Views: 6 Author: Site Editor Publish Time: 2019-06-04 Origin: Site
Electrolytic capacitors, which are important components of electronic products, play an indispensable role in switching power supplies. Their service life and operating conditions are closely related to the life of switching power supplies.
In a large number of production practices and theoretical discussions, when the capacitor in the switching power supply is damaged, especially when the electrolytic capacitor is topped and the electrolyte overflows, the power supply manufacturer suspects that there is a problem with the quality of the capacitor, and the capacitor manufacturer says that the power supply is not properly designed, and the two parties are arguing.
The following analysis of the service life and safety of electrolytic capacitors provides some judgments for electronic engineers.
Arrhenius:
1.1 Arrhenius equation
The Arrhenius equation is an empirical formula used to describe the relationship between the reaction rate of a chemical and temperature. The inside of the electrolytic capacitor is composed of a metal such as aluminum and an electrolyte, and the life of the electrolytic capacitor is closely related to the Arrhenius equation.
Arrhenius equation: k=Ae-Ea/RT or lnk=lnA—Ea/RT (graphing method)
●K chemical reaction rate
●R is the molar gas constant
●T is the thermodynamic temperature
●Ea is the apparent activation energy
●A is the frequency factor
1.2 Arrhenius conclusion:
According to the Arrhenius equation, the temperature increases and the chemical reaction rate (life consumption) increases. Generally, for every 10 °C increase in ambient temperature, the chemical reaction rate (K value) will increase by 2-10 times. For every 10 °C increase in the operating temperature of the capacitor, the life of the capacitor is doubled. For every 10 °C drop in the operating temperature of the capacitor, the lifetime is doubled. Therefore, the ambient temperature is an important factor affecting the life of the electrolytic capacitor.
Electrolytic capacitor life analysis:
2.1 Formula:
According to the conclusion of the Arrhenius equation, the formula for calculating the service life of electrolytic capacitors is as follows:
●L When the ambient temperature is T, the life of the electrolytic capacitor (hour)
●L0 rated life of electrolytic capacitor at maximum temperature (hour)
●T0 electrolytic capacitor rated maximum operating temperature (deg ° C)
●T ambient temperature (deg °C)
●T0-T temperature rise (deg°C)
2.2 Analysis:
According to formula (1)
When the working temperature of the electrolytic capacitor is operated at the highest operating temperature (ie T0=T), the minimum service life of the electrolytic capacitor calculated by the formula (1) is L=L0×20=L0 is equal to the rated life, such as 8000 hours, 8000/ 8760 = 0.9 years.
When the working temperature of the electrolytic capacitor is lower than the maximum operating temperature by 10 °C, the service life of the electrolytic capacitor calculated by the formula (1) is L=L0×2 [T0-(T0-10°C)]/10°C=L0×21 is equal to 2 times the rated life, that is, 16,000 hours, 16000/8760 = 1.8264 years. It can be seen that the calculation formula of the life of the electrolytic capacitor conforms to the conclusion of the Arrhenius equation.
Electrolytic capacitor life calculation:
In electronic products, factors affecting the life of electrolytic capacitors are ambient temperature T and ripple current Irms.
The load power of the capacitor is proportional to the ripple current. The larger the load, the larger the ripple current (the deeper the electrolytic charge and discharge), the more heat is generated when the internal oxide film is decomposed, and the electrolyte consumption during repair is more. The greater the ripple current, the greater the heat generated, so the heat generated by the ripple current should be considered in the calculation of the life of the electrolytic capacitor.
3.1 Ripple current calculation
1) Capacitance capacity
2) Charging time
3) Discharge time
4) Charging current
5) Discharge current
6) Ripple current
3.2 Power loss calculation
3.3 Electrolytic Capacitor Heating Formula
The temperature rise of the container center temperature T0 and the ambient temperature T when the heat balance is reached is determined by the heat dissipation method (air heat dissipation, container heat dissipation) and the dissipated power PD, and is described by thermal resistance, Thermal Resistance Rq, unit (°C/W) ):
● △T plus ripple current I when the electrolytic capacitor itself generates heat (deg ° C)
● I actual working ripple current (A rms),
●β heat dissipation coefficient (W/°C Cm2)
●S electrolytic capacitor surface area (cm2)
●R electrolytic capacitor equivalent impedance (ESR Ω)
3.4 Synthetic ripple current calculation
Since the ripple current contains the ripple current of various frequency waveforms in the actual circuit, the calculation of the actual circuit ripple current should be obtained by the synthesized ripple current Irms:
3.5 rated working temperature
The industry standard for electrolytic capacitors, at the rated temperature T0, plus the maximum heat generated by the rated ripple current I Δt ≤ 5 deg ° C
Therefore, when the actual ripple current is Ir, the heat of the capacitor itself is
●△t is the maximum temperature rise (deg°C) allowed by the capacitor when the rated ripple current is applied at the rated temperature.
●Ir capacitor rated ripple current (Arms)
●I is the (calculated) actual working ripple current (Arms)
3.6 Electrolytic Capacitor Life Calculation
From the above analysis, the calculation formula of the life of the electrolytic capacitor after considering the ripple current is finally:
●T0 is the rated temperature (such as 105 °C)
● Δt is the maximum allowable temperature rise of 5 °C at rated temperature
●T is the ambient operating temperature (such as 55 ° C)
● ΔT is the heating value generated by the ripple current at T temperature (for example, 20 ° C)
Example:
A capacitor ED33uF/200V/105°C, rated life L0=8000 hours, allowing ripple current I=195mA/120Hz, applied in 110V/60Hz circuit with 55°C environment.
4.1 Triangle wave
4.2 sine wave
4.3 Synthesis
4.4 fever
4.5 Life expectancy
1) Does not consider the service life of ripple current
2) Consider the actual service life of the ripple current
in conclusion:
From the calculation of the above example, the influence of ripple current on the life of electrolytic capacitor is very large. When designing electrolytic capacitors, circuit engineers should consider not only the ambient temperature of capacitor operation, but also the circuit ripple current versus electrolytic capacitor life. The effect of the current, as long as possible to extend the life of the electrolytic capacitor.
The circuit's capacitive or strong sensibility affects the safe switching of the triode, etc., which increases the transistor loss, increases the heat, and superimposes a high single-point ripple current on the electrolytic capacitor. The charging and discharging ripple current becomes narrower and higher. Finally, the electrolytic capacitor is severely heated until it is damaged, which is manifested as topping, steaming, liquid leakage or explosion.
Try to choose a good quality electrolytic capacitor, a good sealing performance capacitor, and cut off the electrolytic capacitor with half the service life. A safe working environment for electrolytic capacitors and reasonable design is the solution to the problem of electrolytic capacitors falling, steaming and leaking to prolong life.
