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A brief analysis of the causes of failure, explosion, and burnout of tantalum capacitors

by:Shenmao     2021-03-20
I often encounter many customers discussing the explosion of tantalum capacitors, especially in the switching power supply, LED power supply and other industries. The burning or explosion of tantalum capacitors is the most headache for Ru0026D technicians, making them puzzled. Because of the danger of failure modes of tantalum capacitors, many Ru0026D technicians dare not use tantalum capacitors anymore. In fact, if we can fully understand the characteristics of tantalum capacitors and find the reasons for the failure of tantalum capacitors (in the form of burnout or explosion), tantalum capacitors are not so terrible. After all, the benefits of tantalum capacitors are obvious. In general, the reasons for the failure of tantalum capacitors can be divided into two categories: quality problems of tantalum capacitors and circuit design problems. The quality problems are not considered first. This time we will analyze the circuit design problems first. Circuit design and product selection require that the product performance parameters of tantalum capacitors can meet the characteristics of the circuit signal. However, often we cannot guarantee that the above two tasks are well done. Therefore, in the course of use, there will inevitably appear such and other failure problems; now it is briefly summarized as follows: 1. The failure caused by the high voltage of the low-impedance circuit. There are only two types of circuits used in tantalum capacitors: circuits with resistance protection and Low impedance circuit without resistance protection. For circuits with resistance protection, the resistance will have the effect of stepping down and inhibiting the passage of large currents. Therefore, the operating voltage can reach 60% of the rated voltage of the tantalum capacitor. There are two types of circuits without resistance protection: 1. The input of the front stage has been rectified and filtered, and the output is a stable charging and discharging circuit. In this type of circuit, the capacitor is used as a discharge power source. Because the input parameters are stable and there is no surge, Even though it is a low-impedance circuit, the voltage that can be used safely can still reach 50% of the rated voltage, which can guarantee a high degree of reliability. 2. The power supply part of the complete electronic machine. Capacitors are used in parallel in this type of circuit. In addition to filtering the input signal, it often also has the requirement of discharging according to a certain frequency and power. Because it is a power supply circuit, the loop impedance of this type of circuit is very low. Ensure that the output power density of the power supply is sufficient. In this type of switching power supply circuit [also known as DC-DC circuit], a high-intensity spike pulse with a duration of less than 1 microsecond will be generated in the circuit every time it is turned on and off. The pulse voltage value can reach at least 3 times the stable input value, and the current can reach more than 10 times the steady state value. Due to the extremely short duration, the energy density per unit time is very high. If the voltage of the capacitor is too high, the pulse voltage actually applied to the product will far exceed the rated value of the product and be broken down. Therefore, the allowable operating voltage of the tantalum electrolytic capacitor used in this type of circuit cannot exceed 1/3 of the rated value. Regardless of the circuit impedance type of the circuit, it is derated by 50%. In the DC-DC circuit with the lowest loop impedance, There may be an instant breakdown short circuit or explosion phenomenon as soon as it is turned on. How much capacitors used in such circuits should be derated, must consider the level of circuit impedance, the size of input and output power, and the AC ripple existing in the circuit The value of the value. Because the circuit impedance can determine the magnitude of the transient surge amplitude of the switch. The lower the internal resistance, the more derating should be. The magnitude of the derating must not be generalized. The derating must be determined through accurate reliability calculations. 2. The peak output current of the circuit is too large (appropriate voltage used). The maximum DC current impact that the tantalum capacitor can safely withstand during operation has the following mathematical relationship with the product's own equivalent series resistance ESR and rated voltage UR: Iu003dUR/1 +ESR If a low-capacity tantalum capacitor is used in a circuit with a large peak output current, the product may burn out due to current overload. This is very easy to understand. 3. Failure caused by high ESR of tantalum capacitor and high AC ripple in the circuit. When a tantalum capacitor with too high ESR is used in a filter circuit with excessive AC ripple, even if the voltage is much lower than It should be derated. Sometimes, a sudden breakdown may still occur at the moment of power-on; the main reason for this type of problem is that the ESR of the capacitor and the AC ripple in the circuit are seriously mismatched. The capacitor is polarized. When passing through the AC ripple, the product will generate heat, and products of different shell sizes can maintain thermal balance with different allowable calories. Since the ESR values u200bu200bof products of different capacities are relatively different, therefore, tantalum capacitors of different specifications can be safely withstood The AC ripple value is also very different. Therefore, if the AC ripple in a circuit exceeds the AC ripple value that the capacitor used can safely withstand, the product will experience thermal breakdown. Similarly, if the circuit is The AC ripple of the tantalum capacitor is constant, and the actual ESR value of the selected tantalum capacitor is too high, the same phenomenon will occur in the product.
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