Chargers For Power Supply Tools

Power adapters generate electromagnetic interference to conductive interference, and conductive interference is divided into common mode interference and differential mode interference two types. Common mode interference is usually the most obvious, it is greater than the differential mode interference generated by the radiation type EMI. currently the most effective way to suppress conducted EMI is to use passive filtering technology.

EMI filters can effectively suppress grid noise and improve the anti-interference capability and reliability of the power adapter. The PFC stage of the single-stage PFC converter operates in discontinuous conductive mode, and its input current waveform is a pulsating triangular wave, so the AC AC input front-end needs to add an EMI filter to filter out high-frequency ripples. The EMI filter of the switching power supply is installed between the power line and the switching power supply, which can be used to suppress the conducted interference led from the power line as well as to reduce the conducted interference introduced from the power grid, which plays an important role in improving the reliability of the power adapter.

The single-stage power supply filter is sensitive to both source and load impedance, and when operating under actual source and load impedance conditions, it is prone to gain rather than attenuation. This gain usually occurs in the frequency range of 150 KHz to 10 MHz and can be as large as 10 to 20 dB. Thus installing an unsuitable filter on a product may increase the intensity of the emissions or make the sensitivity worse.

Filters with two or more stages keep the internal nodes at relatively constant impedance, so the impedance dependence on the load and source is not very large, and can provide performance close to the 50/50 ohm index. Of course, these filters are larger and more expensive.

Most power supply filters use a common mode inductor and an X-type capacitor connected between the phase lines to handle differential mode interference. If the filter is used to address the low frequency, high intensity interference generated by switching power supply circuits, differential mode attenuation is usually required to be greater than that provided by the X-type capacitor, and then a differential mode inductor is required. Because of the saturation of the magnetic core, it is difficult to obtain a large inductance in a small size, making these filters generally large and expensive.

Most power filters utilize Y-type capacitors that are connected between the phase and ground lines. To ensure that the leakage current of the Y-capacitors used in power filters does not exceed the permissible leakage value from the ground line as defined by the relevant safety standards, these capacitors have a value of about a few microfarads. Generally Y-type capacitors should be connected to wires with high noise interference (e.g. rectifiers in power adapters, etc.).

For switching power supplies used in medical equipment (IEC 60601 standard), especially in contact with the patient's body, the ground leakage current values are required to be quite low, so that the use of any kind of Y-type capacitor is not possible. The filters used in this case require larger inductors or are cascaded in multiple stages, making them larger and more expensive.

In larger power systems, ground leakage from a large number of small Y-capacitors produces high ground currents, which can lead to ground voltage differences, resulting in "hum" AC and transient highs on the interconnecting cables between different devices. The best solution nowadays is to use an equipotential 3D ground lap, but this is not possible in many older installations. Therefore, switching power supplies used on equipment in large systems should use filters with little or no Y-capacitance.

It is best to use power supply filters that meet safety approvals for safety, reliability, temperature range, voltage and current ratings, and proper application of safety standards as certified by the manufacturer.

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