1) Service Life Calculation

For calculating the service life of electrolytic capacitors, while the original text provided a formula expansion, the definitions of some symbolic parameters were missing, making it unusable. The most commonly used formula is shown below:

L = L0 * 2^((T0 - T)/10)

Where:

• L0is the rated service life at the rated temperature (this is directly given in the specification sheet, e.g., guaranteed 2000 hours of normal operation at 105°C and rated voltage).

  
  

• T0is the maximum operating temperature.

  
  

• Tis the actual operating temperature.

  
  

From the above formula, it can be deduced that for every 10°C decrease in temperature, the service life doubles

. To accurately estimate the actual service life, the parameter for actual operating temperature Tmust consider factors such as in-vehicle day-night temperature variations, the capacitor's temperature rise due to ripple current, and the temperature rise of the product containing the capacitor during operation. These factors overall exhibit periodic fluctuations.

The original text also mentioned three factors affecting capacitor life: ambient temperature, self-heating (due to ripple current), and applied voltage

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Ripple current generates heat when acting on the ESR, accelerating the evaporation of the electrolyte inside the electrolytic capacitor and shortening its service life. Applying a voltage exceeding the rated voltage can cause a decline in the capacitor's electrical parameters or even damage.

Adhering to the principle of focusing on the major aspects during selection, the primary consideration here is the ambient temperature factor. I will utilize a dynamic temperature model to facilitate estimating the overall ambient temperature value. This model currently does not account for superimposed factors such as the capacitor's temperature rise due to ripple current and the temperature rise generated during the operation of the product containing the capacitor. Therefore, the final life estimate will be biased, generally on the high side.

Using a derived dynamic model for cabin temperature change over 24 hours, the formula it derived was complex. Thus, I asked it to directly list the calculated temperature values for 0~24 hours based on the following assumptions, as shown in the table.

Assumption Conditions:

• Ambient temperature: Daily average temperature 30°C, amplitude 5°C, peak occurs at 15:00

  
  

• Solar radiation: Sunshine from 06:00-18:00, peak 1000 W/m²

  
  

• Vehicle parameters: Dark color, medium insulation performance

  
  

• Initial condition: Cabin temperature at 00:00 = Ambient temperature

  
  

Taking the root mean square (RMS) of the ambient temperature values and cabin temperature values from the table yields 29.7°C and 45.1°C, respectively. Substituting the actual operating temperature Tinto the aforementioned life calculation formula allows for estimating the actual service life Lof the electrolytic capacitor.

The above calculation process provides a way of thinking. In most design scenarios within projects, the calculation process might be simpler and more straightforward, often directly taking 85°C as the actual operating temperature to calculate the service life of the electrolytic capacitor. Certainly, the estimation result will have a larger deviation from the actual value, but the parameter redundancy (derating principle) is undoubtedly satisfied.

Regarding the impact of temperature changes inside a vehicle on the service life estimation of circuit components within in-vehicle electronic modules, it is a topic substantial enough for a separate study. My analysis and estimation above are not accurate either. The fundamental intention behind presenting them is to make everyone aware of the design premise: 'The temperature inside the vehicle is not constant but varies periodically; simultaneously, components operate cyclically. All parameters and influencing factors are dynamic, not constant.'

Finally, it's worth noting that in actual product development, electrolytic capacitor selection is typically based on the product's required service life. The goal is to calculate the maximum allowable operating temperature for the electrolytic capacitor to meet that service life. For example, if the product's service life is predetermined (e.g., 3 years), the maximum allowable temperature rise for the capacitor can be calculated using the transformed formula T = T0 - 10 * log2(L / L0), and thus the maximum actual operating temperature Tfor the capacitor can be determined

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2) ESR Calculation

Calculating ESR necessarily involves mentioning the loss tangent tanδ (also called the dissipation factor). The tanδ value given in specifications is usually under specific conditions (e.g., 20°C, 100kHz). tanδ increases with higher measurement frequencies and decreases with higher measurement temperatures. A larger loss tangent indicates greater loss in the capacitor. Capacitors with a large loss tangent are not suitable for high-frequency operation.

Polar coordinate representation of the loss angle

3) Ripple Current

Ripple current generates heat on the capacitor's Equivalent Series Resistance (ESR), causing an internal temperature rise within the capacitor

. Excessive temperature rise accelerates the drying out of the electrolyte, shortening the capacitor's life, and may even lead to capacitor failure. Therefore, the purpose of calculating the ripple current is to ensure that its Root Mean Square (RMS) value is within the capacitor's rated limits, thereby controlling the temperature rise.

Generally, the specification sheet provides the maximum allowable ripple current value under reference conditions (e.g., 100kHz, 20°C) and the corresponding frequency compensation coefficients. This allows for a rough estimation of the maximum allowable ripple current at different frequencies.

With the ESR and Ripple Current, the power loss generated by the electrolytic capacitor itself can be estimated. Combined with the thermal resistance parameter, the temperature rise ΔT can ultimately be determined.

I rarely use the above estimation思路 myself because these estimations heavily rely on mature component models and the completeness of specification parameters, which are often weaknesses of domestic component manufacturers (compared to large international manufacturers). Therefore, the practical design approach is to apply significant derating (substantial parameter redundancy) and conduct careful verification (validating through durability tests whether the electrolytic capacitor can withstand the conditions).

This also implies wasted hardware costs and increased verification risks, a somewhat helpless current situation. Because the later a risk appears, the cost increases exponentially, and the project manager's anger also increases explosively.

4) Residual Voltage

If the circuit design is unreasonable, not only electrolytic capacitors but other types of capacitors may also risk retaining residual voltage due to the lack of a voltage discharge path. When this accumulates to a certain value, it can lightly affect the normal logic of the circuit or heavily potentially break down the capacitor.

Based on the above, the design principle is to start from the capacitor and check whether there are parallel resistors on the same topological path, chip pins with paths to ground, or even specially added transistor switch discharge paths.

DFM Considerations

Through-hole electrolytic capacitors are rarely used now; surface-mount (SMT) packages are basically used. I think the reason might not primarily be hardware design requirements but rather the propagation of considerations from SMT factory manufacturing processes to the design end. Because besides large connectors indeed requiring the additional process of wave soldering, having fewer through-hole components on the circuit board increases the potential for cost reduction and efficiency improvement in arranging wave soldering/manual soldering processes and production costs.

Most SMT factory production lines schedule different projects to share a set of production lines. The more uniform the component soldering process, the fewer process engineers or operators are needed, the lower the probability of potential production emergencies, and the more likely production orders are to be completed on schedule according to the schedule.

Getting back on track, the DFM considerations for surface-mount electrolytic capacitors are as follows:

1. Pay attention to component height and potential mechanical interference. During the schematic design phase, ensure to collect statistics on component heights. This is to allow structural engineers to reserve sufficient clearance space and avoid components contacting the housing.

   
   

2. Layout Principles. For projects where board area is not tight, it's best not to place other components too close to the electrolytic capacitor. The factory might successfully mount them, but during the debugging phase, it becomes difficult: wanting to remove a nearby SMT resistor or capacitor, the hot air gun is hard to use (afraid of melting the plastic base), and the soldering iron is even harder to maneuver—no space. Experienced Layout engineers usually have empirical values for placement spacing. However, during the PCB Layout review, it's best to pay attention to this design detail. After all, once the board is received, the hardware engineers themselves have to bear the responsibility for earlier design issues.

   
   

3. Reinforcement with Adhesive (Potting/Staking). Vibration endurance tests in mechanical stress testing are very unfavorable for all components on the board that are large in volume, relatively heavy, and have small solder leads. Serious consequences include components detaching during vibration tests due to their own weight and insufficient soldering strength. So not only aluminum electrolytic capacitors but also the SMT soldering of power inductors need close attention. Perhaps the reinforcement effect of through-hole aluminum electrolytic capacitors might actually be better. Therefore, in actual PCBA production, an step of applying adhesive is often added to reduce component vibration amplitude and minimize the risk of pin breakage or pad lifting. Besides adhesive reinforcement, it's also important to check whether insufficient solder paste on the pads leads to weak solder joints. If the R&D team has the condition to follow up in the factory, they can assess during the first article inspection whether the solder paste volume is sufficient and request the production line to adjust the stencil solder paste volume.

   
   

Conclusion

As one of the most commonly used components, aluminum electrolytic capacitors are mostly used for energy storage, followed by low-frequency filtering. They are often used in power modules, especially at the primary power input stage of the entire hardware circuit, which is also the place most severely affected by external transient interference.

The TVS diode and Pi-filter circuit before the electrolytic capacitor are used to suppress surges caused by external interference. Here, it is necessary to evaluate whether the preceding stage circuit can effectively block the interference and whether the selected electrolytic capacitor can withstand the required surge voltage (generally not exceeding 1.1 to 1.115 times the rated voltage).