Guidelines for Selecting Patch Capacitors and Their Advantag

發布時間：2019-02-21 15:39
With the development of technology, the volume of MLCC can be made smaller and smaller, reaching 0201, and the volume value is increasing. It has the advantages of saving space, restraining corrugation and prolonging life. However, the small ESR (equivalent series resistance) of MLCC has a negative effect. Because of the abnormal vibration and anti-resonance, special attention should be paid when replacing MLCC.

With the development of technology, the large capacity MLCC with the number of 10-100 mF or more can be commercialized, so that the electrolytic capacitor can be replaced. The lifetime of electrolytic capacitors is 10 years, but almost no factor can shorten the lifetime of MLCC.

The electrolytic capacitor in the output capacitor is constantly replaced by MLCC.

More and more decoupling capacitors are replaced by analog circuits.

Cautions for Use of Various Capacitors

Electrolytic capacitors with large capacity but short life due to their own heating

The ESR of capacitors varies with frequency.

If the ESR of capacitor is expressed as R and the ripple current is expressed as I in a certain frequency, the power of RI2 will lose heat and the capacitor will heat itself.

The electrolytic capacitor has the characteristics of large capacity, but because of its high ESR, it has the disadvantage of excessive heating caused by ripple current.

The acceptable upper limit of ripple current for capacitors is called allowable ripple current. The life of the capacitor will be shortened if the ripple current exceeds the allowable one.

Note: ESR and ripple current

The ideal capacitor is a component with only electrostatic capacity, but in reality, it also has resistance and inductance components due to the electrodes. The components of resistance that can not be identified in the circuit diagram are called ESR (equivalent series resistance), and the components of inductance are called ESL (equivalent series inductance).

Although DC current is a current flowing in one direction with a certain voltage, noise current and other AC components are overlapped in the DC current of power supply circuit. For example, the DC current obtained by rectifying commercial AC (full-wave rectification) contains microwave (ripple) pulsating current twice the cycle of commercial AC. At the same time, in the switching mode DC-DC converter, the DC voltage overlaps the pulsating current with switching period. These are called ripple currents.

The life of aluminium electrolytic capacitor is 10 years

Aluminum electrolytic capacitors are widely used in electronic devices because of their large static capacity and low price, but they have a long service life, so special attention should be paid to them when they are used. Aluminum electrolytic capacitors generally have a life of about 10 years. This is due to the electrolyte drying up (evaporation) resulting in electrostatic capacity reduction (capacity loss).

The amount of electrolyte disappearance is related to temperature, which basically conforms to the theory of chemical reaction rate known as "Arenius law". The law indicates that if the service temperature rises by 10 C, the service life will become 1/2 of the original one. If the service temperature falls by 10 C, the service life will become 2 times of the original one, so the law of 10 2 times. Therefore, the service life will be further shortened when the ripple current is used under the condition that the heat of the ripple current is larger.

At the same time, the drying up of electrolyte will also increase ESR. When the DC voltage overlaps with the ripple voltage, it is necessary to pay attention to the peak value of the ripple voltage not exceeding the rated voltage (withstand voltage). The rated voltage of the capacitor is about 3 times of the input voltage.

Aluminum electrolytic capacitors are widely used in electronic devices because of their large static capacity and low price, but they have a long service life, so special attention should be paid to them when they are used. Aluminum electrolytic capacitors generally have a life of about 10 years. This is due to the electrolyte drying up (evaporation) resulting in electrostatic capacity reduction (capacity loss).

The amount of electrolyte disappearance is related to temperature, which basically conforms to the theory of chemical reaction rate known as "Arenius law". The law indicates that if the service temperature rises by 10 C, the service life will become 1/2 of the original one. If the service temperature falls by 10 C, the service life will become 2 times of the original one, so the law of 10 2 times. Therefore, the service life will be further shortened when the ripple current is used under the condition that the heat of the ripple current is larger.

At the same time, the drying up of electrolyte will also increase ESR. When the DC voltage overlaps with the ripple voltage, it is necessary to pay attention to the peak value of the ripple voltage not exceeding the rated voltage (withstand voltage). The rated voltage of the capacitor is about 3 times of the input voltage.