Selection and identification of large aluminum electrolytic capacitors in industrial power supplies

1. Introduction

With the development of science and technology, the improvement of social needs and the improvement of the environment, China's industrial power supply is in a period of rapid growth. The application field of large aluminum electrolytic capacitors is constantly expanding and the demand is increasing. In industrial power supply, whether it is used for input circuit or output circuit, the main purpose is to provide a high energy storage and low resistance design, but how to correctly choose a capacitor is not a trivial matter. Choosing a good capacitor requires knowledge of various aspects such as: mechanics, heat transfer and electricity.

2. Structural characteristics of large aluminum electrolytic capacitors

The core package of a large aluminum electrolytic capacitor is made of an anode aluminum foil that has been corroded and formed into an oxide film, a cathode aluminum foil that has been corroded, and an electrolytic paper in the middle. It is wound by a large winding device according to a certain process. The core package is placed in a pressure vessel, impregnated with working electrolyte, and then sealed in an aluminum shell. The figure below is a typical structure diagram of an Eastern and Western capacitor.

As can be seen from the figure, the difference between the two is that Asian manufacturers use a fixing agent to fix the core package, while European and American manufacturers use packaging materials with fixing columns, using cathode aluminum foil to extend the core package and directly contact the aluminum shell. From the perspective of thermodynamic heat transfer, this structure has better heat dissipation performance. According to customer requirements, our company has mature supporting experience for these two structures.

3. Selection of Large Aluminum Electrolytic Capacitors

[page]When industrial power supply manufacturers select capacitors, they can first look up the specifications they need from the capacitor manufacturer's samples according to their own design needs. We must be careful not to make the capacitor work at full load to avoid early failure of the capacitor when abnormalities occur in the circuit. We recommend that the working voltage of the capacitor be 60%~80% of the rated voltage. Secondly, they can also provide their own design requirements (including electrical requirements, structural dimensions, installation methods and use environment) to the capacitor manufacturer and ask them to help with the selection, so that they can better choose products that are suitable for them and have a better cost performance.

The following table lists the main technical parameters of Jianghai CD138 series and a European manufacturer:

Where tan δ =RESR/ (1/wC) = wC RESR (w refers to 2πf, c refers to capacitance, and R refers to equivalent series resistance)
RESR=ESR (120 Hz)

Although the raw material of aluminum electrolytic capacitors, anode aluminum foil, has a certain discrete range and electrical properties, Asian capacitor manufacturers manage the marking of anode aluminum foil and cut each roll of aluminum foil and wind it into core packages, so the consistency of electrical properties is very good, such as electrostatic capacitance, loss tangent and leakage current. Especially after using it for a period of time, it is more advantageous to use it as a capacitor bank.

The figure below is the electrical performance change curve of Jianghai CD138 series and a European manufacturer's 400V6800μF product life test.

[page]IV. Certification of Large Aluminum Electrolytic Capacitors

After determining the capacitor model and specifications based on the above technical requirements, how to evaluate it?

The service life formula determines the expected service life according to the specified operating voltage and temperature of the capacitor. The operating temperature is the specified average ambient temperature plus the temperature rise caused by ripple current and leakage current. Compared with the ripple current power, the leakage current power is very small and can be ignored. Example 1 uses a sample capacitor with a thermocouple installed at the central hot spot to measure the temperature rise caused by ripple current (a common method in the East), and Example 2 uses power loss to calculate the temperature rise (European and American calculation method). If the power is calculated, the power at each key frequency must be calculated and these powers added up to form the total power.

Example 1: Actual machine test or simulation test, measure the center temperature and calculate the life span.

Given: Jianghai CD138 450V 3300μF (85℃ product)
Ripple current I _R (120Hz, 65℃) = 19A
Maximum operating voltage V' = 400V
Maximum allowable temperature at the center of the capacitor T ₀ = 93.5℃
Life span L ₀ = 8000 hours

Test method: embed a thermocouple in the center of the capacitor and seal the cover of the capacitor, as shown below:

Set the temperature of the high temperature box to 85℃, connect the positive lead to the positive pole of the ripple power supply, connect the negative lead to the negative pole of the power supply, connect the thermocouple to the temperature recorder, and put it into the high temperature box. Apply the rated voltage of 450V (superimposed ripple current, i.e., 120Hz sinusoidal AC current 19A/piece) until the temperature is balanced, and record the measured value T=75℃.

Calculation: Product life expectancy L = L ₀ × 2 ^{( T0-T ) /10} × (WV/V') ^2.5 = 8000 × 2 ^{( 93.5-75 ) /10} × (450/400) ^2.5 = 38000 hours

Example 2: Based on the capacitor manufacturer's information and actual usage conditions, calculate the center temperature and estimate the lifespan.

Given: A European manufacturer
's 450V5600μF (85℃ product) capacitor has an ambient temperature of Ta=65℃,
ripple current I _R1 (360Hz)=15A, ripple current I _R2 (3kHz)=26A
, ESR1 (360Hz, 85℃)=4.6mΩ, ESR2 (3kHz, 85℃)=3.2mΩ,
thermal resistance R=2.8℃/W,
service life L ₀ =20000 hours
Calculation: P=I _R1 ² ESR1+ I _R2 ² ESR2=15 ² ×4.6×10 ^-3 +26 ² ×3.2×10 ^-3 =3.2W
, capacitor center temperature T=Ta+P×R=65+3.2×2.8=74℃,
this temperature fully meets the 85℃ use condition.
The expected life of the capacitor is L=20000×2 =42000 hours

It should also be pointed out that some European and American capacitor manufacturers define the life of capacitors in terms of expected life, which is completely different from the high temperature load life (laboratory life) defined in the East. If a comparison is required, it is best to do so under the same conditions.

1. Series and parallel use

It is best to use capacitors from the same production batch to ensure similar leakage currents, or use capacitors with higher rated voltages to withstand voltage imbalances between products produced by different manufacturers. At the same time, ensure that the capacitors in series are well ventilated and have the same thermal environment.

For equalizing resistors, in principle, the current flowing through the resistor should be more than 5 times greater than the leakage current of the capacitor to avoid unbalanced voltage division on the capacitor or even overvoltage failure due to improper resistor selection.

2. Capacitor installation

The screws of the capacitor terminals have a certain torque. If the torque is too large, the contact will be poor, causing the contact resistance in the circuit to increase and the terminals to heat up abnormally, which will affect the service life of the capacitor. In severe cases, the screws will break and the terminal threads will be damaged.

When connecting each capacitor, it is best to choose a copper plate with a certain thickness. On the one hand, it can avoid the heat caused by large current passing through, and on the other hand, it can expand the heat dissipation area of ​​the capacitor and minimize the distributed inductance.

VI. Conclusion

Aluminum electrolytic capacitors are important components of industrial power supplies. Choosing professional and excellent capacitor manufacturers and choosing capacitors suitable for their own industrial power supplies are topics that industrial power supply manufacturers pay more attention to. We hope that our experience can be of some inspiration and reference. If there are any errors, please criticize and correct them.