Other Filters–– 26 ––[Notes]This is the PDF file of text No.TE04EA-1. No.TE04EA-1.pdf 98.3.20Differential and CommonMode Noise Noise is classified into two types according to the conductionmode. The first type is differential mode noise which is conducted onthe signal (VCC) line and GND line in the opposite direction toeach othe. This type of noise is suppressed by installing a filter onthe hot (VCC) side on the signal line or power supply line, asmentioned in the preceding chapter. The second type is common mode noise which is conducted onall lines in the same direction. With an AC power supply line, forexample, noise is conducted on both lines in the same direction.With a signal cable, noise is conducted on all the lines in the cablein the same direction. Therefore, to suppress this type of noise, EMI suppression filters264.1. Differential and Common Mode NoiseLoadSignalsourceNoisesource NLoadSignalsourceNoisesource N■ Differential mode noise ■ Suppression method of differentialmode noiseStraycapacitanceStraycapacitanceReference ground surface LoadSignalsourceNoisesource NSignalsourceNoisesourceStraycapacitanceStraycapacitanceReference ground surfaceSuppresses noise.LoadNReference ground surfaceNLine bypasscapacitorMetallic casingSignalsourceNoisesourceLoad■ Common mode noise ■ Suppression method of commonmode noise (1)■ Suppression method of commonmode noise (2)are installed on all lines on which noise is conducted.In the examples shown above, the following two suppressionmethods are applied.1. Noise is suppressed by installing an inductor to the signal lineand GND line, respectively.2. A metallic casing is connected to the signal line using acapacitor. Thus, noise is returned to the noise source in thefollowing order; signal/GND lines ➝ capacitor ➝ metalliccasing ➝ stray capacitance ➝ noise source.4. Other Filters–– 27 ––[Notes]This is the PDF file of text No.TE04EA-1. No.TE04EA-1.pdf 98.3.20Noise Suppression by CommonMode Choke Coils (1) Common mode choke coils are used to suppress common modenoise. This type of coil is produced by winding the signal or supplywires one ferrite core.Since magnetic flux flows inside the ferrite core, common modechoke coils work as an inductor against common mode current.Accordingly, using a common mode choke coil provides largerimpedance against common mode current and is more effectivefor common mode noise suppression than using several normalinductors.274.2. Noise Suppression by Common Mode Choke CoilsCommon mode choke coils work as a simple wire against differential mode current (signal),while they work as an inductor against common mode current (noise).Common modecurrentNormal mode currentMagnetic flux caused by differential mode current cancelseach other, and impedance is not produced.Magnetic flux caused by common mode current is accumulated,producing impedance.(b) Equivalent circuit (a) StructureCommon mode choke coils are suited for common mode noise suppressionbecause a coil with large impedance is easily achieved.(c) Effect against common mode noiseSince magnetic flux caused by common mode current is accumulated, a high amount ofimpedance is produced.(1) When two normal inductors are used (2) When a common mode choke coil is usedZ Noise sourceStraycapacitanceStraycapacitanceLoadReference ground surfaceZNNoise sourceStraycapacitaStraycapacitanceLoadReference ground surfaceZZN4. Other Filters–– 28 ––[Notes]This is the PDF file of text No.TE04EA-1. No.TE04EA-1.pdf 98.3.20Noise Suppression by CommonMode Choke Coils (2)284.2. Noise Suppression by Common Mode Choke Coils(d) Effect on differential mode currentSince magnetic flux caused by differential mode current cancels out, impedance is notproduced.A decrease in impedance due to magnetic saturation does not easily occur,even if the current flow is large.Common mode choke coils are suited for noise suppression on lines withlarge current flows, such as AC/DC power supply lines.The distortion of the waveform is less.Common mode choke coils are suited for noise suppression on lines wheresignal waveform distortion causes a problem, such as video signal lines.(1) When two inductors are used (2) When a common mode choke coil is used+-Input waveform(Before filtering)Output waveform(After filtering)Output waveform(After filtering)+-1 10 100 1000Frequency (MHz)Impedance (Ω)10000100010010100000PLM250H10PLM250S20PLM250S30PLM250S40PLM250S50(Differential mode)H10S20S30S40S50(Common mode)(e) Examples of impedance characteristicsof DC common mode choke coils Since magnetic flux cancels out inside the ferrite core, impedanceis not produced for differential mode current. The magneticsaturation problem is small. Common mode choke coils are suitedfor common mode noise suppression on lines with large currentflow, such as AC/DC power supply lines. Since they do not affectsignal waveform, they are also suited for common mode noisesuppression on lines where signal waveform distortion causes aproblem, such as video signal lines. The above graph shows examples of impedance characteristicsof DC common mode choke coils. Actual characteristics alsocontain differential mode impedance, and this must be consideredwhen using common mode choke coils in circuits where the signalwaveform is significant.The distortion of the waveform is large The distortion of the waveform is small4. Other Filters–– 29 ––[Notes]This is the PDF file of text No.TE04EA-1. No.TE04EA-1.pdf 98.3.20Example of Noise Suppression inDC CircuitThe above drawing shows an example of noise suppression in theDC circuit.DC power supply input sectionA common mode choke coil is installed in the input section ofthe DC power supply line to suppress common mode noise.(This coil can be replaced with two ferrite bead inductors.)Differential mode noise is suppressed by installing a threeterminalcapacitor and ferrite bead inductor in the supply line.Video signal output sectionCommon mode noise transmitted to the video signal outputsection is suppressed by using a common mode choke coil.294.3. Example of Noise Suppression by using Common mode Choke CoilsVideo signal outputCommon mode choke coilSuppresses common modenoise.Common mode choke coilSuppresses common modenoise.Ferrite bead inductorSuppresses differentialmode noise.DC power supplyinput sectionThree-terminal capacitorSuppresses differentialmode noise.4. Other Filters–– 30 ––[Notes]This is the PDF file of text No.TE04EA-1. No.TE04EA-1.pdf 98.3.20Example of Noise Suppression onAC Power Supply LineSwitching power supply LoadCommon mode choke coilSuppresses common modenoise.Line bypass capacitor(Y-capacitor)Suppresses common modenoise.Across-the-line capacitor(X-capacitor)Suppresses differentialmode noise.Across-the-line capacitor(X-capacitor)Suppresses differentialmode noise. The above drawing shows an example of noise suppression onan AC power supply line.Common mode noise is suppressed by using a common mode chokecoil and capacitor (line bypass capacitor or Y-capacitor) installedbetween each line and the metallic casing. The Y-capacitor returnsnoise to the noise source in the following order; Y-capacitor ➝metallic casing ➝ stray capacitance ➝noise source. Differential mode noise is suppressed by installing capacitors(Xcapacitors) across the supply line.304.3. Example of Noise Suppression by using Common mode Choke Coils