Technical Information Site of Power Supply Design

2019.10.09 Switching Noise - EMC

Basics of EMC - Summary

Summary

This is the final entry in the series "Switching Noise--EMC Basics", spanning 21 articles. Starting from "Basics of EMC", and focusing on switching power supply applications, we have discussed basic topics related to "Noise Countermeasures (procedures and summary)", "Dealing with Noise Using Capacitors", "Dealing with Noise Using Inductors", and "Other Noise Countermeasures". In conclusion, we summarize the key points of each of these articles.

＜Basics of EMC＞

Key Points:

・EMC (electromagnetic compatibility) means supporting both EMI (electromagnetic interference) and EMS (electromagnetic susceptibility) capabilities.

・EMI means the interference due to the emission of electromagnetic waves.

・EMS means an immunity to EMI.

Key Points:

・As the frequency is raised, the overall spectrum amplitude increases.

・When rise/fall times are slowed (increased), the frequency from which -40dB/dec attenuation begins is lower, and the spectrum amplitude is attenuated.

・When the duty cycle is changed, even-numbered harmonics appear, but there is no effect on the spectrum peak. The fundamental wave spectrum is attenuated.

・When only the rise time is slowed, tr components are attenuated from a lower frequency.

Key Points:

・Electromagnetic interference or EMI can broadly divided into "conducted emission" and "radiated emission".

・Conducted emission can be further classified into two types, differential (normal) mode noise, and common mode noise.

・With respect to radiation caused by conducted emission, the important factors are the loop area of the line in the case of differential mode noise, and the line length in the case of common mode noise.

・It should be born in mind that under the same conditions, radiation due to common mode noise is far greater than that due to differential mode noise.

Key Points:

・Crosstalk occurs between parallel wires.

・The causes of crosstalk are capacitive (electrostatic) coupling due to stray (parasitic) capacitance, and inductive (electromagnetic) coupling due to mutual inductance.

Key Points:

・In a loop in which currents are suddenly turned on and off during switching, high-frequency ringing (switching noise) occurs due to parasitic components.

・This switching noise can be reduced through optimization of the PCB wiring and other measures, but the noise that remains is conducted to an input power supply as common mode noise, and so measures to prevent leakage are necessary.

＜Noise Countermeasures＞

Key Points:

・The further along product development advances, the more the techniques and means available to address noise are limited, and the more expensive the available options become.

・By conducting thorough studies and evaluations early in the product development stage, it is possible to implement noise countermeasures with a considerable margin for error.

・It is extremely important that noise types and properties be understood, and that measures suited to the respective noise types be implemented.

・Noise countermeasures are implemented through the procedures of first ascertaining frequency components, then grasping the sources and conduction paths of the noise, then reinforcing the circuit ground, and finally adding components to deal with the noise.

Key Points:

・In order to reduce differential mode noise, the area of the loops of large-current paths is decreased, and optimal decoupling and an input filter are added.

・It is important that differential mode noise, which is a noise source, be suppressed to the extent possible; this will result in reduced common mode noise as well.

・To reduce common mode noise, wires are shortened to suppress crosstalk, and common mode paths are interrupted (filtered).

Key Points:

・Input filters for switching power supplies are provided to address common mode noise and differential mode noise respectively.

・Common mode filters are used to handle common mode noise.

・To address differential mode noise, a filter is constructed from components such as capacitors, inductors, beads, and resistors.

＜Dealing with Noise Using Capacitors＞

Key Points:

・Capacitors for use in dealing with noise should be selected based on the frequency characteristic of the impedance rather than the capacitance.

・When the capacitance and the ESL are smaller, the resonance frequency is higher, and the impedance in the high-frequency region is lower.

・The larger the capacitance, the lower is the impedance in the capacitive region.

・The smaller the ESR, the lower is the impedance at the resonance frequency.

・The smaller the ESL, the lower is the impedance in the inductive region.

Key Points:

・Noise amplitudes can be reduced by lowering the impedance at the frequency of the targeted noise.

・A capacitor to be used to address noise is selected for its impedance frequency characteristic rather than for its capacitance value.

Key Points:

・There are two main points to consider for the effective use of decoupling capacitors: (1) Use of multiple capacitors, and (2) lowering capacitor ESL values.

・When using multiple capacitors, the effect differs depending on whether the capacitance values are all the same or are different.

Key Points:

・There are two main points to consider for the effective use of decoupling capacitors: (1) Use of multiple capacitors, and (2) lowering capacitor ESL values.

・By lowering the capacitor ESL, high-frequency characteristics are improved, and high-frequency noise can be more effectively attenuated.

・There are capacitors which, even for the same capacitance, have ESL values that are lowered through innovations in the size and structure.

Key Points:

・The relationship between the Q factor and the frequency-impedance characteristic should be understood to use capacitors with different Q factors selectively according to the objective.

・For a high-Q capacitor, the impedance drop is sharp in a narrow band. A low-Q capacitor has a more gentle decline over a broader frequency band.

・The thermal relief pattern of a wiring board and other factors cause inductance components to be increased, shifting the resonance frequency to the low-frequency side.

・Trial mounting when studying noise countermeasures may not result in the effect obtained for the modified board during studies, if the mounting method is not in keeping with the actual modifications.

・If the capacitance change rate is large, the resonance frequency may shift, and noise attenuation at the desired frequency may not be obtained.

・In applications with harsh or fluctuating temperature conditions, the possibility of using capacitors with better temperature characteristics, such as devices with CH/C0G characteristics, should be studied.

＜Dealing with Noise Using Inductors＞

Key Points:

・An inductor exhibits an inductive characteristic (the impedance increases as the frequency rises) up to a resonance frequency.

・Beyond the resonance frequency, the inductor exhibits a capacitive characteristic (the impedance decreases as the frequency rises).

・At frequencies higher than the resonance frequency, the inductor does not function as an inductor.

・When the inductance L is small, the resonance frequency of the inductor is high.

・The impedance of an inductor at the resonance point is limited by parasitic resistance components.

Key Points:

・Inductors used to deal with noise can be broadly divided into filters based on winding-type inductors and ferrite beads, which convert noise into heat.

・Relative to general inductors, ferrite beads have a high resistance component R and low Q value.

・Inductors in general can tolerate comparatively large DC superposition currents, and within this range, the impedance is not affected very much by the DC current.

・Ferrite beads tend to saturate easily when a DC current is passed, and saturation causes the inductance to fall and the resonance point to shift to higher frequencies.

・Filters based on general inductors can be selected with a wide range of inductance values.

・Ferrite beads have low Q values, and so are an effective means of dealing with noise over a relatively broad range of frequencies.

Key Points:

・Common mode filters are used for elimination of common mode noise.

・Common mode filters use self-induction action to prevent passage of common mode currents.

Key Points:

・Depending on the board wiring layout, crosstalk may detract from the efficacy of filters.

・Noise generating from a GND line may depend on how the capacitors of a Π filter are grounded.

・These problems can be avoided through an appropriate board wiring layout.

＜Other Noise Countermeasures＞

Key Points:

・RC snubber circuits reduce voltage spikes, occurring due to parasitic capacitances and parasitic inductances, by using resistors to convert the voltages into heat.

・The addition of a snubber circuit may possibly reduce circuit efficiency, and so the trade-off between noise level and efficiency must be studied.

・A resistance converts noise voltages into heat, and so attention must be paid to the allowable dissipation of the resistance.