Power adapter When assessing long-term performance and service life, the primary consideration should be the expected lifespan of electrolytic capacitors. Over time, the internal electrolyte evaporates, causing the capacitance to decline; once it falls below a certain threshold, the capacitor is deemed “dead” and no longer usable. Most manufacturers specify a rated lifetime during which the capacitance decreases by only 20% relative to its initial value. However, it’s important to note that the initial capacitance itself already represents a 20% deviation—typically, the initial tolerance is ±20%. Consequently, at the end of the capacitor’s life, its capacitance will be 0.8 × 0.8 = 0.64 of the original value, meaning it has dropped by 36% (not 20% + 20% = 40%). For example, if the nominal capacitance is 100 µF, at the end of its life, in the worst-case scenario, the actual capacitance could be as low as 64 µF. Therefore, if the power adapter you’re designing uses relatively small‑value capacitors, premature failure is likely, and the hold‑up time will quickly become inadequate. Even under normal operating conditions, input ripple can become so large that its average value drops significantly, particularly when the supply voltage is low. As a result, over time, thermal stress on the components intensifies, efficiency deteriorates, output ripple increases, and the power adapter may fail to operate properly. Thus, if your calculations indicate that a 100 µF capacitor is required, you should actually select an electrolytic capacitor with an initial rating of C = 100 / 0.64 ≈ 156 µF—always choosing a value 56% higher than the calculated requirement.
So what happens when an electrolytic capacitor exceeds its rated lifespan? In other words, suppose that by the time a 100 µF capacitor reaches the end of its life, its capacitance has dropped to 64 µF—your power supply would still be functioning normally. Can you really expect it to keep running properly for several more years? Of course not. The end of an electrolytic capacitor’s life is defined by two key criteria: a 20% reduction in capacitance and a 200% increase in the dissipation factor. The dissipation factor (DF) is defined as the ratio of the equivalent series resistance (ESR) to the capacitive reactance at a frequency of 120 Hz. Thus, for a 100 µF capacitor with an ESR of 1 Ω, the DF at 120 Hz is ESR × 2πfC = 0.075. Keep this value in mind, and remember that DF is directly proportional to both ESR and capacitance (as well as frequency). This will help you quickly grasp the relationship between ESR and DF. Note that DF is also referred to as the tangent of the loss angle or simply the loss angle. Generally speaking, for capacitors, if the rated capacitance is 10 µF or greater, the standard test frequency is 120 Hz; otherwise, it is 1 kHz. However, for electrolytic capacitors, this test frequency is always fixed at 120 Hz. You can verify this in the datasheet of the selected electrolytic capacitor.
If we assume that, at the end of its life, an electrolytic capacitor’s capacitance has dropped by 20% and its dissipation factor (DF) has likewise decreased by 20%, calculations show that its equivalent series resistance (ESR) has increased by 200/0.8 = 250%. In other words, by the time the capacitor reaches the end of its lifespan, even if its capacitance has declined only slightly, its ESR will have grown by a factor of 2.5. Consequently, if the capacitor is tasked with suppressing high‑frequency AC ripple—such as in the input or output filtering of buck or boost switching power supplies—its self‑heating will rise by 2.5 × 2.5 = 6.25 times. What, then, are the implications? Naturally, efficiency will be significantly compromised; but remember that heat is the primary driver of electrolyte drying out in electrolytic capacitors. As a result, thermal runaway becomes imminent: the more heat generated, the higher the ESR; the higher the ESR, the more heat is produced—creating a vicious cycle.
Let’s summarize several key factors in the capacitor aging process.
1. Sealing performance of capacitor terminals. No joint can achieve perfect sealing; over time, evaporation will gradually occur. We can select suppliers that offer high‑quality components. In principle, a capacitor can be fully sealed—e.g., by completely encapsulating it in epoxy resin or an ultra‑strong adhesive. However, electrolytic capacitors are designed with venting ports to release vapor under high pressure, much like a pressure cooker. Despite these extensive safety measures, we still observe severe explosions in some electrolytic capacitors.
2. Ambient temperature. Heat may originate from surrounding power devices or from internal thermal losses within the device itself. If the temperature is relatively high, the evaporation rate increases, thereby reducing the device’s lifespan.
3. Core temperature. Since thermal conductivity within the capacitor is not ideal, we assume the presence of localized hotspots inside the capacitor. In the worst-case scenario, the temperature at these hotspots is taken as the operating temperature used in lifetime calculations.
4. Damping resistor ESR. Clearly, the ESR affects internal thermal losses, which can raise the temperature and accelerate the evaporation process.
5. Frequency. Since ESR is a function of frequency, frequency will indirectly affect the capacitor’s lifespan.
6. Ripple Current. The most critical parameter in an electrolytic capacitor datasheet is the rated ripple current. This typically refers to the root-mean-square (RMS) value of the current at a frequency of 120 Hz and a temperature of 105°C. Essentially, the rated ripple current indicates that, when the ambient temperature is at the maximum rated temperature of 105°C and the capacitor carries a current equal to the specified RMS value, its expected lifetime will match the manufacturer’s specified life expectancy. Under these conditions, the capacitor’s lifespan generally ranges from 2,000 to 10,000 hours. In switch-mode power supplies, electrolytic capacitors rated for 85°C are rarely used, as they cannot meet the required lifetime specifications at higher ambient temperatures.
