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Power Adapter Measures To Suppress Electromagnetic Interference
Nov 26, 2018

2.1 AC input EMI filter

Filtering is a method of suppressing conducted interference. Connecting a filter to the input of the power supply can suppress the noise from the power grid from invading the power supply itself, and can also suppress the interference generated by the power adapter and fed back to the grid. As an important unit to suppress the conducted interference of power lines, the power supply filter plays an extremely important role in the electromagnetic compatibility design of equipment or systems. The power input terminal usually uses an EMI filter circuit. The circuit can effectively suppress low frequency differential mode disturbance and high frequency mode common mode disturbance at the input end of the AC power source. In the circuit, the differential mode capacitors Cx1 and Cx2 (also called X capacitors) connected across the power supply are used to filter out differential mode interference signals. Generally, ceramic capacitors or polyester film capacitors are used, and the capacitance value is usually 0.1 to 0.47F. The common mode capacitors Cy1 and Cy2 (also known as Y capacitor) grounded in the middle of the line are used to short-circuit the common mode noise current, which is usually C1=C2#2200pF. The suppression inductors L1 and L2 are usually taken as 100~130H. The common mode choke L is composed of two coils which are equivalent and wound in the same direction on one magnetic core. The inductance L#15~25mH is usually required. When the load current crosses the common mode choke, the magnetic lines of force generated by the coils connected in series on the live line are opposite to the lines of magnetic force generated by the coils connected in series on the zero line, which cancel each other out in the core. Therefore, even in the case of a large load current, the magnetic core is not saturated. For the common mode interference current, the magnetic fields generated by the two coils are in the same direction, which will exhibit a large inductance, thereby attenuating the common mode interference signal.

2.2 using absorption circuit

The main cause of EMI in the power adapter is the sharp change in voltage and current, so it is necessary to reduce the rate of change of voltage and current in the circuit (du/dt and di/dt) as much as possible. The absorption circuit can suppress EMI. The basic principle is to provide a bypass when the switch is turned off, and absorb the energy accumulated in the parasitic distribution parameters, thereby suppressing the occurrence of interference. The RC snubber circuit as shown in Figure 2(a) can be connected in parallel across the switch. During the turn-on and turn-off of the switch or diode, the reverse spike current and spike voltage generated in the tube can be buffered. get over. The buffer absorption circuit can reduce the amplitude of the spike voltage and reduce the rate of change of the voltage waveform, which is very beneficial for the safety of the semiconductor device. At the same time, the buffer absorption circuit also reduces the spectral composition of the radio frequency radiation, which is beneficial to reduce the energy of the radio frequency radiation.

Clamp circuits are primarily used to prevent the risk of breakdown of semiconductor devices and capacitors. Taking into account the protection of the clamp circuit and the efficiency requirements of the power adapter, the breakdown voltage of the TVS tube is selected to be 1.5 times the induced voltage of the primary winding. When the voltage on the TVS exceeds a certain amplitude, the device is turned on quickly, thereby discharging the surge energy and limiting the amplitude of the surge voltage to a certain amplitude. A saturable core coil or a microcrystalline magnetic bead may be connected in series on the positive electrode lead of the drain of the switch tube and the output diode. The material is generally cobalt. When the normal current is passed, the core is saturated, and the inductance is very small. Once the current is flowing in the opposite direction, it will generate a very large back EMF, which effectively suppresses the reverse surge current of the diode.

2.3 Shielding measures

An effective way to suppress radiated noise is to shield. The electric field can be shielded by a material having good electrical conductivity, and the magnetic field can be shielded by a material having a high magnetic permeability. In order to prevent the magnetic field leakage of the transformer and make the primary and secondary coupling of the transformer good, the closed magnetic ring can be used to form the magnetic shielding. For example, the leakage flux of the can core is much smaller than that of the E type. For the power adapter's cable, the power cable should use a shielded wire to prevent external interference from being coupled into the circuit. Or use EMC components such as magnetic beads and magnetic rings to filter out high-frequency interference from power supplies and signal lines. However, it is important to note that the signal frequency cannot be disturbed by the EMC component, ie the signal frequency is within the passband of the filter. The outer casing of the power adapter also needs to have good shielding characteristics, and the joints must comply with the shielding requirements specified by EMC. Through the above measures, the power adapter is protected from external electromagnetic environment and external electronic equipment.

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