Low-Frequency EMI Shielding in Circuit Boards
In both conducted and radiated EMI, there can be signals of low-frequencies, mid-frequencies, and high frequencies.
Low-frequency EMI shielding materials are selected to alter or block unwanted electromagnetic field distribution from interfering with sensitive equipment.
Highly permeable materials with low reluctance, such as mu-metal or steel, are particularly effective in low-frequency EMI shielding.
Low-frequency EMI shielding is one technique to prevent low-frequency EMI from interfering with circuit operation
Electromagnetic interference problems are a serious concern in any power electronic system. The EMI generated in power electronic circuits ranges from low-frequency to high frequency. There are various techniques to mitigate EMI, depending on which frequency class the EMI belongs to.
EMI filters and EMI shielding are the two most commonly-used techniques to mitigate EMI. Depending on the EMI frequency, the design of the filters and shields change. For low-frequency EMI, low-frequency EMI shielding is one technique that is used to prevent this type of EMI from interfering with circuit operations.
In this article, we will review the types of EMI, with a focus on low-frequency EMI and the best ways to mitigate it.
The Types of EMI
Power electronic converters are common in every electronic device we use today. These power converters change the magnitude and frequency of voltages and currents in a circuit using switching devices. The switching action in power converters makes them vulnerable to EMI. At the same time, they affect nearby devices by emitting EMI.
The EMI generated in power electronic converters can be classified into two types:
- Conducted EMI - Usually, switching converters supply conducted EMI to the power source to which it is connected. In the case of inverters, the conducted EMI interferes with the DC power supply, which can be a DC-DC converter or a battery source. The magnitude of conducted EMI is greater than that of radiated EMI. The conducted EMI is subdivided into differential mode EMI and common mode EMI.
- Radiated EMI - EMI signals that are emitted from the circuit fall under radiated EMI. The radiated EMI impacts the source device as well as the devices in its vicinity.
In both conducted EMI and radiated EMI, there can be signals of low-frequencies, mid-frequencies, and high frequencies. Each type of frequency requires slightly different EMI-mitigation techniques. Low-frequency EMI shielding is one such technique that is effective at mitigating lower-frequency band EMI.
Compared to high-frequency EMI, low-frequency EMI is harder to prevent from reaching sensitive circuits. This type of EMI can sometimes reach the power frequency level or go even lower. As the frequency of electromagnetic waves decreases, the field strength increases and reflection and absorption losses decrease. If designing EMI filters for low-frequency EMI mitigation, then the size of the inductors involved will be large. Considering all these factors, special shielding techniques for low-frequency EMI are required.
Low-Frequency EMI Shielding
Low-frequency EMI shielding is the technique in which low-frequency EMI signals are blocked from interfering with the source circuit as well as the neighboring circuits by using enclosures made of conductive materials. The materials chosen are usually of high permeability so that they can divert electromagnetic fields from the devices of interest. Low-frequency EMI shielding is effective for reducing the electromagnetic field strength by around 95%. Considering the cost and size, it is the best method of EMI shielding.
Low-Frequency EMI Shielding Materials
The strength of the electric and magnetic fields is high at low frequencies, so highly permeable materials are used for low-frequency EMI shielding. The selection of materials depends on the frequency and the strength of the electromagnetic fields needing to be blocked.
Low-frequency EMI shielding materials are selected to alter or block unwanted electromagnetic field distribution from interfering with sensitive equipment. The typical materials used for EMI shielding are copper and aluminum. Highly permeable materials with low reluctance, such as mu-metal or steel, are particularly effective in low-frequency EMI shielding.
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