How does an input EMI filter work?
Most electronics contains an EMI filter, either as a separate
device, or embedded in circuit boards. Its function is to reduce
high frequency electronic noise that may cause interference with
other devices. Regulatory standards exist in most countries that
limit the amount of noise that can emitted.
EMI, or Electro-Magnetic Interference, is defined as unwanted
electrical signals and can be in the form of conducted or radiated
emissions. Conducted EMI is where the noise travels along the
electrical conductors and radiated EMI is where the noise travels
through the air as magnetic fields or radio waves.
EMI is generated from the switching of electrical current and comes
from a variety of sources including electronic power supplies.
Power supplies convert an input voltage into regulated and isolated
(in most cases) DC voltages to run a host of electronic components.
That conversion is performed at high frequencies ranging from
several kHz to more than a MHz. LED lighting, computers, motor
drivers, DC relays and battery chargers all rely on power supplies
to operate.
An EMI filter for a power supply normally consists of passive
components, including capacitors and inductors, connected together
to form LC circuits. The inductor(s) allow DC or low frequency
currents to pass through, while blocking the harmful unwanted high
frequency currents. The capacitors provide a low impedance path to
divert the high frequency noise away from the input of the filter,
either back into the power supply, or into the ground connection.
In addition to assisting to meet EMI regulations, the filter also
has to meet safety standards. The inductor temperature rise is
measured and for mains operation, the minimum electrical spacing
between line, neutral and ground are controlled. This reduces the
risk of fire and electrical shock. The capacitors are also
individually safety certified, depending on their position in the
circuit. Special “X” capacitors have to be used across the input
terminals and “Y” capacitors from the AC circuit to ground.
