How to select the filter capacitor -
How to select the filter capacitor -
How to select the filter capacitor this article mainly introduced how to select the power filter capacitor, this article about 1150 words, read the full article 12 minutes.
How to select power filter capacitance, need to grasp the essence and the method.
1, the theoretical ideal capacitance impedance decrease with the increase of frequency (
1 / jwc)
, but due to the inductive effect of pins on both ends of the capacitor, the capacitor should be considered a LC series resonant circuit, device from the resonance frequency of FSR parameters, which means that the frequency is greater than the SFR, capacitance becomes a inductance, if the capacitance to earth filtering, when frequency beyond the SFR, for interference suppression is compromised, so I need a smaller capacitance in parallel to ground.
The reason is that small capacitance, SFR value is big, the high frequency signal, provides a path to.
So often we understand it in the power supply filter circuit: large capacitor filter low frequency, small capacitance filter high frequency, the fundamental reason lies in the SFR (
Since the resonant frequency)
Values are different, think about why?
If want to from this Angle, you can also understand why why power supply filter capacitance on the anchor as near as possible.
2, so in actual design, we often have a question, how do I know how much is the SFR capacitance?
Even if I know the value of SFR, how can I choose different SFR value of capacitance value?
Selection is a capacitor or two capacitor?
SFR value of the capacitance and the capacitance, inductance and capacitance of the pin, so the same values of 0402060 3, or into the capacitive SFR value is not the same, of course, access to the SFR value has two: 1)
The device Data sheet, such as 22 pf, 0402 capacitance values of SFR around 2 g 2)
Measure the resonant frequencies directly by network analyzer, think about how to measure the S21?
After know the SFR capacitance value, using the simulation software, such as RFsim99, choose one or two circuit is whether you have the working frequency band of the power supply circuit has enough noise rejection ratio.
After the simulation, it is the actual circuit experiment, such as debugging mobile receiving sensitivity, LNA power filtering is the key, good power supply filter can improve a few dB.
Capacitance is the nature of exchanges, dc, in theory the power filter capacitor is bigger, the better.
But due to lead and PCB wiring, capacitance is the parallel circuit inductance and capacitance, actually,
Resistance and capacitance itself, sometimes cannot be ignored)
It is introduced the concept of resonance frequency, omega = 1 / (
1/2 under resonant frequency capacitance is capacitive, resonance frequency capacitance is perceptual.
So they generally large capacitor filter low frequency wave, small capacitance filter high frequency wave.
This also explains why the same capacity value of STM encapsulation capacitor filter frequency is higher than DIP package.
The following is a reference capacitance value of capacitance resonance frequency table DIP (
55 more reliable, it is two capacitors in parallel, one large and one small difference in general requirements of more than two orders of magnitude in order to obtain larger filter frequency band.
Capacitance to earth filtering, need a smaller capacitance in parallel to ground, the high frequency signal, provides a pathway to.
Power supply filter capacitance of the anchor should be as near as possible.
In theory the power filter with capacitance, the better, the greater the generally large capacitor filter low frequency wave, small capacitance filter high frequency wave.
Reliable, it is two capacitors in parallel, one large and one small difference in general requirements of more than two orders of magnitude in order to obtain larger filter frequency band.
The selecting principle of filter capacitor after the rectifier bridge is pulsating dc, range is very large.
Behind the size is commonly used in two capacitance;
Large capacitor to stabilize output, it is well known on both ends of the capacitor voltage can't mutation, so you can make the output smooth;
Small capacitance is used to filter out high frequency interference, make the output voltage is pure;
Capacitance is smaller, the higher the resonance frequency, can filter the disturbance frequency is higher;
Large capacitance, the heavier load, the ability to absorb current, the more the more the the volume of this large capacitance.
Small capacitance, thumb, 104 commonly can.
Link address: this paper label: how to select filter capacitance of the capacitor filter on a: high pressure of resistor and the next article: thermistor common problems