Components for Suppressing Noise and Interference in Switching Power Adapters

When electronic device manufacturers use switching power adapters, they often encounter issues such as unresponsive touchscreens, striped interference on display screens, or a "buzzing" current noise in audio signals. Today, Shenzhen Boerze Power Technology Co., Ltd., a switching power adapter manufacturer, will discuss the necessity of noise filters in power supply equipment.

Just as power adapters vary in design, the noise filters used in them also come in different types. There are several reasons for incorporating noise filters in switching power adapters:

1. Preventing external electromagnetic noise from interfering with the adapter's control circuitry.
2. Shielding the adapter's load from external electromagnetic noise.
3. Suppressing electromagnetic interference (EMI) generated by the adapter itself.
4. Mitigating EMI from other devices that propagates through the power supply.

Switching power adapters contain not only driving circuits but also protection circuits, control circuits, and input/output voltage detection circuits, making their design quite complex. These circuits are primarily built using dedicated or general-purpose power ICs. The most effective way to prevent malfunctions caused by external noise is to employ noise filters.

On the other hand, some of the noise entering the power supply's input can appear at the output. This noise induces voltage in the device's circuits, leading to operational errors. Noise filters can also prevent this issue.

Terminal noise and radiated interference are additional concerns. The adapter itself generates terminal noise during switching operations, and clock-frequency switching in the device can also produce noise at the input, leading to radiated interference. Since this can disrupt other devices, strong countermeasures are necessary. Recently, many countries have implemented noise regulations-such as the FCC in the U.S. and FTZ in Germany-making compliance mandatory for product sales. For these reasons, switching power adapters must incorporate noise filters.

Noise transmission can be categorized into two types: common-mode noise (current flowing to ground) and differential-mode noise (current flowing between lines). While common-mode noise is the primary focus of noise suppression, differential-mode noise dominates at low frequencies. Therefore, the appropriate noise filter must be selected based on the noise composition.

Common-mode inductors typically use toroidal, E-core, or U-core ferrite cores. Toroidal cores are suitable for high-current, low-inductance applications, offering a longer magnetic path without gaps. Fewer windings can achieve higher inductance, making them ideal for superior frequency characteristics. E-core inductors, on the other hand, exhibit lower magnetic flux leakage, making them preferable when minimizing interference with nearby circuits is critical.

Differential-mode inductors usually employ powdered metal cores, which perform well in the lower frequency range (tens of kHz to a few MHz). Their DC superposition characteristics ensure stable inductance even under high current, making them the best choice for differential-mode noise suppression.