Ways to Reduce Power-Supply Noise
Noise is undesired harmonic energy resulting from fast changes in voltage and current in electronic equipment and circuits. In a power supply, this is primarily caused by the high frequency power conversion switching circuits (e.g. PWM). This noise can conduct onto and radiate from PCB traces and wires and cables. Several common practices can be employed to reduce the noise so it does not propagate into other equipment and systems.
EMI Filters utilizing inductors and capacitors can be used at the input and output ports of the power supply. Filtering at the input is governed by international EMI standards that limit the noise that can be propagated onto the power supply network. Output filtering typically is done to reduce radiation from the output circuits and cabling feeding the various loads of the power supply.
Bypassing is the simplest form of filtering, where capacitors are used to reduce circuit noise, specifically that related to the supply and return pins. To increase the bandwidth effectivity of the capacitors, SMD and ceramic capacitors are used. Placement near the noise source as well as reducing impedance from long leads/trace circuits is critical.
Reducing Inductance and Circuit Loops
Trace inductance and circuit loop control are inter-related. Utilization of power and ground planes can go a long way to reducing radiation and coupling from circuit loops.
Decoupling is similar to bypassing in which a circuit is isolated from the noise of another circuit. It will reduce the amount of supply trace shared between circuits.
Decoupling consists of a high impedance element along the supply line to ensure that the current noise will flow through the low impedance bypass element. It will act as a low pass filter so that the high-frequency content of any current that does pass through the series element will be reduced.
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