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How to understand the leakage current indicator in the power supply?

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.05.07
Introduction

In daily work, hardware design engineers often come into contact with the indicator of leakage current, which is divided into leakage current and withstand leakage current. This article will briefly describe the differences between the two types of leakage currents in switch mode power supplies, and briefly analyze the common problems of abnormal leakage currents. At the same time, corresponding verification methods and solutions for prevention and control are provided to ensure system reliability.

1.What is the difference between the withstand voltage leakage current and the leakage current (contact current) in the power supply?

In layman's terms, withstand voltage leakage current refers to the leakage current flowing through the internal insulation of a product when it is not in a charged state during a high-voltage test of several thousand volts. The withstand voltage leakage current is relatively large, usually ranging from a few milliamperes to several tens of milliamperes, such as 2000VAC/5ma;

There are two indicators of leakage current: contact leakage current and ground leakage current. When the human body comes into contact with the product casing, it is ground contact leakage current, and when it touches a certain output terminal, it is application end contact leakage current. The leakage current is relatively small, usually a few milliamps, such as 240VAC/0.75ma; Of course, there are also applications with high leakage current requirements, such as the medical industry: medical equipment is required to test leakage current ≤ 100uA under normal conditions.
The leakage current consists of two parts: one part is the conductive current I1 flowing through the insulator; The other part is through the displacement current I2 of the safety capacitor (Y capacitor leakage current calculation I=2 * π * f * C * Vrms, leakage current increases with increasing power frequency).

2.How to understand leakage current? What are the standard requirements?

The leakage current in the power manual, also known as leakage current, is mainly caused by the incomplete insulation performance of internal components of the power supply, generally less than 0.75ma. The main factor affecting this indicator is the capacitance and number of Y capacitors inside the product. The larger the capacitance value and number of Y capacitors, the larger the corresponding leakage current measured.

Leakage current is mainly used to verify the safety of the power supply in contact with the human body. Various industry certifications have limited value standards for leakage current. For example, in industry, it is generally required to be less than 0.75ma, while in the medical industry, it is required to be less than 0.1ma.

3. Common problems with leakage current

If the product design meets the specifications, what would cause the leakage current test to exceed the standard?

(1) Testing method issue: The higher the input voltage, the greater the leakage current tested, and the higher the input frequency, the greater the leakage current tested; Multiple power sources are tested together, and the leakage current will be superimposed.

(2) Insulation failure: If the insulation material is damaged, aged, or damp, it can cause current to leak out of the circuit, resulting in leakage current.

(3) Grounding problem: If there is a problem with the grounding wire of the circuit, such as a broken or poor contact of the grounding wire, it can cause current to leak out through other paths, forming leakage current.

How to solve the problem of excessive leakage current?

(1) Troubleshooting testing method: Check if the input power supply, input frequency, wiring method, and impedance network matching are correct;

(2) Appearance inspection: visible signs of component wear or skin damage to the naked eye;

(3) Reduce the number of Class I power supplies used, and different voltage requirements can be converted using the DCDC power supply scheme.

How to improve the demand for smaller leakage current indicators and what are the impacts?

(1) Reducing the number of Y capacitors used, or minimizing the use of capacitance values as much as possible, may affect EMC indicators, such as deteriorating RE performance.

4.How to understand withstand voltage leakage current? What are the standard requirements?

The withstand voltage and leakage current indicators in the power manual mainly refer to the current flowing through the isolation devices between the input and output terminals, input and housing, and output terminals and housing during the withstand voltage test process of the product. Generally, the industrial demand is less than 10ma.


Voltage withstand test

Voltage withstand test is one of the main methods to test the ability of power supplies to withstand transient overvoltage of products, with the aim of verifying the reliability of power supplies in environments similar to lightning strikes and electrical faults. The withstand voltage index is related to the safety certification and overvoltage level that the power supply meets.

If the product design meets the specifications, what would cause the leakage current test to exceed the standard?

(1) Test method issue: If the voltage resistance exceeds the specification, for example, if the specification is 3KV, a 3.5KV voltage resistance test needs to be conducted; Multiple power sources tested simultaneously, with superimposed leakage currents; The setting parameters of the withstand voltage tester are abnormal.

(2) Aging test: Isolation withstand voltage test is a destructive experiment. After multiple long-term withstand voltage tests, the product may be damaged to a certain extent and the leakage current may exceed the standard.

(3) External influence: The insulation layer is damaged and the leakage current exceeds the standard due to product transportation or other collisions.

How to improve the requirement for smaller voltage withstand and leakage current indicators, and what are the impacts?

(1) Reduce the number of Y capacitors used, or minimize the use of capacitance values as much as possible, but it may affect EMC indicators, such as deteriorating RE performance;

(2) Increase the insulation strength of the transformer.

Summary

This article will briefly describe the differences between the two types of leakage currents in switch mode power supplies, and briefly analyze the common problems of abnormal leakage currents. At the same time, corresponding verification methods and solutions and prevention methods are provided to ensure system reliability. Jinshengyang serves customers with a professional sales and technical service team, providing pre-sales selection, technical exchanges during sales, and professional after-sales support, providing customers with one-stop solutions.
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