This article is reproduced from Power Consortium
An impedance proportional to the frequency of the inductor , the capacitance of the impedance inversely proportional to frequency . Therefore, inductance can be choked by high frequency, low-pass capacitors can be choked. Suitable combination of the two, can filter various frequency signals. As the rectifier circuit, the capacitor and the inductor in the load or in series with the load, the AC ripple can be filtered off .
Genus voltage filtering capacitor filter , the ripple voltage is directly stored to smooth the output voltage, the output voltage is high, close to the peak AC voltage; for small current, the smaller the current the better the filtering effect.
Filtering inductor current filtering genus , is by electromagnetic induction current by smoothing the output current is low, the output voltage is lower than the effective value of the AC voltage; for high current, the greater the current the better the filtering effect. Many characteristics of capacitance and inductance are just the opposite.
In general, the role of the electrolytic capacitor is to filter out the low-frequency signal current, but even the low-frequency signal whose frequency is also divided into several orders of magnitude. Thus in order to be suitable at different frequencies, also divided into a high frequency electrolytic capacitor capacitance and low capacitance (high frequency here is relative).
Primarily low frequency filter capacitor for filtering the rectified mains filter or a transformer, which is consistent with the operating frequency of the mains 50Hz; high frequency filter capacitor main work in the filtered rectified power supply switching, the operating frequency of several thousand to several Hz Wan Hz. When we When the low frequency filter capacitor for a high frequency circuit , since the low-frequency filter capacitor bad characteristics, it is a large resistance at a high frequency discharge, high equivalent inductance. Thus polarization due to frequent the electrolytic solution and have a greater amount of heat during use. While higher temperatures will enable the interior of the gasification electrolyte capacitor capacitance in the pressure increases, eventually leading to the capacitor bulge and burst .
The size of the power supply filter capacitor, usually do design, pre-use 4.7u, for filtering a low frequency, two with 0.1U, for filtering high frequency , 4.7uF capacitive effect is to reduce the output ripple and low frequency interference , 0.1uF of capacitance should be high-frequency interference due to the transient load current variations caused . Usually front of the bigger the better, two capacitance values differ by about 100 or so times. Filtering power, switching power supply, depending on your ESR (equivalent series resistance capacitance) how much, and preferably the selected frequency capacitance at its self-resonant frequency . Large capacitor to prevent surge, more like a mechanism than as flood large capacity reservoir; small capacitor filter high frequency interference, any device can be equivalent to a resistance, inductance, capacitance of series-parallel circuit, and will have a self-resonance, only in this self-resonance frequency, the minimum equivalent resistance, the best filtering!
Equivalent model of a capacitor in series an inductance L, a resistance R and capacitance C,
inductance L actions lead capacitance , resistance R represents the capacitance of the active power loss , a capacitor C.
Thus equivalent to the condition series LC circuit seeking the resonant frequency of the series resonant WL =. 1 / the WC , W is = 2 the PI F, whereby this equation F =. 1 / (2pi * LC) . , The center frequency of the series LC circuit reactance shown as the minimum resistance, the play center frequency filtering effect. Lead inductance due to its size and different thickness and length, inductance grounding capacitor is generally about 10nH 1MM, depending on the needs of the ground frequency.
Capacitive filter design parameters to be considered:
the ESR
the ESL
withstand voltage of
the resonant frequency
# So how do you choose power supply filter capacitor it?
How to select the power supply filter capacitor, and grasp the essence of the method, in fact, it is not difficult
1, the theoretically ideal capacitive impedance decreases with increasing frequency (1 / jwc), but due to inductive effects pin across the capacitor, then the capacitance should be seen as a series resonant LC circuit, i.e., self-resonant frequency FSR device parameters, which indicates the frequency value is greater than the SFR, the capacitance becomes an inductor , if the capacitance of the filter, when the frequency exceeds SFR, the suppression of interferences greatly reduced, so that the need for a small capacitance in parallel with the ground the reason is that small capacitance, the SFR value is large, the high frequency signal provides a path to ground.
Therefore, the filter circuit in the power supply, we have often appreciated that: a large capacitance low frequency filter, a high-frequency filter small capacitance, the fundamental reason that the SFR (self-resonant frequency) different values, think about why? If you want from this perspective, one can understand why the power supply filtering capacitor to ground as close as possible to the foot of why.
2, then in the actual design, we often have questions, how do I know how much SFR capacitor? Even if I know the value of SFR, how do I choose a different capacitance values SFR value of it? It is to select a capacitor or two capacitors?
SFR value and capacitance of the capacitor is related to and about the pin inductance capacitor , so 0402,0603, or SFR-line capacitance of the same value of capacitance values is not the same, of course, obtain SFR value there are two ways:
- Device Data sheet, such as 22pf, 0402 SFR capacitance value of about 2G
- Directly through a network analyzer to measure their self-resonant frequency, think about how to measure S21?
SFR know capacitance values, simulation software, such as RFsim99, selected from one or two circuits is whether the operating frequency band of the power supply circuit you have sufficient noise rejection. After the simulation, that is, an actual circuit testing, debugging, such as a mobile phone when the reception sensitivity, the LNA's power filtering is critical, good power filter can often be improved several dB.
Nature capacitor is a through AC, DC blocking, power supply filtering capacitor theory, the better. However, since the lead and PCB layout reason, the capacitance is actually a parallel circuit of inductance and capacitance, (as well as the resistance of the capacitor itself, may also not be ignored) which introduced the concept of resonance frequency: ω = 1 / (LC) 1 / 2
below resonance capacitors is capacitive, the capacitance than the resonant frequency is inductive. thus typically large capacitance filtered low frequency wave, high frequency wave of small filter capacitor.
This also explains why STM encapsulated capacitor filter frequency value higher than the capacity of the same DIP.
As for how much the capacitance in the end, which is a resonant frequency of the reference capacitor
| Capacitance values | DIP(MHz) | STM(MHz) |
|---|---|---|
| 10PF | 800 | 1.6(GHz) |
| 100PF | 200 | 500 |
| 1000PF | 80 | 160 |
| 0.01uF | 25 | 50 |
| 0.1uF | 8 | 16 |
| 1uF | 2.5 | 5 |
But just a reference only, mainly by old engineer said that experience
more reliable approach is a freshman small two capacitors in parallel,
the general requirements differ by more than two orders of magnitude in order to obtain a greater filtering band.
I read this article, but also to be a rough summary of it:
- Capacitance to ground filtering, requires a smaller capacitor in parallel to ground, the high frequency signal provides a path to ground.
- Power supply filtering capacitor as close as possible to the foot.
- Theoretically the better power supply filtering capacitor, a large capacitance is generally filtered low frequency wave, high frequency wave of small filter capacitor.
- Reliable approach is two capacitors connected in parallel to one large and small, generally require a difference of two orders of magnitude or more, to obtain a greater filtering band.
Filter capacitor selection principle #
After the bridge was later rectified pulsating DC, a large fluctuation range. Usually the back with two capacitors size;
large capacitance for stabilizing the output voltage across the capacitor is not known mutations, it is possible to output smooth;
small capacitance is used to filter out high frequency interference, the output voltage pure;
the smaller the capacitance, the higher the resonance frequency, the higher frequency interference can be filtered out;
Select ## capacity
- Large capacitance, the heavier the load is, the stronger the absorption current, the greater the necessary large capacitance of the capacitance
- Small capacitance, empirically, typically 104 to
Specific case
- AC220-9V again after full-bridge rectifier, filter capacitor is required to add how much? After another 78LM05 need to add capacitance is how much?
The former capacitor voltage should be greater than 15V, the capacitance should be greater than 2000 micro hair. The latter capacitor voltage should be greater than 9V, capacity should be more than 220 micro hair.
2. a filter capacitor single-phase bridge rectifier circuit, the output voltage of 24V, current is 500mA, requirements:
(1) Select the rectifier diode;
(2) Select the filter capacitor;
(3) further: a buck capacitor filter or by pressure?
(1) Since the full-wave bridge, each diode current so long as to reach half of the load current on the line, the diode current is greater than the maximum 250mA; the output voltage of the capacitive filter type bridge rectified input AC voltage is equal to 1.2 times the RMS , so your circuit input AC voltage effective value should be 20V, the diode is subjected to the maximum back pressure root of 2 times that voltage, therefore, should be greater than the diode voltage 28.2V.
(2) Select the filter capacitor:
1, voltage is greater than 28.2V;
2, find the size of C: Formula RC≥ (3-5) × 0.1 sec in this problem R = 24V / 0.5A = 48 EU
it is possible to obtain C≥ (0.00625-0.0104) F, i.e. the value of C should be greater than 6250μF.
(3) the filter capacitor voltage is increased.
Filter capacitor selection principles #
In the power supply design, the filter capacitor selection principle is: C≥2.5T / R & lt
where: C is the filter capacitance in the UF;
T is the time in S
R & lt load resistance is, the unit is Ω
Of course, this is just a general principle of selection, in practical applications, such as conditions (space and costs) to allow, are selected C≥5T / R.
The size of the filter capacitor ## to select
### PCB plate capacitor selection
When the PCB has contacts, relays, buttons and other components. Will produce a greater spark discharge operation thereof, RC snubber circuit must absorb a discharge current. R generally take. 1 2 k [Omega, C takes 2.2 4.7uF.
Generally about 10PF capacitor used to filter out high-frequency interference signals, to filter out the ripple about 0.1UF low-frequency disturbance, may also play a role in regulation.
What capacity filter capacitors specific choice depends on your PCB main operating frequency and harmonic frequencies may affect the system , you can check the relevant vendor information or capacitance reference library software vendors, selected according to specific needs . As for the number you might not see your specific needs, and to pay more one or two too good, temporarily useless may not posted first, then select the capacitance according to the actual situation of debugging . If you are on the main PCB, then the operating frequency is relatively low, can add two capacitors, and a ripple in addition to consideration, taking into account in addition to a high-frequency signal. If there will be a relatively large instantaneous current, it is recommended together with a larger tantalum capacitors.
In fact, the filter should also include two aspects, that is, you said large capacitance and small capacitance, is decoupling and bypass. I do not say principle, practical point, typically 0.1uF decoupling digital circuits can, for the 10M; 20M or more with 1 to 10 uF Tolerance, better remove high frequency noise. If in accordance with C = 1 / f. Bypass generally relatively small, and generally used is 0.1 or 0.01uF.
Speaking of capacitors, variety is called it will make people dizzy, bypass capacitors, decoupling capacitors, filter capacitors, etc., in fact, call anyway, its principle is the same, namely the use of AC signal exhibits low impedance characteristic, can be seen by this equation the equivalent impedance of the capacitor out: XCAP. 1 = / 2лfC , the higher the frequency, the smaller the larger the capacitance of the capacitive impedance.
In the circuit, if the capacitance play a major role it is to provide a low impedance path to the AC signal, called bypass capacitor; if mainly to increase the power and ground AC-coupled, reducing the impact on the AC power signal, can be called for the decoupling capacitor; if a filter circuit, the filter capacitor can be called; except for the DC voltage of the capacitor can be used as an energy storage circuit, by using the battery charge and discharge functions. The reality, often capacitance role is multi-faceted, we need not spend too much time considering how to define. In this paper, we unify these apply to high-speed PCB design is called bypass capacitor capacitance.
