June is approaching, and the weather is getting hotter. When using capacitors, it has gradually been observed that the actual measured leakage current of aluminum electrolytic capacitors is increasing. Regarding this, let's talk about the basic characteristic of leakage current in aluminum electrolytic capacitors. 

Leakage current is the current that leaks from regions of insulation where current normally does not flow in a circuit. Leakage current can lead to circuit failures, increased power consumption, and heating problems. Capacitors also have leakage current. However, this not only includes the current through the dielectric acting as an insulator but also the current caused by the polarization of the dielectric.

There are two parameters describing the insulation characteristics of a capacitor: 'insulation resistance' (R_iso) and 'leakage current' (i_Leak). The former is used for film capacitors and ceramic capacitors with extremely low leakage current, while the latter is used for electrolytic capacitors with high leakage current. Leakage current can be regarded as an insulation resistance (IR) in parallel with the capacitor (Figure 1), and therefore is sometimes referred to as a resistance (leakage resistance) rather than a current.

Common Questions and Solutions:

1: Can leakage current be completely eliminated?

Answer: Leakage current is a fundamental characteristic of capacitors. It is a basic parameter that can be optimized and reduced through various methods, but can never be completely eliminated. The value of the leakage current depends on the choice of materials, manufacturing processes, and following proper usage to minimize the impact of leakage current.

2: Why does the leakage current increase when the weather gets hot?

Answer: This is because aluminum electrolytic capacitors have poor temperature characteristics. At high temperatures, without being powered, the electrolyte slowly degrades the oxide film, leading to an increase in the basic leakage current. Therefore, manufacturers of aluminum electrolytic capacitors set a storage period, and after a certain period of storage, the capacitor needs to be periodically re-energized to reduce the leakage current. The leakage current is a recoverable parameter.

3: Why does a switching power supply experience delayed startup after being stored for a long time?

Answer: The small capacitor powering the chip (usually with a voltage below 50V and a capacitance below 47μF) can have increased leakage current due to long-term storage. Additionally, because the circuit sets a low startup charging current, usually below 1mA, it can sometimes cause delayed startup. This issue can be resolved by running the unit for a long time to reduce the capacitor's leakage current. After this repair, the power supply can start normally, without affecting the device's normal quality and operation.

4: Why does the circuit board have higher leakage when installed, but shows normal leakage when measured after being removed? 

First, other components may cause measurement errors. Additionally, when the insulation between terminals deteriorates due to humidity or foreign substances adhering to the terminals, current may bypass the dielectric and flow between the electrode terminals. Extra dirt may also cause leakage. The problem can be solved by cleaning the circuit board with board washing solution and then drying it.