“Vaporized electrolyte leaks from the sealing part of the electrolytic capacitor. This phenomenon will accelerate with the increase of temperature. It is generally believed that the leakage rate will increase to twice when the temperature rises by 10°C. Therefore, it can be said that the electrolytic capacitor determines the life of the power supply device.
1. The components that determine the life of the switching power supply
Vaporized electrolyte leaks from the sealing part of the electrolytic capacitor. This phenomenon will accelerate with the increase of temperature. It is generally believed that the leakage rate will increase to twice when the temperature rises by 10°C. Therefore, it can be said that the electrolytic capacitor determines the life of the power supply device.
The depletion of the lubricating oil of the spherical bearing and the bearing, and the wear of the mechanical device parts will accelerate the aging of the fan. In addition, in recent years, the drive circuit of DC fans has begun to use components such as electrolytic capacitors, so it is necessary to take into account factors such as the life of the circuit components.
The current transfer ratio (CTR; Current Transfer Ratio) will gradually decrease over time. As a result, the current of the light-emitting diode will continue to increase, sometimes reaching the maximum limit current, causing the system to lose control.
Most switching power supplies are equipped with capacitor input rectifier circuits. When the power is turned on, surge current will be generated, which will cause the fatigue of the switch contacts, and cause problems such as increased contact resistance and adsorption. Theoretically, during the expected life of the power supply, the number of on-off times of the switch is about 5,000 times.
⑤ Inrush current protection resistor, thermistor power resistor
In order to resist the inrush current generated when the power is turned on, the designer uses the resistor in parallel with SCR and other components. The peak value of the power when the power is turned on is as high as tens to hundreds of times the rated value, which results in thermal fatigue of the resistor and causes an open circuit. Under the same conditions, thermistor power resistors will also experience thermal fatigue.
2. Evaluation and calculation of the life of each component
1. Life performance
The end of the life of an electrolytic capacitor is in the form of wear failure, and the main factors that determine the life are the electrostatic capacity, the tangent of the loss angle (tanδ), and the leakage current. As time goes by, the electrostatic capacity decreases and tanδ increases. The leakage current tends to increase when the voltage is applied, so it has little effect on the life of the load.
2. Judgment of life
The percentage is used to express the rate of change of the electrostatic capacity relative to the initial value. Generally, when it reaches below -20%, the life is over. When the value of tanδ exceeds the specified value, the life ends. Leakage current tends to increase under the condition of zero load, and for the same reason, life ends when it exceeds the specified value.
3. The main reason affecting life span
The main reason for the deterioration of the characteristics mentioned above is the electrolyte. As the temperature rises, the electrolyte vaporizes and leaks through the sealing part of the capacitor, and the internal electrolyte continues to decrease. As the amount of electrolyte decreases, tanδ will gradually increase. As a result, the heat generated when the pulse current passes through increases, which further accelerates the degradation process. This relationship is shown in Figure 4 and Figure 5.
4. Calculation of life span
The approximate life of an aluminum electrolytic capacitor can be deduced from the self-heating temperature caused by the ambient temperature and the pulse current. The following formula shows the relationship between life and ambient temperature.
When measuring the heating value of pulse heating, avoid other heat radiation. In addition, small electrolytic capacitors are very easy to heat up when heated, so it is best to measure the surface temperature.
GaAs-based infrared light-emitting diodes mostly use photocouplers. The degradation of the luminous efficiency of this light-emitting diode will cause the CTR (current transfer rate) to decrease, and other forms of CTR degradation include the peeling of the light-bonded resin on the chip surface. The higher the temperature, the faster the decrease in CTR. At the same time, the larger the diode current, the faster the CTR drops. Figure 6 and Figure 7 indicate the relationship between these factors.
The time it takes for CTR to drop to 50% of the initial value is called the half-life. In the statistics of the power circuit, this is the limit value, so it can be considered that the half-life is the life time. Under normal conditions, the half-life is 50,000 to 100,000 hours, but all photocouplers have the life value shown in Figure 8, so it is best to confirm it before performing life evaluation.
The life of the fan is affected by the degree of wear of the bearings and spherical bearings. The bearing part generates heat due to rotation. Although the fan itself can cool down to a certain degree, it cannot fundamentally solve the heat generation problem. Measure the heating value of the bearing part, the smaller the heating value, the better the quality, so choose the right manufacturer.
The lubricating oil in the bearing part and the wear of the bearing cause the number of revolutions to decrease, increase the noise, and accelerate the end of life. Regarding the reduction of the number of revolutions, the standards of each manufacturer are not the same, but generally the upper limit is 3 to 5% of the initial value. The life span decreases as the temperature rises. An ordinary DC brushless motor has a lifespan of about 40,000 hours at a temperature of 40°C, and an inexpensive metal bearing fan has a life of about 10,000 hours. Figure 9 shows the characteristic value of the fan’s life. In addition, the life of the DC fan is also affected by the internal organs-the motor drive circuit. Aluminum electrolytic capacitors are often used in fans, so it is necessary to disassemble the capacitors for inspection (aluminum electrolysis is 105°C).
④ Anti-surge resistance, thermistor
The resistance is high in stability, the failure rate is below 1FIT, and the life is extremely long, so you don’t need to pay special attention to it in normal use.
Components with surge power like resistors used in the inrush current protection circuit will experience thermal fatigue due to the on/off cycle, resulting in an open circuit. Surge power, duration and cycle times are in the following relationship.
Figure 13 shows the surge withstand characteristics of the attenuated waveform with load. Substituting the largest peak voltage (Vp) of the first waveform into equation (7), Vrms can be obtained, and then substituting into equation (8), the rated power multiple can be obtained. Both of these values and the decay time constant r are applicable to Figure 13. The inner side of the curve is the safety zone. When using ordinary nickel-chromium wire (surge resistant), it can withstand about 30,000 surges.
The time constant is the time value when the effective value of the attenuation waveform drops to 0.368 times of the first waveform, so its value is generally obtained from the picture of the waveform on the resistance value.
① Life performance
As a component of the inrush current protection circuit, it is used in a power supply with a smaller capacity (not exceeding 70W). When the power supply is connected, the current reaches the maximum value, and the resistance value of the thermistor decreases as the temperature rises. Usually the temperature will rise to 70～90℃. Although the thermistor is made of heat-resistant materials, thermal fatigue will still affect its life. Manufacturer’s life specification: When the maximum allowable current is passed, the life of the intermittent load is 10,000 cycles. However, when the thermistor is used to protect against inrush current, after the power supply is turned on, the current passing through the resistor will reach 10-20 times the maximum allowable current, so the service life of the power cycle will also be shortened.
② Life judgment
The resistance value changes with the passage of time, and when the rate of change exceeds the specified value, the life span is terminated. When the thermistor is used to protect the inrush current, the resistance value will gradually become larger. Table 3 lists the thermistor life performance specifications.
⑤ Relay, switch
There are two lifespans of relays and switches: one is mechanical life, and the other is electrical life. The former is determined by the degree of wear and tear of mechanical components, including phenomena such as decreased flexibility of switches, prolonged operating time and reset time of relays. The latter is mainly affected by the increase in insulation resistance and contact resistance of the contacts. Among the above-mentioned deterioration forms, the most important thing to pay attention to is the contact arc phenomenon caused by the surge voltage of the inductive load, and the contact deterioration problem caused by the inrush current. Generally speaking, the greater the switching voltage and current, the shorter the contact life. The lower the power factor, the shorter the life span. Figure 14 shows the life performance of the relay.