Reliability selection of military electrolytic capacitors

In military electronic products, electrolytic capacitors occupy a large proportion, and their reliability plays a vital role in the whole machine. According to the statistics of the relevant departments in China, the failures caused by the choice and application of capacitors account for about 55 ~ 85% of the total failure rate of capacitors. The failure caused by the quality of the capacitor itself accounts for about 15 ~ 45%. It can be seen from the above data that the main reason for capacitor failure is related to improper selection and use. Therefore, the selection and application of electrolytic capacitors is of great significance to ensure the quality and reliability of military products.

1 selection of electrolytic capacitor

In military electronic products, in order to ensure the reliability of the whole system, the following principles should be followed when selecting electrolytic capacitors:

(1) Try to select components listed in the QPL for Military Electronic Components;

(2) Choose preferred LIST of Components (PPL) as far as possible;

(3) Correctly select the quality grade of components;

(4) Standard and general components should be selected as far as possible, and new varieties and non-standard components should be carefully selected;

(5) When providing the list of components, the meaning of the component mark must be clear to avoid that the purchased components do not meet the reliability requirements of the whole system.

When selecting capacitors, designers should understand the main technical performance of electrolytic capacitors, and compare its capacitance, working voltage, leakage current, loss Angle tangent value, temperature characteristics, impedance frequency characteristics, reverse voltage resistance, storage performance and price. At the same time, we should pay attention to the material used by the capacitor, the most commonly used material is Z5U, this material has stable performance, high dielectric coefficient, large capacitor capacity, self-resonant frequency can reach 1 ~ 20MHz; Up to 50 MHz. Another commonly used material is NPO, which has good high frequency characteristics, but low dielectric constant, small capacity, generally not used below 10 MHz.

Electrolytic capacitors are divided into aluminum electrolytic capacitors and tantalum electrolytic capacitors. Tantalum electrolytic capacitors are divided into solid tantalum electrolytic capacitors and liquid tantalum capacitors. The three kinds of capacitors have their own characteristics and should be selected differently.

The advantages of aluminum electrolytic capacitors are that aluminum is widely distributed in China, low price, large production, light weight and self-healing characteristics. Its shortcoming is medium loss is big, insulation performance is poor (leakage current is big), capacity deviation is big, and have shelving effect. In addition, the non-gas sealing element with aluminum shell is generally used, so the reliability is poor. Adapt to the environment temperature range is narrow, especially the negative temperature characteristics, generally -20℃(individual products can reach -40℃, military STANDARD CDK series is -55℃). It is suitable for general civilian electronic products with low requirements on environmental conditions and part of ground military electronic whole machines with low requirements on reliability.

Compared with aluminum electrolytic capacitors, tantalum electrolytic capacitors have the advantage of wide ambient temperature range, generally -55℃ ~ +125℃; The insulation resistance of tantalum oxide film is twice that of aluminum oxide film, so the leakage current is small. Within the operating voltage range of semiconductor devices (below 150 V), tantalum capacitors with the same capacity are twice smaller than aluminum electrolytic capacitors, and have advantages of small loss, stable performance, long life and high reliability. Tantalum electrolytic capacitors are suitable for military electronic equipment, but their disadvantages are high price and limited by cost, so the consumption is very little. Because CA42 tantalum capacitors are coated with epoxy resin, the price of solid tantalum capacitors is greatly reduced, so some civil complete machines have also been used in batches. As CA42 epoxy solid tantalum capacitor is a non-sealed element, it is not suitable for use in military equipment with bad environmental conditions, so the fully sealed tantalum capacitor should be used in military products.

Electrolytic capacitors have large dielectric loss, large capacity error and poor insulation performance, which are only suitable for filtering in power supply of all kinds of 50-100 Hz electronic equipment, bypass and coupling capacitance in low-frequency circuits, or low-frequency circuits with low capacity error requirements.

Liquid tantalum capacitors have low leakage current, and the product of capacitance per unit volume and operating voltage is very large, which is especially suitable for large capacity circuits under medium and high voltage conditions. However, this kind of capacitor has acidic liquid, if leakage occurs, it may short-circuit the PCB connection, thus causing serious failure of the whole machine. Therefore, it is forbidden to use liquid tantalum capacitors in military electronic equipment. However, with the development of science and technology, some units have basically solved the sealing problem of liquid tantalum capacitors, liquid tantalum capacitors have reached the requirements of national military standards, and their reliability has reached the level of six. At present, it has been used in aerospace products, and other military products can be referenced. However, liquid tantalum capacitors must be sealed and screened for leakage before operation, and large derating is adopted in design. To ensure its reliability.

Reliability application of electrolytic capacitors

Reducing the working voltage of electrolytic capacitor is the most effective method to prolong the service life and improve the reliability of capacitor. Because the failure rate of an electrolytic capacitor is proportional to the quadratic ratio of the applied voltage to the rated voltage of the capacitor. Electrolytic capacitors are used in power supply filter circuit in most cases. When the input voltage changes or the load suddenly opens, the voltage at both ends of the filter capacitor will change accordingly. If the derating design is not carried out, it is likely to cause breakdown of the capacitor. In addition, the AC voltage from the input is not sinusoidal voltage, generally the peak voltage of the non-sinusoidal wave is higher than the sinusoidal voltage, which can have a great impact on the life and reliability of the capacitor. Therefore, during the design, the working voltage of the electrolytic capacitor should be significantly derated. The voltage derating range of electrolytic capacitor depends on the reliability requirement of the whole machine and the specific circuit of capacitor. Generally can be divided into three levels, the first level is 50% of the rated voltage; The secondary is 60%; Level 3 is 70%. High voltage and large capacity capacitors should choose a larger derating range. The larger the capacitor capacity, the larger the oxide film area, the greater the probability of medium defects, the lower the reliability.

2.2 Fully consider ripple voltage

Generally electrolytic capacitors are divided into positive and negative poles. Special attention should be paid to its operating characteristics when used in pulsating circuits with both DC and AC voltage components. The electrolytic capacitor must meet the voltage polarity requirements specified at both ends of the electrolytic capacitor. When used in the interstage coupling or pulse circuit, places of dc voltage on the capacitor and superimposed ac voltage component amplitude, in some cases may make negative peak voltage of the ac component voltage value more than just flow, so that the polar capacitor in reverse working state, this will make the capacitor leakage current surge, damaging the positive working characteristic and cause failure. Therefore, when there is a pulsating AC component at both ends of the capacitor, the sum of the ac peak voltage and the applied DC voltage should not exceed the rated operating voltage of the capacitor. The reason is that the temperature rise of the capacitor caused by the AC component is much more serious than the DC component, so it is necessary to strictly control the size of the ripple, generally should not exceed 20% of the rated working voltage of the capacitor. For tantalum electrolytic capacitors, it should also be controlled within 10%. Since the ripple voltage can polarize the electrolyte and greatly affect the loss resistance RS, it is necessary to control the ripple peak voltage applied to the capacitor effectively. The allowable AC component according to the general technical conditions refers to the allowable value under the condition of power frequency 50 Hz. If the frequency exceeds the above conditions, it can be calculated according to the following formula:

Where, Ssh is the surface area of capacitor shell (unit cm2), t is the allowable temperature rise at a certain ambient temperature (℃), f is the sine frequency of pseudo-ripple voltage (Hz), C is the capacitance (μ f), tgδ is the tangent value of loss Angle at the actual frequency.

To ensure reliable operation of the capacitor, the ripple voltage applied to the capacitor should be less than the ripple peak voltage calculated by the above equation. As a general polar capacitor can not withstand the reverse voltage, so. Polar capacitors are not allowed in polarity conversion or pure AC circuits. Instead, non-polar tantalum capacitors (such as CA74 fully sealed solid tantalum capacitors) should be used. In fact, two polar tantalum capacitors are connected back to back, even though one of the capacitors is always in the positive polarity state in the AC circuit.

2.3 Working frequency of electrolytic capacitor

Electrolytic capacitors are most suitable for filtering power supply at power frequency or for bypass or inter-stage coupling in low frequency circuits. And the lower the impedance, the more reliable the circuit. An electrolytic capacitor operates as an electrolytic cell, with one electrode as the electrolyte. Because the resistance of electrolyte is much higher than that of normal metal electrode, the series equivalent resistance of electrolytic capacitor is large. Under dc or low frequency conditions, the equivalent series resistance RS and equivalent inductance L are negligible compared with the dielectric insulation resistance scale RP of the actual capacitor. With the increase of frequency, the equivalent series resistance RS and equivalent inductance L both increase. The increase of equivalent series resistance RS is due to "skin effect". The equivalent inductance L is caused by a magnetic field proportional to the frequency. In general, the value of capacitive reactance XC decreases and the value of inductive reactance XL increases as the frequency increases, indicating that (XC-XL)2 will decrease with increasing frequency, and the impedance Z=RS when the frequency increases to a certain frequency point (XC-XL)2=0. The capacitor will then resonate. This is the resonant point at which the capacitor presents pure resistance to the circuit. When the frequency is higher than the resonant point of the capacitor, the capacitor has actually become an inductor and can not play the role of the capacitor, so it is required that the use frequency of the general electrolytic capacitor should not exceed 20 kHz. As the dielectric constant of most materials is affected by frequency, its capacity will decrease greatly with the increase of frequency. So when using more than 20 kHz frequency, the allowed by the ac component is very small, the capacity of capacitor loss is very serious, the general can choose a high frequency porcelain interface or mica capacitor in parallel with as high frequency channel, electrolytic capacitor, because of the large capacity can be used as a low frequency channel, its capacity should be greater than the high frequency capacitor capacity of more than 100 times. Since most of the characteristics of capacitors are affected to some extent by frequency, it is required that the frequency should be more than 1/2 of the resonant frequency. Too high a frequency not only consumes a lot of power in the circuit. Moreover, the inner core of the capacitor can be heated, thus affecting the reliability of the capacitor. Generally electrolytic capacitors should work below 10 kHz. If the frequency is used above 10 kHz, the effective capacity will decrease rapidly until the capacitance impedance becomes pure resistance. In general, the frequency characteristics of solid tantalum capacitors are better than liquid tantalum capacitors when their capacity varies with frequency in the range of 100 Hz ~ 100kHz. However, liquid tantalum capacitors in the range of 1 kHz ~ 3 kHz, their capacity decreases with frequency by 6%. When the frequency increases by more than 10 kHz, the decrease will be as high as 65%. This is mainly determined by the material properties of the medium. If the electrolytic capacitors cannot meet the requirements of use, a four-wire electrolytic capacitor or capacitor with other media can be used.

2.4 Properly reduce the ambient temperature

At the same voltage, the leakage current and loss of electrolytic capacitor increase with the increase of temperature. When the temperature rises from room temperature 25℃ to 85℃, the leakage current will often increase by 3 times. The leakage current and loss of the capacitor are the main reasons for heating the capacitor, so reducing the ambient temperature of the capacitor is a favorable measure to prolong the service life of the capacitor and improve the reliability. Generally, the design can be carried out according to the requirements of temperature derating. The rated temperature of aluminum electrolytic capacitor should be reduced by 20 ~ 40℃; The rated temperature of solid tantalum capacitor can be reduced by 15 ~ 25℃. The rated temperature of liquid tantalum capacitors should be reduced by 15 ~ 30℃. Aluminum electrolytic capacitors are generally rated at -20℃ because the loss increases sharply at negative temperature.

In use, with the increase of ambient temperature, capacitor leakage also increases, when there is a surge current through, leakage current may occur "avalanche" phenomenon and damage the capacitor. To prevent this phenomenon, the impedance of the circuit should be increased so that the impedance of the circuit is not less than 3 ω /V. Otherwise the reliability will be reduced accordingly.

2.6 Reliability of capacitor installation and welding

If the capacitor is a recent factory product, and the weldability has reached the requirements, generally do not need to dip tin pretreatment. And if the storage time is long before use, tin immersion treatment, tin immersion treatment should be controlled in the technical specifications of the sealing 3.2mm, and avoid too long or too high temperature, resulting in sealing melting or lead and electrode welding. For chip capacitors, also should avoid the use of high activity, strong acid flux, so as not to clean unclean, infiltration, corrosion and diffusion and affect the reliability of the product. At the same time, the welding temperature and time of chip capacitor should be controlled (generally 260℃/10 seconds). The capacitor should be installed away from the heating element. For larger capacitors, capacitor pins can not be used to install, in order to prevent the lower lead breakage or seal damage in vibration or impact, if necessary, should also design a clamping device to fix. When installing, the marked surface of the capacitor should also be exposed as far as possible for observation.

3 conclusion

Electrolytic capacitor is one of the most basic components of military whole system and irreplaceable electronic components. The selection of electrolytic capacitors must be carried out in accordance with the principle of electronic components, from the environmental conditions used by the whole machine, electrical performance requirements, volume weight, reliability requirements and cost comprehensive tradeoff. The quality and reliability of military electronic products can be guaranteed only by selecting and applying electrolytic capacitors correctly and reasonably.