CapXon UJ series aluminum electrolytic capacitors

Life estimation and reliability design

6.1 Life cycle model

The lifespan of aluminum electrolytic capacitors is mainly affected by temperature and follows Arrhenius' law. CapXon provides a useful life curve (see the Useful Life chart on page 8 of the PDF), with the core formula being:

L=L0×two T0−Ta ten

L=L0×2 ten T0−Ta

among which

L0=5000 hours (at 105 ℃, under rated ripple current conditions)

T0 = 105℃

Ta=Hot spot temperature inside the capacitor (ambient temperature+ripple current self heating)

In practical applications, it is necessary to consider the temperature rise (Δ T) caused by ripple current. Usually, Δ T ≤ 5 ℃ can ensure the service life. By using derating (reducing ripple or lowering ambient temperature), the lifespan can be doubled. For example, at an ambient temperature of 85 ℃ and a ripple current of 80% of the rated value, with an internal temperature of approximately 88 ℃, the estimated lifespan is 5000 × 2 ^ ((105-88)/10) ≈ 5000 × 2 ^ 1.7 ≈ 5000 × 3.25 ≈ 16250 hours. If the device works for 10 hours a day, its theoretical lifespan exceeds 4 years.

6.2 Engineering measures to improve reliability

Parallel use: Using multiple small capacitors in parallel instead of a large capacitor can reduce the ripple current load of each capacitor, while the ESR is lower after parallel connection, resulting in a smaller overall ripple.

Leave sufficient heat dissipation space: The horn capacitor should avoid being in close contact with heating power devices (such as rectifier bridges and MOSFETs), and air vents should be opened on the PCB.

Voltage derating: At 105 ℃, it is recommended to reduce the operating voltage to within 80% of the rated value. For example, a 400 V capacitor is used for a 320 V DC bus.

Regular monitoring: The lifespan can be predicted by measuring the appearance, capacitance value, and ESR of the capacitor top. At the end of its lifespan, the UJ series has a capacity change of within ± 20% (after 2000 hours of durability testing), and after 5000 hours, the capacity change may exceed -20%. It is recommended to replace it at this time.

Common fault modes and troubleshooting methods

Possible causes of malfunction using UJ series solutions

The top of the capacitor bulges (pressure relief valve action), causing excessive ripple current or overvoltage and resulting in excessive internal gas pressure. Choose a model with a higher ripple current level; Check if the bus voltage exceeds the surge specification; Increase the number of parallel capacitors

Leakage (electrolyte seeping out from the bottom or top) during long-term high-temperature operation or sealing failure to ensure that the working temperature is below 105 ℃ and improve heat dissipation; Choose AEC-Q200 version to cope with harsh environments

The capacitance value significantly decreases (>30%), the electrolyte dries up, and the lifespan is terminated. Re evaluate the thermal load; Choose the UJ series with a longer lifespan (5000h); Reduced usage voltage/ripple

Abnormal increase in ESR (>2 times the initial value), increase in internal contact resistance, or local overheating measurement to determine if the actual ripple current exceeds the standard; Check if the capacitor installation is loose and causing poor contact

Low frequency (50/120 Hz), excessive ripple, insufficient capacitor capacity or high ESR, increase capacity; Confirm if the frequency multiplier Kf is correctly applied (allowing for lower ripple at low frequencies)

Installation and welding precautions

8.1 Mechanical Installation

The pins of the horn capacitor usually need to be inserted into the pre opened circular holes on the PCB, and then soldered by wave soldering or manual soldering. Suggest inserting it all the way through, leaving a gap of 0.5~1mm between the bottom of the capacitor and the PCB, so that the soldering flux gas can escape during soldering.

For large capacitors with a diameter of ≥ 30 mm, it is strongly recommended to use clamps or adhesives to fix them on the PCB or chassis to resist vibration. The UJ series has passed the IEC 60068-2-6 vibration test (10~55 Hz, 0.75 mm amplitude, 10g), but the installation method directly affects the actual reliability.

8.2 Welding conditions

Manual soldering: Iron temperature ≤ 350 ℃, time ≤ 5 seconds.

Wave soldering: preheating temperature ≤ 100 ℃, soldering temperature ≤ 260 ℃, time ≤ 10 seconds.

Excessive temperature or prolonged time can damage the internal sealing structure, leading to early leakage.

8.3 Storage and Aging

Unused capacitors should be stored in their original packaging at an ambient temperature of 0-35 ℃ and a relative humidity of ≤ 75%.

If stored for more than 2 years, it is recommended to apply rated voltage through a 1 k Ω resistor for aging treatment for 1-2 hours before use to repair the oxide film and reduce leakage current.

Practical case: Upgrade and replacement of PFC output capacitor for charging piles

9.1 Original design issues

A 3.3 kW AC charging station PFC output stage uses two 680 µ F/450 V capacitors in parallel (ordinary 105 ℃/2000h product). At an outdoor temperature of 45 ℃ in summer, the measured surface temperature of the capacitor reached 78 ℃, with a ripple current of about 3.5 A (sharing 1.75 A per capacitor). The rated ripple of the original capacitor is 1.9 A @ 105 ℃. Although there is some margin when converted to 78 ℃, some products still have bulges after 1.5 years.