Methods for Characterizing DC Inrush Current

Introduction

Inrush current or input surge current refers to the momentarily large surge current that occurs when you turn on the power to a device. Inrush current often occurs in devices with a large amount of parallel capacitance at the input, such as DC-to-DC converters, or devices with extremely low impedance under transient conditions, such as electric motors. Inrush current can be several times larger than the device’s steady-state current. For instance, electric motors can have inrush current levels 25 times greater than their steady-state current levels.

When you characterize inrush current, you capture either a high-resolution digitized profile of a device’s inrush current or a reliable peak current reading. This application note presents techniques for using a high-resolution oscilloscope and a high-precision digital multimeter to accurately capture inrush current, providing valuable insights into system behavior.

The high-precision digital multimeter (DMM) used for this DC inrush current measurement is part of a set of Keysight Smart Bench Essentials Plus instruments. They come equipped with proven pro-level measurement technologies designed to minimize measurement errors resulting from real-world factors to provide the accuracy and precision required for this application. The instruments also come with large color screen displays and graphical interfaces to visualize, analyze, and share test results quickly.

The high-resolution oscilloscope used in this application is also part of the Smart Bench Essentials Plus instruments. It comes with a custom ASIC and built-in 14-bit analog-to-digital converter to provide very high vertical display resolution and low noise floor to easily catch transient in-rush current spikes for this application.

Why Inrush Current Measurement Is Essential

When an electrical device is powered on, it often draws a surge of current, known as inrush current. This brief but intense current spike can be several times higher than the device’s normal operating current. While inrush current is a natural occurrence, uncontrolled surges can lead to various issues, including component damage, power instability, and reduced system efficiency.

To manage these risks, it helps to understand the key benefits of accurately measuring and managing inrush current.

Protecting sensitive components

Electronic devices, such as microcontrollers, sensors, and integrated circuits, are designed to operate within specific current and voltage limits. Excessive inrush current can cause thermal stress, degrade components over time, or even lead to immediate failure. Measuring inrush current enables engineers to design protective circuits, such as inrush current limiters or soft-start circuits, to prevent damage.

Example: In an automotive electronic control unit, repeated exposure to high inrush currents could shorten the lifespan of onboard power regulators and degrade reliability. Proper measurement and tuning of inrush current help maintain system reliability.

Preventing circuit breaker tripping and fuse blowing

Power distribution systems, especially in industrial and commercial applications, rely on circuit breakers and fuses to protect against overcurrent conditions. If the inrush current exceeds the circuit breaker’s threshold, the breaker may trip unexpectedly, causing unnecessary downtime or disruptions.

Example: LED lighting systems often experience a high inrush current when switched on. Without proper characterization, an array of LEDs may trip circuit breakers, leading to operational failures in commercial buildings or stadium lighting systems.

Ensuring power supply stability

Sudden inrush currents can create voltage drops or fluctuations in power distribution networks, affecting other connected devices. Characterizing inrush current helps engineers design power supplies and distribution systems that can handle these transients without compromising performance.

Example: In data centers, server power systems must accommodate multiple devices turning on simultaneously. Uncontrolled inrush current could cause voltage sags, leading to unexpected reboots or failures of critical infrastructure.

Optimizing energy efficiency and thermal performance

Higher inrush currents lead to increased energy dissipation, which can generate excess heat and reduce overall system efficiency. Measuring and managing inrush current ensures that devices operate within their optimal thermal and electrical limits.

Example: Industrial motor drives used in manufacturing facilities draw large inrush currents at startup. By measuring and controlling this current, engineers can implement soft-start mechanisms that reduce wear and improve energy efficiency.

Meeting regulatory and safety standards

Many industries have stringent regulations regarding power quality, electromagnetic compatibility, and electrical safety. Accurately characterizing inrush current ensures compliance with industry standards such as IEC, UL, and IEEE guidelines.

Example: Power adapters and battery chargers for consumer electronics must meet regulatory requirements for inrush current limits to prevent electrical hazards and ensure user safety.

This application note also covers methods for characterizing inrush current.