The concept of filter capacitor
Characteristics of filter capacitors
5. Low-frequency filtering and high-frequency filtering capacitors
Effect of power supply filter capacitor size on voltage
Capacitor filter circuit principle
The larger the capacity of the filter capacitor, the better?
Selection and calculation of power supply filter capacitor
How to select power supply filter capacitor
The concept of filter capacitor
- An energy storage device installed at both ends of the rectifier circuit to reduce the AC pulsation ripple coefficient and improve efficient and smooth DC output. This device is usually called a filter capacitor.
- The capacitance of the filter capacitor should be determined based on the load resistance and output current. When the filter capacitor reaches a certain capacity, increasing the capacitor capacity will have harmful effects on some other indicators.
Inductor filtering and capacitor filtering
- The impedance of an inductor is directly proportional to frequency, and the impedance of a capacitor is inversely proportional to frequency. Therefore, the inductor can block the passage of high frequencies, and the capacitor can block the passage of low frequencies. Many properties of capacitors and inductors are exactly opposite.
- Capacitor filter:
It is a voltage filter that directly stores the pulsating voltage to smooth the output voltage. The output voltage is high, close to the AC voltage peak; it is suitable for small currents. The smaller the current, the better the filtering effect.
It belongs to current filtering, which relies on electromagnetic induction generated by current to smooth the output current. The output voltage is low, lower than the effective value of the AC voltage; it is suitable for large currents. The larger the current, the better the filtering effect.
Characteristics of filter capacitors
1. Low temperature rise
- The harmonic filter loop is composed of capacitors connected in series with reactors, forming the lowest impedance at a certain harmonic order to absorb a large amount of harmonic current. The quality of the capacitor will affect the stable absorption effect of the harmonic filter, and the service life of the capacitor will depend on the Temperature has a great relationship. The higher the temperature, the shorter the service life. Filtering full film capacitors have the characteristics of low temperature rise, which can ensure their service life.
2. Low loss
3. Security
4. Convenience
5. Low-frequency filtering and high-frequency filtering capacitors
Low frequency filter capacitor:
- It is mainly used for mains power filtering or filtering after transformer rectification. Its operating frequency is the same as the mains power supply of 50Hz. The pulsating voltage frequency of ordinary electrolytic capacitors used in 50 Hz power frequency circuits is only 100 Hz, and the charge and discharge time is on the order of milliseconds.
- In order to obtain a smaller pulsation coefficient, the required capacitance is as high as hundreds of thousands of microfarads. Therefore, the goal of ordinary low-frequency aluminum electrolytic capacitors is to increase the capacitance. The capacitance, loss tangent value and leakage current of the capacitor are the identification factors. The main parameters of its advantages and disadvantages.
High frequency filter capacitor:
- It mainly works in filtering after rectification of switching power supply, and its operating frequency is from several thousand Hz to tens of thousands Hz. Its sawtooth voltage frequency is as high as tens of thousands of hertz or even tens of megahertz.
- At this time, capacitance is not its main indicator. The standard for measuring the quality of high-frequency aluminum electrolytic capacitors is the "impedance-frequency" characteristic. It is required to have a low equivalent impedance within the operating frequency of the switching power supply, and at the same time have a good filtering effect on the high-frequency spike signals generated when the semiconductor device is working.
Effect of power supply filter capacitor size on voltage
Capacitor filter circuit principle
- When the input pulsating voltage Ui is higher than the voltage across the filter capacitor, the capacitor is charged. When the input pulsating voltage Ui is lower than the voltage across the filter capacitor, the filter capacitor begins to discharge and assumes the responsibility of providing power to the load, compensating for the input pulsating voltage. The downward trend of ui, thereby reducing the ripple degree of the ripple voltage (ripple coefficient).
- We have also been taught that the larger the filter capacitor is, the smaller the filtered output voltage ripple will be.
The larger the capacity of the filter capacitor, the better?
- First of all, there is no doubt that the larger the capacity, the higher the cost, but more importantly, when the filtering capacity is large to a certain extent, the benefits brought by the capacitor capacity will be less.
- First, as mentioned above in the bridge rectifier filter, when the capacity of the filter capacitor is from 10uF to 100uF, the ripple voltage improvement is 64V-22V=42V, and from 100uF to 1000uF, the ripple improvement value is 22V-4.24V=19.6V, and from The ripple improvement value from 1000uF to 4700uF is only 4.24-1.35=2.89V, as shown in the figure below:
It is obvious that the larger the capacity of the filter capacitor, the corresponding ripple voltage decreases. However, the larger the filter capacitor is, the fewer benefits can be obtained. From an economic point of view, the smaller the marginal benefit. (low cost performance), not worth doing;
- Second, the necessity of excessive filtering capacity. If there is no need to execute something, then we don’t need to execute it. This seems to be nonsense, but this is also the applicability rule followed in circuit design (as long as it is enough). The purpose of the filter capacitor is to reduce the AC pulsating voltage (ripple coefficient), not to output a stable voltage;
- Third, the feasibility of the filter capacitor being too large. If the capacity of the filter capacitor is too large, the charging current (ripple current) will also be larger. Excessive ripple current is a fatal damage to the circuit system. Ripple current (Ripple current) is defined as: the RMS value (effective value) of the AC ripple current that the capacitor can withstand under the highest operating temperature condition, and the specified ripple is in the frequency range (100Hz ~ 120Hz ) sine wave.
The manifestation of ripple current in voltage is ripple voltage (ripple). The maximum allowable ripple current that a capacitor can withstand is limited by parameters such as temperature, loss angle, and AC frequency. It is usually represented by IR in the data sheet, as follows The ripple current shown in the figure (the figure below is from the VISHAY aluminum electrolytic capacitor 038 RSU data sheet):
Selection and calculation of power supply filter capacitor
- The size of the power supply filter capacitor is usually used in design. The front stage uses 4.7u, which is used to filter low frequencies. The second stage uses 0.1u, which is used to filter high frequencies. The 4.7uF capacitor is used to reduce output pulsation and low-frequency interference. The 0.1uF capacitor is used to filter the low frequency. The capacitor should reduce high-frequency interference caused by instantaneous changes in load current. Generally, the bigger the front one, the better. The difference between the two capacitance values is about 100 times.
- Power supply filtering and switching power supplies depend on how big your ESR (equivalent series resistance of the capacitor) is, and the best choice for high-frequency capacitors is at their self-resonant frequency. Large capacitors prevent surges, and the mechanism is just like a large reservoir with stronger flood control capabilities; small capacitors filter high-frequency interference, and any device can be equivalent to a series-parallel circuit of resistors, inductors, and capacitors, thus creating self-resonance. Only at this self-resonant frequency, the equivalent resistance is the smallest, so filtering is the best!
How to select power supply filter capacitor
- Theoretically, the impedance of an ideal capacitor decreases as the frequency increases (1/jwc). However, due to the inductance effect of the pins at both ends of the capacitor, the capacitor should be regarded as an LC series resonant circuit. The self-resonant frequency is the device's self-resonance frequency. FSR parameter, which means that when the frequency is greater than the FSR value, the capacitor becomes an inductor. If the capacitor is filtered to the ground, when the frequency exceeds the FSR, the suppression of interference will be greatly reduced, so a smaller capacitor is needed in parallel to the ground. The reason is that the small capacitor and large SFR value provide a path to ground for high-frequency signals.
So in the power supply filter circuit, we often understand it this way: large capacitors filter low frequencies, and small capacitors filter high frequencies. The fundamental reason is that the SFR (self-resonant frequency) values are different. Think about why? If you think about it from this perspective, you can understand why the capacitor and the ground pin in the power supply filter should be as close as possible to the ground.
- How do you know what the SFR of a capacitor is? How to choose capacitor values with different SFR values? Should I choose one capacitor or two capacitors?
The SFR value of a capacitor is related to the capacitance value and the pin inductance of the capacitor, so the SFR values of 0402, 0603 or plug-in capacitors with the same capacitance will not be the same. Of course, there are two ways to obtain the SFR value: 1 ) Device Data sheet, such as the SFR value of the 22pf0402 capacitor is around 2G, 2) Directly measure its self-resonant frequency through a network analyzer?
- After knowing the SFR value of the capacitor, use software simulation, such as RFsim99, to choose one or two circuits based on whether the working frequency band of your power supply circuit has sufficient noise suppression ratio. After the simulation is completed, it is the actual circuit test.
- The essence of a capacitor is to pass AC and block DC. Theoretically, the larger the capacitor used for power supply filtering, the better. However, due to the wiring and PCB wiring, the capacitor is actually a parallel circuit of an inductor and a capacitor (and the resistance of the capacitor itself, which cannot be ignored sometimes). This introduces the concept of resonant frequency: ω=1/(LC)1/ 2
Below the resonant frequency the capacitor is capacitive, and above the resonant frequency the capacitor is inductive. Therefore, generally large capacitors filter low-frequency waves, and small capacitors filter high-frequency waves.
This can also explain why the capacitor filtering frequency of STM packages with the same capacitance is higher than that of DIP packages.
- Capacitor-to-ground filtering requires a smaller capacitor connected in parallel to ground, which provides a path to ground for high-frequency signals.
- In power supply filtering, the capacitor should be as close to the ground as possible.
- Theoretically speaking, the larger the capacitor used for power supply filtering, the better. Generally, large capacitors filter low-frequency waves, and small capacitors filter high-frequency waves.
- A reliable approach is to connect two capacitors, one large and one small, in parallel, generally requiring a difference of more than two orders of magnitude to obtain a larger filtering frequency band.