stage requires design changed, so it is recommended that a pattern be prepared in the design stage to allow for installation of a filter.