Electromagnetic interference filter

Introduction

The function of the electromagnetic interference filter is to keep the noise generated by the internal generated by the electronic device does not leak, while preventing noise generated by the AC line outside the electronic device to enter the device.

Electromagnetic interference filter is typically composed of a network of passive electronic components, including capacitive and inductance, which form an LC circuit.

Working principle

Because the frequency of harmful electromagnetic interference is much higher than the normal signal frequency, the electromagnetic interference filter is a high frequency that is selectively blocking or shunting. Play a role. Basically, the induction portion of the electromagnetic interference filter is designed as a low-pass device to pass the AC line frequency, and it is also a high-frequency shutoff device, other portions of the electromagnetic interference filter use the capacitance to be separated or diversally harmful high frequency Noise makes these harmful high frequency noise can't reach the sensitive circuit. Thus, the electromagnetic interference filter significantly reduces or attenuates all harmful noise signals to enter or leave the protected electronic devices.

Electromagnetic interference filter is typically placed on the front end of the switching power source and the grid, is a low pass filter composed of a series reactor and a parallel capacitor. As shown in Figure 1, the noise source isometric impedance is ZSource, and the grid is equivalent impedance is Zsink. Satons filter indicators (FSTOP and HSTOP) can be implemented by first-order, second or third-order low-pass filter, and the calculation of filter transfer functions is usually approximately lower frequencies, that is, for N-order filters, ignore all Ωk Related items (when k output impedance does not match the interference brought by filtering characteristics.