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Internal and external interference of power adapters

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.05.16
The interference of power adapters is generally divided into two categories: first, the interference formed by internal components of switching power supplies; The second is the interference caused by external factors in the switching power supply. Both involve both human and natural factors.


65W Desktop Power Adapter

Internal interference of power adapter: The EMI generated by switching power supply is mainly caused by high-order harmonic current interference generated by the basic rectifier and peak voltage interference generated by the power conversion circuit.

There is a distributed capacitance between the switch tube and its heat sink, the casing, and the leads inside the power supply. When the switch tube flows through a large pulse current (generally a rectangular wave), the waveform contains many high-frequency components; At the same time, the device parameters used to turn off the power supply, such as the storage time of the switching power transistor, the large current of the output stage, and the reverse recovery time of the switching rectifier diode, can cause instantaneous short circuit in the circuit, generating a large short-circuit current. In addition, the load of the switching transistor is a high-frequency transformer or energy storage inductor. At the moment when the switching transistor is turned on, there is a large surge current in the transformer primary, causing peak noise.

The transformer in the high-frequency transformer switching power supply is used for isolation and voltage transformation, but due to leakage inductance, it will produce electromagnetic induction noise; At the same time, under high-frequency conditions, the distributed capacitance between the layers of the transformer will transfer high-order harmonic noise from the primary side to the secondary side, while the distributed capacitance of the transformer to the outer shell forms another high-frequency path, making it easier for the electromagnetic field generated around the transformer to couple and form noise on other leads.

When a rectifier diode is used for high-frequency rectification on the secondary side, due to the reverse recovery time factor, the charge accumulated by the forward current cannot be immediately eliminated when the reverse voltage is applied (due to the presence of charge carriers and current flow). Once the slope of the reverse current recovery is too large, the inductance flowing through the coil generates a spike voltage, which will generate strong high-frequency interference under the influence of transformer leakage inductance and other distribution parameters, with a frequency of several tens of MHz. Capacitors, inductors, and wire switching power supplies, due to operating at higher frequencies, can cause changes in the characteristics of low-frequency components, resulting in noise.

External interference of power supply: External interference of switching power supply can exist in a "common mode" or "differential mode" manner. The type of interference can vary from short duration peak interference to complete power loss. This also includes voltage changes, frequency changes, waveform distortion, sustained noise or clutter, and transients.
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