1、 EMC filter planning tips
The filter in EMC planning usually refers to the low-pass filter composed of L and C. The selection of filter structure is determined by the "maximum mismatch criterion". In any filter, there is high impedance at both ends of the capacitor and low impedance at both ends of the inductance. Fig. 1 shows the matching relationship between the structure of the filter with the loss of the maximum mismatch criterion and ZS and ZL. In each case, 2 structures and corresponding attenuation slopes are given (n represents the total number of capacitance and inductance elements in the filter).
In between: l and R are divided into the length and radius of the lead. The parasitic inductor will have series resonance with the capacitor, that is, self resonance. At the self resonance frequency fo, the decoupling capacitor has the least impedance and the best decoupling effect. However, for the noise component with frequency f higher than f / O, the decoupling capacitance is electrically rational, and the impedance increases with the decrease of frequency, which greatly reduces the decoupling or bypass effect. In theory, the self resonant frequency f0 of decoupling capacitor should be selected according to the highest frequency Fmax of noise, and the best value is fo = Fmax.
High voltage chip capacitor
The selection of decoupling capacity is in digital details. The capacity of decoupling capacity is usually budgeted as follows:
2、 EMC grounding plan
Grounding is the most ineffective way to control the source of disturbance, which can handle 50% of EMC results. It is connected to the earth on a trivial basis and can suppress electromagnetic interference. The metal part of the shell is indirectly connected to the ground, and it can also provide leakage path of electrostatic charge to avoid electrostatic accumulation.
Pay attention to the following points in the ground wire planning:
(1) In the low frequency circuit, the mission frequency of the signal is less than 1MHz, the influence of the wiring and the inductance between the devices is small, but the circulating current composed of the grounding circuit has great influence on the disturbance, so the single point grounding should be selected. When the signal mission frequency is greater than 10MHz, the ground wire impedance becomes large. At this moment, the ground wire impedance should be reduced as much as possible, and the nearest multi-point grounding should be selected. When the mission frequency is 1-10mhz, if one point of grounding is selected, the length of the ground wire shall not exceed 1 / 20 of the wavelength, otherwise multi-point grounding method shall be selected.
(2) Separate the digital circuit and the analog circuit. There are both high-speed logic circuit and linear circuit on the circuit board. They should be separated as far as possible. The ground wires of the two should not be mixed. They should be connected with the ground wires at the power supply end. It is necessary to increase the space product of linear circuit as much as possible.
(3) If the ground wire is very thin, the ground potential will change with the change of the current, resulting in the instability of the punctual signal level of the electronic equipment and the deterioration of the anti noise function. Therefore, the ground wire should be thickened as much as possible so that it can pass through the allowable current of three positions on the printed circuit board. If possible, the width of the earth wire shall be greater than 3mm.
(4) When the ground wire of printed circuit board which is only composed of digital circuit is trivial, making the ground wire into a closed loop can clearly advance anti noise ability. The reason lies in: there are many integrated circuit components on the printed circuit board, especially in the case of components with more power consumption, because of the restriction of the thickness of the ground wire, a large potential difference will occur on the ground junction, which will cause the anti noise ability to decline. If the grounding structure is formed into a loop, the potential difference will be reduced, and the anti noise ability of the forward electronic equipment will be improved.
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