Attention in use of capacitor
A. what is good capacitance?
1. The larger the capacitance, the better.
Many people tend to use large capacity capacitors when replacing capacitors.We know that the larger the capacitance, the greater the capacity to compensate for the current provided by the IC.Not to mention the increased volume of capacitance, which increases costs and affects air flow and heat dissipation.The key lies in the parasitic inductance on the capacitor, and the capacitor discharge circuit will resonate at a certain frequency point.At the resonance point, the capacitance has a low impedance.Therefore, the discharge circuit has the smallest impedance and the best effect of energy supplement.However, when the frequency exceeds the resonance point, the impedance of the discharge circuit begins to increase, and the capacity of capacitance to provide current begins to decline.The larger the capacitance value of the capacitor, the lower the resonance frequency, and the smaller the frequency range that the capacitor can effectively compensate the current.In terms of the capacity of capacitors to provide high-frequency current, the idea that larger capacitors are better is wrong.
2. For capacitors of the same capacity, the more small capacitors in parallel, the better
Voltage tolerance, temperature tolerance, capacitance and ESR (equivalent resistance) are important parameters of capacitance, and the lower the ESR, the better.ESR is related to capacitance, frequency, voltage, temperature, etc.When the voltage is fixed, the larger the capacity, the lower the ESR.The use of several small capacitors and connections in the board card design is limited by the PCB space, so some people think that the more small resistances in parallel, the lower the ESR, the better the effect.Theoretically, this is the case, but considering the impedance of capacitor solder joints, the effect of using multiple small capacitors in parallel is not necessarily outstanding.
3. The lower the ESR, the better.
Combined with our improved power supply circuit above, for the input capacitor, the capacity of the input capacitor is a bit larger.Relative capacity requirements, ESR requirements can be appropriately reduced.Because the input capacitance is mainly voltage withstand, followed by absorption MOSFET switch pulse.For the output capacitance, the requirement of withstand voltage and capacity can be appropriately reduced a little.The ESR is a little bit more demanding, because there is enough current to be excessive.However, it is important to note that ESR is not as low as possible. Low ESR capacitance causes the switching circuit to oscillate.The complexity of vibration elimination circuit will increase the cost.In the design of board card, there is usually a reference value, which is used as the parameter of element to avoid the increase of cost caused by vibration elimination circuit.
4. Good capacitance means high quality.
"Capacitive theory" was once in full vogue, and some manufacturers and media deliberately made it a selling point.In board card design, the level of circuit design is the key.And some manufacturers can use two-phase power supply than some manufacturers use four-phase power supply more stable products, blindly using high-priced capacitors, may not be able to make a good product.To measure a product, we must consider the full range of multi-angle, must not exaggerate the role of the capacitor intentionally or unintentionally.
B. Capacitor blasting surface interview
There are two types of blasting: input capacitor blasting and output capacitor blasting.
In the case of the input capacitor, which I mean C1, C1 filters the current received by the supply.The input capacitor blast is related to the quality of the input current.Too much burr voltage, too high peak voltage, unstable current, etc., make the capacitor too frequently charged and discharged. The capacitor under such working environment for a long time will increase the internal temperature quickly.Bursting occurs when the limit of the discharge is exceeded.
For the output capacitor, as I mentioned in C2, the current adjusted by the power module is filtered.Here the current after a filtration, relatively smooth, the possibility of the occurrence of blasting is relatively small.However, if the ambient temperature is too high, the capacitor is also prone to bursting.Blast, too.The use of garbage will naturally explode, ah retribution.Who would know the cause of the past, see the effect of the present;Who would know the future, see the present.
Causes of electrolytic capacitor blasting:
There are many reasons for capacitor blasting, such as the current is larger than the allowable stable wave current, the applied voltage exceeds the working voltage, the reverse voltage, frequent charge and discharge, etc.But the most immediate cause is high temperatures.We know that an important parameter of the capacitor is the temperature tolerance, which is the boiling point of the electrolyte inside the capacitor.When the internal temperature of the capacitor reaches the boiling point of the electrolyte, the electrolyte begins to boil, and the pressure inside the capacitor increases.Therefore, temperature is the direct cause of capacitor blasting.The designed service life of the capacitor is about 20,000 hours, which is also greatly affected by the ambient temperature.The service life of capacitor decreases with the increase of temperature. The experiment proves that the service life of capacitor will be halved for every 10℃ increase of ambient temperature.The main reason is that temperature accelerates the chemical reaction and degrades the dielectric over time, thus ending the capacitor life.In order to ensure the stability of the capacitor, the capacitor should be tested in a high temperature environment for a long time before the board is inserted.High-quality capacitors can work for thousands of hours, even at 100 ° c.At the same time, the lifetime of the capacitor we are referring to is that the capacity of the capacitor will not exceed 10% of the variation in the standard range during the use of the capacitor.Capacitive life refers to the problem of capacitive capacity, not the burst that occurs upon design life.It is just not possible to guarantee the capacity standard of capacitor design.
Therefore, in a short period of time, the normal use of the plate capacitor will occur in the case of blasting, this is the capacitor quality problem.In addition, abnormal use may also occur in the case of capacitor blasting.For example, hot-plugging computer accessories will also cause sharp changes in the local circuit current and voltage of the board card, thus causing capacitor failure.