As an important component of electronic products, electrolytic capacitor
s play an indispensable role in switching power supplies. Its service life and working conditions are closely related to the service 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 design is improper, and the two sides are in dispute. The following analyses the service life and safety of electrolytic capacitors, and provide some basis for the judgment of electronic engineers. 1 Arrhenius (Arrhenius) 1.1 Arrhenius equation The Arrhenius equation is an empirical formula used to describe the relationship between the reaction rate of a chemical substance and the temperature. The inside of the electrolytic capacitor is composed of metal aluminum and other chemical substances such as electrolyte, so the life of the electrolytic capacitor is closely related to the Arrhenius equation. Arrhenius equation formula: ku003dAe-Ea/RT or lnku003dlnA-Ea/RT (graphic 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 of 1.2. Arrhenius conclusion According to the Arrhenius equation, the chemical reaction rate (life consumption) increases as the temperature rises. Generally speaking, for every 10°C increase in the ambient temperature, the chemical reaction rate (K Value) will increase by 2-10 times, that is, when the operating temperature of the capacitor increases by 10°C, the life of the capacitor is doubled. When the operating temperature of the capacitor drops by 10°C, the life of the capacitor is doubled. Therefore, the ambient temperature affects the life of the electrolytic capacitor. Important factor. 2 Analysis of the service life of electrolytic capacitors 1) Formula: According to the conclusion of the Arrhenius equation, the formula for calculating the service life of electrolytic capacitors is as follows: ●The service life of the electrolytic capacitor when the ambient temperature is T (hour) ●L0 The rating of the electrolytic capacitor at the maximum temperature Life (hour) ●T0 rated maximum operating temperature of electrolytic capacitor (deg℃) ●T ambient temperature (deg℃) ●T0-T temperature rise (deg℃) 2)Analysis: According to formula (1), it can be seen that when the working temperature of electrolytic capacitor is at When working at the highest operating temperature (ie T0u003dT), the minimum service life of the electrolytic capacitor calculated by formula (1) is Lu003dL0×20u003dL0, which is equal to the rated life, such as 8000 hours, 8000/8760u003d0.9 years. When the operating temperature of the electrolytic capacitor is 10°C lower than the maximum operating temperature, the service life of the electrolytic capacitor is calculated by formula (1) as Lu003dL0×2[T0-(T0-10°C)]/10°Cu003dL0×21, which is equal to 2 times the rated life, that is, 16000 hours, 16000/8760u003d1.8264 years. It can be seen that the calculation formula for the service life of electrolytic capacitors conforms to the conclusion of the Arrhenius equation 3. In the calculation of the service life of electrolytic capacitors in electronic products, the factors that affect the life of electrolytic capacitors include ambient temperature T and ripple current Irms. The load power borne by the capacitor is proportional to the ripple current. The larger the load, the greater the ripple current (the deeper the electrolytic charge and discharge), the greater the heat generated when the internal oxide film decomposes, and the more electrolyte is consumed during repair. See Figure 1 The larger the ripple current, the greater the heating, so the heating caused by the ripple current should be considered in the calculation of the life of the electrolytic capacitor. 3.1 Ripple current calculation 1) Capacitor capacity 2) Charging time 3) Discharging time 4) Charging current 5) Discharging current 6) Ripple current 3.2 Calculation of power loss 3.3 Electrolytic capacitor heating formula reaches thermal equilibrium when container center temperature T0 and ambient temperature T The temperature rise is determined by the heat dissipation method (air heat dissipation, container heat dissipation) and the dissipated power PD, which is described by thermal resistance, thermal resistance (Thermal Resistance) Rq, unit (℃/W): ● When △T plus ripple current I Self-heating of electrolytic capacitor (deg℃) ●I actual working ripple current (A rms), ●β heat dissipation coefficient (W/℃ Cm2) ●S surface area of u200bu200belectrolytic capacitor (cm2) ●R equivalent impedance of electrolytic capacitor (ESR Ω) 3.4 Calculation of synthetic ripple current. Because in the actual circuit, the ripple current contains ripple currents of various frequency waveforms, the calculation of the actual circuit ripple current should be obtained from the synthetic ripple current Irms: 3.5 Rated operating temperature of electrolytic capacitor According to industry regulations, at the rated temperature T0, plus the allowable rated ripple current I, the maximum heat generated △t≤5 deg℃, so when the actual ripple current is Ir, the heat generated by the capacitor itself is ●△t plus the rated temperature The maximum allowable temperature rise of the capacitor at the rated ripple current (deg℃) ●Ir Rated ripple current of the capacitor (Arms) ●I is the (calculated) actual working ripple current (Arms) 3.6 The calculation of the life of the electrolytic capacitor can be seen from the above analysis, The calculation formula for the life of the electrolytic capacitor after considering the ripple current is finally: ●T0 is the rated temperature (for example, 105℃) ●Δt is the maximum allowable temperature rise at the rated temperature of 5℃ ●T is the ambient operating temperature (for example, 55℃) ●ΔT It is the heating value generated by the ripple current at temperature T (such as 20℃). 4 For example, a capacitor ED33uF/200V/105℃, rated life L0u003d8000 hours, allowable ripple current Iu003d195mA/120Hz, in an environment of 55℃ Application in 110V/60Hz circuit.
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