Selection of filter capacitance

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After the rectifier bridge is pulsating DC, the fluctuation range is very large. After the general use of the size of two capacitors

The large capacitance is used to stabilize the output and it is well known that the voltage on the capacitor cannot be mutated, so the output is smoothed

Small capacitance is used to filter out high-frequency interference, so that the output voltage is pure

The lower the capacitance, the higher the resonant frequency, the higher the interference frequency can be filtered

Capacity selection:

(1) Large capacitance, the heavier the load, the higher the ability to absorb the current, the larger the capacity of this capacitor will be greater

(2) small capacitance, with experience, General 104 can

2. Other people's experience (from the Internet)

1, capacitance to ground filtering, need a small capacitance in parallel to the ground, the high-frequency signal provides a ground access.

2, Power filter in the capacitance to the foot as close as possible to the ground.

3, theoretically, the power filter is larger and better, the general large capacitance filter low-frequency wave, small capacitance filter high-frequency wave.

4, a reliable approach is to be a large number of small two capacitors in parallel, the general requirements of two orders of magnitude above, in order to obtain a larger filter band.

Concrete case: Ac220-9v After the full bridge rectification, the need to add the filter capacitance is how big? How big is the capacitance to be added after 78lm05?

The former capacitor voltage should be greater than 15V, capacitance capacity should be greater than 2000 micro-hair. The latter capacitor voltage should be greater than 9V, the capacity should be more than 220 micro-hair.

2. There is a capacitor filter single-phase bridge rectifier circuit, the output voltage is 24V, the current is 500mA, requires:
(1) Select rectifier Diode;
(2) Select filter capacitance;
(3) Another: Capacitor filter is buck or booster?

(1) Because the bridge is a full wave, so that each diode current as long as the load current is half the line, so the maximum diode current is greater than 250mA , the output voltage of the capacitor filter bridge rectifier is equal to 1.2 times times the effective value of the input AC voltage, so your circuit input AC voltage valid value should be 20V, and the maximum back pressure of the diode is twice times the square root of this voltage, so, the diode should withstand more than 28.2V.
(2) Select the filter capacitance: 1, the voltage is greater than 28.2v;2, the size of C: Formula Rc≥ (3--5) x0.1 seconds, the subject of r=24v/0.5a=48 Europe
Therefore, C≥ (0.00625--0.0104) F can be obtained, that is, the value of C should be greater than 6250μf.
(3) The capacitance filter is to raise the voltage.

Selection principle of filter capacitance

In power supply design, the principle of selecting the filter capacitance is: c≥2.5t/r

Wherein: C is the filter capacitance, the unit is UF;

T is frequency, unit is Hz

R is the load resistor, measured in Ω

Of course, this is only the general principle of selection, in the actual application, such as the conditions (space and cost) allow, are selected c≥5t/r.

3. Selection of the size of the filter capacitor
PCB Plate Capacitor Selection

When the PCB has contactors, relays, buttons and other components. When they are operated, they produce a large spark discharge and must be
An RC absorption circuit is used to absorb the discharge current. General R take 1~2kω,c take 2.2~4.7μf

The general 10PF capacitance is used to filter out high-frequency interference signal, 0.1UF to filter out the low-frequency ripple interference, but also
Can play a role in voltage regulation

The specific choice of filter capacitance depends on the main operating frequency on your PCB and the harmonics that may affect the system
Frequency, you can check the relevant manufacturer's capacitance data or reference vendor to provide database software, according to the specific needs
Choose. As for the number is not necessarily, see your specific needs, more than one or two is also very good, temporarily useless can
Do not post first, according to the actual debugging situation and then select the tolerance value. If your PCB main operating frequency is relatively low, add two
Capacitor on it, a consideration of the removal of Ripple, a high-frequency signal to consider. If there is a large instantaneous current, it is recommended
Add a larger tantalum capacitor.

In fact, the filter should also contain two aspects, which is what you call the large capacitance and Yung value, is decoupling and bypass.
Principle I will not say, practical point, the general digital circuit decoupling 0.1uF can be used for 10M or less; 20M above 1 to
10 uf, remove high-frequency noise better, probably press c=1/f. Bypass generally is relatively small, generally according to the resonant frequency
Typically 0.1 or 0.01uF

When it comes to capacitance, a variety of names can make people dizzy, bypass capacitors, decoupling capacitors, filter capacitors, and so on, which
In any case, its principle is the same, that is, the use of the AC signal to show low impedance characteristics, which can be
It is shown by the equivalent impedance formula of the capacitance: XCAP=1/2ЛFC, the higher the operating frequency, the larger the capacitance value of the capacitor
The smaller the impedance. In a circuit, if the primary function of the capacitor is to provide a low impedance path to the AC signal, it is called a side
If the main purpose is to increase the AC coupling between the power supply and ground, and reduce the influence of the AC signal on the power supply, you can
Referred to as a decoupling capacitor, if used in a filter circuit, it can also be called a filter capacitor, in addition, for the direct current
Voltage, the capacitor can also be used as a circuit to store energy, using the impulse discharge to play the role of the battery. And in reality, the capacitance is often
The role is manifold, we can not spend too much thought to consider how to define. In this article, we unify these
Capacitors used in high-speed PCB designs are called bypass capacitors.

The nature of the capacitor is through the AC, DC, theoretically speaking, power supply filter to the larger the better.

But because of the lead and PCB cabling, in fact the capacitance is the shunt circuit of inductance and capacitance,

(There is also the resistance of the capacitor itself, sometimes not negligible)

This introduces the concept of resonant frequency: ω=1/(LC) 1/2

Capacitance is capacitive below the resonant frequency, and the capacitance above the resonant frequency is inductive.

Thus the general large capacitance filter low frequency wave, small capacitance filter high frequency wave.

This also explains why the capacitance filtering frequency of the STM package with the same capacitive value is higher than the dip package.

As for how much capacitance is used, this is a reference

Capacitance Resonant Frequency

Capacitor value DIP (MHz) STM (MHz)

1.0ΜF 2.5 5

0.1μf 8 16

0.01μf 25 50

1000pF 80 160

250 PF 500

-PF 1.6 (GHZ)

But only for reference, in the words of old engineers--mainly by experience.

A more reliable approach is to parallel a small two capacitors,

The general requirements of two orders of magnitude above, in order to obtain a larger filter band.

In general, large capacitors filter out low-frequency waves, small capacitors filter out high-frequency waves. The capacitance is inversely proportional to the square of the frequency you want to filter.
。

The specific capacitance can be selected using the formula C=4PI*PI/(R * f * f)

How to choose the Power filter capacitor, master its essence and method, in fact, it is not difficult.

1) theoretically ideal capacitance whose impedance decreases with increasing frequency (1/JWC), but due to inductance effect at both ends of the capacitor
, the capacitance should be regarded as an LC serial resonant circuit, the self-resonant frequency is the device's FSR parameter, which indicates that the frequency is greater than
FSR value, the capacitance becomes an inductor, if the capacitance to the ground filter, when the frequency exceeds the FSR, the interference suppression will be a big hit
discount, so need a smaller capacitor in parallel to the ground, can you think about why?

The reason is that the small capacitance, the SFR value is large, the high-frequency signal provides a ground access, so in the power filter circuit we often
Often this understanding: large capacitance for low frequency, small capacitance for high frequency, the fundamental reason is the SFR (self-resonant frequency) value is different, of course
You can think about why? If you think from this angle, you can understand why the capacitance in the Power filter
As close to the ground as possible.

2) So in the actual design, we often have doubts, how do I know the capacitance of the SFR how much? Even if I knew the SFR value
, how do I choose the capacitance value of the different SFR values? Is it a capacitor or two capacitors?

The SFR value of the capacitance is related to the capacitance value, which is related to the pin inductance of the capacitor, so the same capacitance of the 0402,0603, or the plug-in electric
The SFR value will not be the same, of course, there are two ways to obtain the SFR value, 1) device data sheet, such as the 22pf0402 capacitor
SFR value around 2G, 2) through the Network analyzer directly measured its self-resonant frequency, think how to measure? S21?

Once you know the SFR value of the capacitor, use software emulation, such as RFsim99, to select one or two circuits that are the work of your power supply circuit.
Whether the frequency band has sufficient noise rejection ratio. After the simulation, that is the actual circuit test, such as the commissioning of the phone receiver sensitivity,
The LNA's power filter is the key, and a good power filter can often improve several db.

In general, the selection of output filter capacitance is mainly to obtain a good filtering effect, the output voltage ripple and the working mode of the chip (boost or buck) and the principle of operation, single-phase and multiphase calculation method is different. For example, if using the ltc3406b chip, vout≈ IL (esr+1/8fcout), where Vout is the output voltage ripple, IL is the ripple current of the inductor, ESR is the output filter capacitance of the internal resistance, F is the switching frequency, The cout is the capacitance of the output filter capacitor. With this formula, it is easy to calculate the required capacitance parameters. Filter capacitance range is too wide, here is a brief talk about power bypass (decoupling) capacitor. The choice of filter capacitor depends on whether you are using a local power supply or a global power supply. To the local power supply is to play the role of transient power supply. Why do we need to add a capacitor to power it? This is because the device's demand for current varies rapidly with the demand for the drive (e.g. DDR Controller), and the distribution parameters of the circuit are considered in the high frequency range. The presence of distributed inductors hinders the drastic change of the current, which reduces the voltage on the chip's power foot-that is, the noise is formed. Moreover, the current feedback power supply has a reaction time-that is, to wait until the voltage fluctuations occur for a period of time (usually Ms or US) to make adjustments, for the NS level of current demand changes, this delay, also formed the actual noise. Therefore, the function of the capacitor is to provide a low inductance (impedance) route to meet the current demand for rapid change. Based on the above theory, the calculation of capacitance will be calculated according to the energy of the capacitor to provide the current change. Choose the type of capacitor, you need to follow its parasitic inductance to consider-that is, the parasitic inductance is less than the distribution inductance of the power path.

Selection of filter capacitance