RFI Filter is a key component for solving high-frequency electromagnetic interference (EMI), which is especially indispensable in high power and high frequency equipment such as industrial chargers.

Industrial chargers are often faced with complex electromagnetic environments: fast switching of switching power supplies, high-current pulses, and parallel operation of multiple devices generate high-frequency noise, which not only affects charging efficiency, but can also damage sensitive circuits.

The application scenarios of RFI filters mainly include:

Power input: Suppresses common mode and differential mode noise in the power grid and prevents external interference from intruding into the equipment through the power line.

Signal transmission port: Protect communication modules (such as CAN bus or Wi-Fi module) from high-frequency radiated interference to ensure data transmission stability.

Chapter 2: Sources of RFI Radio Frequency Interference

The root causes of RF interference in industrial chargers can be divided into two categories: internally generated and externally coupled:

Power device high-frequency switching: IGBT, MOSFET and other power devices in the switching process (frequency can be up to 100kHz or more) generated by the transient voltage/current mutation, through the parasitic inductance formation of high-frequency harmonics. For example, the current change rate (di/dt) of the IGBT of an 800V high-voltage fast-charging pile exceeds 10A/ns when it is switched off, resulting in broadband interference in the 30MHz - 1GHz band. This type of interference propagates through conduction paths (e.g., power lines) or radiation paths (e.g., parasitic capacitive coupling of cables), and requires the use of penetrating capacitor filters to block the high-frequency loop.

Electromagnetic radiation coupling: Long distance cables (>3 meters) form an antenna effect at high frequencies, receiving external RF signals (e.g., 5G base stations, Wi-Fi devices in the 2.4 GHz band) or radiating

internal noise. In industrial environments, wireless communication modules of neighboring devices (e.g. Bluetooth, ZigBee) may trigger cross-interference.

Grounding system defects: Excessive ground impedance (>0.1Ω) or improperly designed ground loops cause common mode interference to be conducted through the ground. For example, filters that are not flange-mounted with galvanized steel, where oxidation of the contact surface significantly increases the impedance, allowing high-frequency noise to escape.

Chapter 3: How to Avoid Hazards, Maintenance and Inspection

I. Key Measures to Avoid RF Interference Hazards

Source suppression

Add buffer circuits (e.g. RC absorption network) to the power devices to reduce di/dt and dv/dt in the switching process, and reduce high-frequency harmonic generation. Use shielded cables (shield coverage ≥ 85%) and magnetic ring chokes to block the coupling path of radiated interference.

Type of filtering system optimization:

Conducted interference below 30MHz: prioritize the use of π-type filters (insertion loss ≥40dB).

Radiated interference above 1 GHz: Use metal-cavity feedthrough filters (e.g. TDK BFC series).

Ensure filter impedance matching, e.g. high impedance source matched C-type filter, low impedance source matched L-type filter.

II. Maintenance Points

Physical condition check: Check the contact impedance between the filter housing and the mounting plate every month (target value <5mΩ), and use internal tooth shims to prevent oxidation. Under high-temperature environment, clean the filter cooling holes regularly to avoid dust accumulation leading to excessive temperature rise (>85℃).

Electrical performance testing: Use a spectrum analyzer to measure the insertion loss quarterly: in the 500MHz - 2GHz band, the filter should be replaced if the loss value drops more than 3dB.

Leakage current detection: medical equipment requires <5μA, industrial equipment allows ≤1mA, exceeding the standard may cause electric shock or false operation.

Troubleshooting and Repair

Common Failures

Core saturation: When the rated current is insufficient (e.g. 50A filter is used in 80A system), the inductance value decreases, resulting in low-frequency interference leakage. Need to upgrade the power capacity (1.5 times the peak current selection).