- Basic structure of DC input filter circuits
- Normal (differential) mode noise and common-mode noise
- Trends in DC-DC converters for automotive ECUs
- Factors that increase noise in the FM band
- Features of TDK's common-mode filters for automotive power supply lines and product lineup
- Contact Information
- Related Links
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).
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.
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.
Change in noise when the switching frequency is 2 MHz
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.
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.
(at 100 MHz)
|DC resistance||Rated current||Rated voltage||Insulation resistance|
|(Ω) min.||(Ω) typ.||(mΩ) max.||(A) max.||(V) max.||(MΩ) max.|
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.