while in the view of the high frequency noise filtering, the resonant capacitance is not hope, but the resonance capacitance are not always harmful.
When determine to filter out the noise of the frequency, can, by adjusting the capacitance of the capacity to make resonance point fall on the disturbance frequency.
In practical engineering, to filter out the electromagnetic noise frequency is often as high as several hundred MHz, even more than 1 GHZ.
For such a high frequency electromagnetic noise must use wear heart capacitance can effectively filter out.
Capacitors are not effectively filter out high frequency noise, because two reasons, one reason is that the capacitor lead inductance capacitance resonance, the high frequency signal to present larger impedance, weakened the bypass effect of high frequency signal;
Another reason is that lead the parasitic capacitance between the coupling in the high frequency signal, reduces the filtering effect.
Wear heart capacitance can effectively filter out high frequency noise, because wear heart capacitance does not lead inductance capacitance resonance frequency is too low problem, and wear heart capacitance can be installed directly on the metal panel, have the effect of high frequency isolation using metal panel.
But in the use of wear heart capacitance, attention should be paid to the problem is to install.
Wear heart capacitance's biggest weakness is afraid of high temperature and temperature shock, this in capacitance will wear heart during welding caused great difficulties to the metal panel.
Many of the capacitance in case of damage during the process of welding.
Especially when you need to install a lot of wear heart capacitance on the panels, as long as there is a damage, it is difficult to repair, because in the damage of capacitor will be removed, can cause damage to other nearby capacitance.
The working principle of electromagnetic interference filter for harmful electromagnetic interference frequency is much higher than normal signal frequency, so the electromagnetic interference filter is through the selectively block or shunt harmful high frequency to play a role.
Basically, induction part of emi filter is designed as a low pass device by the ac line frequency, it is a high frequency cut-off device at the same time, the rest of the emi filter using capacitance to shunt or bypass harmful high frequency noise, make these harmful high frequency noise can't reach the sensitive circuits.
So the electromagnetic interference filter is significantly reduced or attenuation all to enter or leave the protected the harmful noise signal of electronic devices.
Electromagnetic interference filter is usually placed in the switching power supply and power grid connected to the front, is made up of reactor in series and parallel capacitor low-pass filter.
As shown in the picture on the right, noise equivalent impedance for equivalent impedance for Zsink Zsource, power grid.
Satons filter indicators,
Fstop and Hstop)
By first and second order or third order low-pass filter, filter transfer function calculation under the high frequency approximation, usually for n order filter, that is to ignore all omega k related items (
When k, n)
And contains only take n omega related items.
Special note to consider input and output impedance mismatch brings to the filtering characteristics of interference.
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