Reprinting forbidden EMC Basics ver.A2 ROHM Co. Ltd. © 2018 ROHM Co.,Ltd. Agenda Reprinting forbidden The Basics of EMC 1. What is EMC? 2. Spectrum Basics 3. Differential Mode Noise & Common Mode Noise 4. Cross Talk 5. Noises from Switch Mode Power Supplies © 2018 ROHM Co.,Ltd. P. 1 Reprinting forbidden 1. What is EMC? What is EMC? Conducted Emission • Without causing electromagnetic interference to other devices, the ability to maintain the inherent performance even subjected to electromagnetic interference from other devices. • Because of the need to sustain both of the capabilities, the term “electromagnetic compatibility” is used. • EMC includes the following EMI and EMS. EMI (Electromagnetic Interference； Emission) • Due to operation of a electronics, giving an electromagnetic interference to other devices or a thing that is such electromagnetic wave. • Circuit designing not to generate EMI is required. • Noise that is propagated through wires and PCB wiring. Radiated Emission • Type of noise that is emitted (radiated) through the air. EMS (Electromagnetic Susceptibility； Immunity) • The immunity that do not cause errors, such as a malfunction, when subjected to an EMI. • Robust circuit designing is required for EMI. © 2018 ROHM Co.,Ltd. P. 2 Reprinting forbidden 2. Spectrum Basics dB A -20dB/dec tw A 2 -40dB/dec ts ts T Pulse Waveform 1 πtw 1 πts dB Upper right:Fourier transformation spectrum of theoretical pules wave form. -20dB/dec Lower right: Fourier transformation spectrum when ts is slower. • The spectrum lowers when ts is slower. • Verify how the spectrum changes with changing parameters(e.g. frequency) by calculation. © 2018 ROHM Co.,Ltd. f When ts is slower. -40dB/dec 1 πtw 1 πts f P. 3 Reprinting forbidden 2. Spectrum Basics Amplitude [V] Voltage [V] © 2018 ROHM Co.,Ltd. Frequency [Multiple] Default Frequency [kHz] n-th harmonic vs Amplitude Waveform Time [s] Amplitude [dBuV] Frequency [Multiple] Time [s] Ampli.：10V f ：2MHz Duty：50% tr /tf：10ns Frequency vs Amplitude [dBµV] Frequency vs Amplitude [dBµV] Amplitude [dBuV] Ampli.：10V f ：400kHz Duty：50% tr /tf：10ns n-th harmonic vs Amplitude Amplitude [V] Voltage [V] Waveform Blue Line:Default Frequency [kHz] P. 4 Reprinting forbidden 2. Spectrum Basics Time [s] Ampli.：10V f ：400kHz Duty：20% tr /tf：10ns Time [s] © 2018 ROHM Co.,Ltd. Amplitude [dBuV] n-th harmonic vs Amplitude Amplitude [V] Voltage [V] Waveform Frequency [Multiple] Blue Line:Default Frequency [kHz] Frequency [Multiple] Frequency vs Amplitude [dBµV] Amplitude [dBuV] Ampli.：10V f ：400kHz Duty：50% tr /tf：100ns Frequency vs Amplitude [dBµV] n-th harmonic vs Amplitude Amplitude [V] Voltage [V] Waveform Blue Line:Default Frequency [kHz] P. 5 Reprinting forbidden 2. Spectrum Basics Waveform Frequency vs Amplitude [dBµV] n-th harmonic vs Amplitude Amplitude [dBuV] Ampli.：10V f ：400kHz Duty：50% tr ：100ns tf ： 10ns Amplitude [V] Voltage [V] Blue Line:Default Frequency [Multiple] Time [s] Higher frequency Lower tr and tf Change in duty cycle ⇒ ⇒ ⇒ Only lower tr ⇒ Frequency [kHz] Spectrum increase for all. Decrease -40dB/dec from low frequency. Even number-th harmonic occurs but not affects to the peak of spectrum. Fundamental wave lowers. tr component decreases at low frequency. * Lower frequency and slower tr and tf make spectrum lower. © 2018 ROHM Co.,Ltd. P. 6 Reprinting forbidden 3. Differential Mode Noise & Common Mode Noise Differential(Normal) Mode Noise Common Mode Noise Case Case PCB PCB Circuit Power Supply Circuit Power Supply Vn Vn Noise Source （Signal Source） Reference GND • A mode in which a noise current flows on the same path as the power supply current. • Noise voltage occurs across power supply lines. © 2018 ROHM Co.,Ltd. Stray Capacitance Reference GND Reference GND • Noise voltage does not occur across power supply lines. • Noise voltage occurs across power supply line and reference GND. • Noise currents flow in the same direction on the power supply positive and negative sides. P. 7 Reprinting forbidden 3. Differential Mode Noise & Common Mode Noise Radiation due to Differential Mode Noise Ed ∝ Id r Id x f 2 r Loop Area: S TX xS RX Radiation due to Common Mode Noise Ec ∝ Ic TX r Cable Length: L Ic x f x L r RX *Equations excerpted from： Detailed Explanation--Electromagnetic Compatibility Engineering, Author: Henry W. Ott, Publisher: John Wiley & Sons © 2018 ROHM Co.,Ltd. P. 8 3. Differential Mode Noise & Common Mode Noise Reprinting forbidden 1m 20cm TX RX 1cm * Example using an approximate expression. Differential Mode Noise Common Mode Noise For a 100 MHz differential mode noise current of 1 µA flowing in a loop of area 20 cm2. The value of the electric field intensity at a distance of 1 m (90°) is: Ed = 1.316 x 10-14 x = 1.316 x 10-14 x = 0.26µV/m Id x f 2 xS r 1µA x (100MHz )2 x (0.2 x 0.01) 1 For a 100 MHz common mode noise current of 1 µA flowing in a 20 m cable The value of the electric field intensity at a distance of 1 m (90°) is: Ec = 1.257 x 10-6 x = 1.257 x 10-6 x Ic x f x L r 1µA x 100MHz x 0.2 1 = 25.1µV/m ・For the same noise current values, the radiation due to common mode noise is far greater (in this example, roughly 100 times greater). ・Using a twisted-wire cable, Area S becomes smaller and a differential mode noise decreases. ・Area S doen not affect to common mode noise. © 2018 ROHM Co.,Ltd. P. 9 Reprinting forbidden 4. Crosstalk Capacitive Coupling (Electrostatic Coupling) Inductive Coupling (Electromagnetic Coupling) Pattern 2 Pattern 1 C Pattern 1 Pattern 2 Is M C Is Vs Vs Noise source (Signal source) R Illustration R Vn M Vs Vn R Vs Equivalent circuit • Illustration of capacitive coupling between patterns due to stray capacitance. • Noise occurring on the side of pattern 1 causes a voltage Vn relative to GND due to capacitive coupling: Vn = jω R C Vs Noise source (Signal source) Illustration Vn R Vn Equivalent circuit • Illustration of inductive coupling between patterns due to mutual inductance. • Noise occurring on the side of pattern 1 causes a voltage Vn relative to GND due to inductive coupling: Vn = jω M Is Crosstalk increases with high frequency, short distance between patterns, and high impedance coupling. *Electromagnetic Compatibility Engineering, Author: Henry W. Ott, Publisher: John Wiley & Sons © 2018 ROHM Co.,Ltd. P. 10 Reprinting forbidden 5. Noise Occurring in Switching Power Supplies ON OFF OFF ON SW2 ON SW1 ON • The thick lines in the diagram on the left indicate the loop in which there are sharp changes in current due to switch ON/OFF operation. • Because high-frequency currents flow in this loop during switching, ringing occurs in the loop. V=L× dI dt Example) When a 1 A current changes in 10 ns in wiring having a 10 nH inductance component, a voltage of 1 V appears. © 2018 ROHM Co.,Ltd. P. 11 5. Noise Occurring in Switching Power Supplies Parasitic C Wiring L L5 VIN CIN Cdecap C1 Switching node waveform Parasitic L Switching noise components L L1 VOUT L2 L4 Driver circuit C2 COUT GND L3 Wiring L Fundamental wave component Parasitic C ・ Wiring inductance L is roughly 1 nH per mm. ・ Rise/fall time for a switching MOSFET is several ns. I =C× Reprinting forbidden dV dt V =L× Intense ringing at 100 to 300 MHz occurs at rises/falls. dI dt • Parasitic components on the mounting board cannot all be eliminated through power supply ICs. • Innovations are necessary in the PCB wiring layout and through decoupling capacitors. © 2018 ROHM Co.,Ltd. P. 12 5. Noise Occurring in Switching Power Supplies Reprinting forbidden LISN Common Mode BATTERY Differential LISN Common Mode Differential mode ⇒ Common mode ・ Switching noise components that remain even after loop optimization are conducted to the power supply side as common mode noise. ・ Measures must be taken to confine noise by inserting high-impedance components such as inductors into transmission lines. ・ Attention must be paid to crosstalk as well. © 2018 ROHM Co.,Ltd. P. 13 © 2018 ROHM Co.,Ltd.
