How Ceramic Capacitors and Electrolytic Capacitors Work
In the circuit design process, capacitors are used for filtering. Sometimes electrolytic capacitors are used, and sometimes ceramic capacitors are used. Sometimes both are used. I would like to ask: what is the role of using electrolytic capacitors? What is the function of using ordinary ceramic capacitors? How to calculate the size of its capacity? How to choose and determine the withstand voltage of electrolytic capacitors? In which cases should electrolytic capacitors be used, in which cases ceramic capacitors should be used, and in which cases should both be used? It was mentioned in the old version of the analog e-book that there is a special formula to calculate the size of the capacitor value, but some ICs and the like have regulations on how to match the capacitor in its Datasheet, I hope it can help you.
Electrolytic capacitors and ceramic capacitors are generally used between the power supply of the IC and the ground to play a filtering role. The ceramic capacitors are used alone for decoupling. Its use is generally explained in the IC. Relevant, take 0.01uf for ceramics.
If I want to replace a certain capacitor with another capacitor, do I have to satisfy both the capacity and withstand voltage? Sometimes, it's hard to find the best of both worlds. Is it possible to give up one of them at this time?
The filter capacitor range is too wide, here is a brief talk about the power bypass (decoupling) capacitor.
The choice of filter capacitor depends on whether you use it in the local power supply or the global power supply. For the local power supply, it is to play the role of transient power supply. Why add capacitors to supply power? It is because the current demand of the device changes rapidly with the driving demand (such as the DDR controller), and in the discussion in the high frequency range, the distribution parameters of the circuit must be considered. Due to the existence of the distributed inductance, the drastic change of the current is prevented, and the voltage on the power supply pin of the chip is reduced - that is, the noise is formed. Moreover, the current feedback power supply has a reaction time - that is, it will not make adjustments until the voltage fluctuation occurs for a period of time (usually ms or us level). For the current demand change of the ns level, this kind of The delay also forms the actual noise. Therefore, the role of the capacitor is to provide a low inductive reactance (impedance) route to meet the rapid changes in current demand.
Based on the above theory, the calculation of capacitance should be calculated according to the energy that the capacitor can provide for current change. When choosing the type of capacitor, you need to consider its parasitic inductance—that is, the parasitic inductance should be smaller than the distributed inductance of the power path.
Discussing issues must start from the essence. First of all, you probably know that capacitors are DC isolation, while inductors are the opposite. All are based on basic principles. At this time, the capacitor has the two most common functions. One is to isolate DC between poles. Some people also call it a coupling capacitor because it isolates DC, but it needs to pass AC signals. The DC path is limited between several stages, which can simplify the very complicated calculation of the operating point, and the second is filtering. Basically these two. As a coupling, the value of the capacitor is not strictly required, as long as its impedance is not too large, so that the signal attenuation is too large.
But for the latter, it needs to be considered from the point of view of the filter. For example, the power supply filtering at the input end requires filtering out low-frequency (such as power frequency) noise and high-frequency noise, so it needs to be used at the same time. Large capacitors and small capacitors. Some people will say, with a large capacitor, why do you need a small one? This is because the large capacitance, the large inductance due to the large plate and the pin end, does not work for high frequencies. Small capacitors are just the opposite. The size can be used to determine the capacitance. As for the withstand voltage, it must be satisfied at all times, otherwise, it will explode. Even for non-electrolytic capacitors, sometimes it does not explode, and its performance is also reduced. It's too much to talk about, let's talk about it first. They are all filtering functions. The aluminum electrolytic capacitor has a relatively large capacity and is mainly used to eliminate low-frequency interference. The capacity is about 1mA current corresponding to 2~3μf, if the requirement is too high, 1mA can correspond to 5~6μf. Non-polar capacitors are used to filter out high frequency signals. Most of the time it is used alone, it is used to remove the lotus root. Sometimes it can be used in parallel with electrolytic capacitors. The high-frequency characteristics of ceramic capacitors are better, but at a certain frequency (about 6MHz, I can't remember clearly), the capacity decreases rapidly.
The role of electrolytic capacitors and precautions for use
1. The role of electrolytic capacitors in circuits
1. Filtering effect. In the power supply circuit, the rectifier circuit turns the AC into a pulsating DC, and a large-capacity electrolytic capacitor is connected after the rectifier circuit, and the rectified pulsating DC voltage becomes Relatively stable DC voltage. In practice, in order to prevent the power supply voltage of each part of the circuit from changing due to load changes, electrolytic capacitors of tens to hundreds of microfarads are generally connected to the output end of the power supply and the power input end of the load. Since large-capacity electrolytic capacitors generally have a certain inductance and cannot effectively filter out high-frequency and pulse interference signals, a capacitor with a capacity of 0.001--0.lpF is connected in parallel at both ends to filter out high-frequency signals. and pulse interference.
2. Coupling effect: In the process of transmission and amplification of low-frequency signals, in order to prevent the static operating points of the front and rear circuits from affecting each other, capacitive coupling is often used. In order to prevent excessive loss of low-frequency components in the signal, electrolytic capacitors with larger capacity are generally used.
Second, the judgment method of electrolytic capacitor
Common faults of electrolytic capacitors include capacity reduction, capacity disappearance, breakdown short circuit and leakage. The change in capacity is caused by the gradual drying of the electrolyte inside the electrolytic capacitor during use or placement, while breakdown and leakage are generally added. The voltage is too high or the quality itself is not good. Judging the quality of the power supply capacitor is generally measured by the resistance file of the multimeter. The specific method is: short-circuit the two pins of the capacitor to discharge, and use the black test lead of the multimeter to connect the positive electrode of the electrolytic capacitor. The red test lead is connected to the negative pole (for an analog multimeter, the test lead is intermodulated when measuring with a digital multimeter). Normally, the test needle should swing in the direction of small resistance, and then gradually return to infinity. The greater the swing of the needle or the slower the return speed, the greater the capacity of the capacitor, and vice versa, the smaller the capacity of the capacitor. If the pointer does not change somewhere in the middle, it means that the capacitor is leaking. If the resistance indication value is small or zero, it means that the capacitor has been broken down and short-circuited. Because the voltage of the battery used by the multimeter is generally very low, it is more accurate to measure the capacitor with low withstand voltage. When the withstand voltage of the capacitor is high, although the measurement is normal, there may be leakage or shock when high voltage is added. wear phenomenon.
3. Precautions for the use of electrolytic capacitors
1. Since electrolytic capacitors have positive and negative polarities, they cannot be connected upside down when used in circuits. In the power supply circuit, the positive pole of the electrolytic capacitor is connected to the output terminal of the power supply when the positive voltage is output, and the negative pole is connected to the ground; when the negative voltage is output, the negative pole is connected to the output terminal, and the positive pole is grounded. When the polarity of the filter capacitor in the power supply circuit is reversed, the filtering effect of the capacitor is greatly reduced, on the one hand, the output voltage of the power supply fluctuates, and on the other hand, the electrolytic capacitor, which is equivalent to a resistor, heats up due to reverse power supply. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will become very small, so that the capacitor will burst and damage due to overheating for a short time after power-on.
2. The voltage applied to both ends of the electrolytic capacitor cannot exceed its allowable working voltage. When designing the actual circuit, a certain margin should be reserved according to the specific situation. When designing the filter capacitor of the regulated power supply, if the AC power supply voltage is 220~ The rectified voltage of the secondary of the transformer can reach 22V. At this time, the electrolytic capacitor with a withstand voltage of 25V can generally meet the requirements. However, if the AC power supply voltage fluctuates greatly and may rise to more than 250V, it is best to choose an electrolytic capacitor with a withstand voltage of more than 30V.
3. Electrolytic capacitors should not be close to high-power heating elements in the circuit to prevent the electrolyte from drying up quickly due to heating.
4. For the filtering of signals with positive and negative polarity, two electrolytic capacitors can be connected in series with the same polarity as a non-polar capacitor.
How to use a multimeter to measure capacitance?
Use the pointer multimeter to measure the capacitance. See the attached picture: The pointer type multimeter can be used to detect the capacitance. The basis is that the electrical barrier of the multimeter is equivalent to a DC power supply with internal resistance, and the capacitance can be charged. As time goes by, the voltage across the capacitor gradually increases. The charging current gradually decreases until it reaches zero. Steps
1. Choose the appropriate gear for the electric block. Generally, if the capacity is below 0.01uF, choose x10k gear; about 1-10uF, choose X1k gear; above 47uF, choose x100 gear or x10 gear.
2. For each test, short-circuit the capacitor with a wire, and then perform the next test after discharge.
3. Electrolytic capacitors have polarity, and the positive electrode has a higher potential than the negative electrode during use. Since the black test lead is connected to the positive electrode of the battery in the watch, the black test lead is connected to the positive electrode of the electrolytic capacitor, and the red test lead is connected to the negative electrode of the capacitor. A good capacitance performance is that the pointer deflects - down during detection, and then gradually returns to the mechanical zero (that is, the resistance is infinite) position.
The deflection of the pointer is related to the electric capacity and the electric barrier, and the larger the capacity, the larger the deflection. In practice, pay attention to the rules and accumulate data. The adjustment method of the mechanical zero of the meter head is to use a flat screwdriver to align the mechanical zero adjustment notch on the meter head when the meter pen is neither shorted nor to measure any device, and rotate left and right to make the meter pointer point to zero. The performance of the capacitor that has lost its capacity is that the detection pointer is not deflected and does not need to be discharged. The performance of the capacitor that loses part of the capacity is that, compared with the standard capacitor, the pointer deflection is not in place. It can be judged by experience or by referring to the standard capacitor of the same capacity and according to the maximum amplitude of the pointer swing.
The reference capacitor does not have to have the same withstand voltage value, as long as the capacity is the same. For example, to estimate a 100uF/250V capacitor, a 100uF/25V capacitor can be used as a reference first, as long as the maximum amplitude of the pointer swing is the same, it can be concluded that the capacity is the same . The performance of leakage capacitance is that the pointer cannot return to the mechanical zero position (that is, the resistance is infinite). It should be noted that there is leakage of electrolytic capacitors larger or smaller, the leakage of low withstand voltage is large, and the leakage of high withstand voltage is small; use x10k to measure the leakage, and use the block below xlk to measure the leakage to determine whether the capacitor is leaking.
For capacitors above 1000uF, you can use the Rxl0 block to quickly charge it first, and initially estimate the capacity of the capacitor, and then change to the Rxlk block to continue the measurement for a while. At this time, the pointer should not return, but should stop at or very close to infinity, otherwise There may be leakage. For some capacitors below tens of microfarads, after the Rxlk block is fully charged, use the Rx10k block to continue the measurement, and the needle should stop at infinity and not return. Except for electrolytic capacitors, the withstand voltage of ceramic, polyester, metallized paper and monolithic capacitors is greater than 40V. Test with a multimeter, no matter which block, a good capacitor should not leak. To measure small-capacity capacitors with a multimeter, the amplification effect of low-power silicon NPN triodes can be used, and the method is shown in Figure 1(f). Use the resistor Rxlk to block, the black test lead is connected to the collector, the red test lead is connected to the emitter, touch the small capacitor to the collector, and the pointer should be deflected. The principle is that when the capacitor is charged, the charging current injects the base current into the base, and this current is amplified by the triode, and the pointer deflection is more obvious.
