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switching power supplies, typically operating at frequencies in the range of 100 to 400 kHz. Power supply noise is strongly associated with the switching of power transistors and may be coupled onto power or signal lines.
This type of noise is present at
the fundamental frequency and (with decreasing magnitude) at harmonics of the switching frequency. This noise can cover a spectrum from as
frame, enclosed, or standalone, and whether the application requires hold-up performance. There are also non-EMC consid-
erations, such as cost, efficiency, size, weight, and safety compliance. Other sources of internally-generated elec- tromagnetic noise include motors and drives. Interference signals generated
by internal sources may be coupled in various ways, such as through para- sitic capacitances, cables or other con- nections, and must be prevented from interfering with internal cir- cuitry returning to the mains. This objective — to reduce and contain EMI energy — is usually achieved through filtering by using a combination of inductive and capacitive elements.
EMC Filter Networks This type of filter, includ-
Common interference-filter networks.
low in frequency as 10 kHz up to around 50 MHz. Bridge rectifier dif- ferential-mode noise at multiples of 120 Hz, as well as high-frequency noise, resulting from parasitic oscil- lation not related to the switching frequency, may also be present. When choosing an AC/DC
power supply, selection criteria, such as the AC input voltage, radiated emissions and immunity compliance are extremely important, as are DC- AC noise isolation, DC-DC noise iso- lation, DC-DC galvanic isolation, whether the power supply is open-
ing topology and component val- ues, depends on the noise-signal frequencies, noise-source char- acteristics, and the impedances of circuitry connected to the fil- ter. Commonly-used filters range from simple capacitive fil-
ters to more complex T and pi filters. Three-terminal feed-through capaci- tors are typically used. The extra elements of a pi filter
give a steeper frequency response for applications that require sharper fre- quency cutoff. An LC filter may be used in conjunction with a low- impedance source and high-imped- ance load, or vice versa. The T filter is a strong choice where the source and load are both of low impedance. Designing an EMC filter net- work to prevent noise from interfer-
ing with signals can be easier when the noise and signal frequencies are very different. On the other hand, designs including inductor and capacitor selection can be more com- plicated when the frequencies are close together. KEMET’s K-SIM allows the
May, 2017
Classifying and Combating Electromagnetic Interference Continued from previous page
sired L-C tank circuits. These tank circuits are the root
cause of anti-resonance artifacts. Reducing the gap between capacitor values minimizes this resonance effect.
Managing noise in electrical sys-
simulation of different capacitors, both individually and in combina- tion. Individual plots are grayed out to highlight the combined result. The
tems, and ensuring EMC compliance for new products, can be a complex challenge and requires reference to multiple standards. These include not only product-acceptance specifica-
Simulation of decoupling filter capacitor network using K-SIM.
simulations show how capacitors can be combined to maintain low noise attenuation across a wide frequency range.
Care must be taken in the selec-
tion of parallel decoupling capacitors as anti-resistance can occur. This anti-resistance reduces the overall effectiveness of the decoupling filter network. When capacitors with dif- ferent values and sizes are placed in parallel, their combined capacitive and inductive reactances form unde-
tions, such as the latest EU EMC directive, but also require knowledge of various safety standards. Successful system noise management depends on engineering experience, as well as extensive familiarity with the applica- ble standards and knowledge of the underlying design principles. Contact: KEMET Electronics,
2835 Kemet Way, Simpsonville, SC 29681 % 864-963-6300 fax: 864-228-4242 Web:
www.kemet.com r
See at EWPTE, Booth 1909
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