In the reliability design of electronic equipment, inrush current is an easily overlooked yet highly destructive "invisible killer." It is a peak current generated instantaneously when equipment powers on, switches loads, or is subjected to external interference. Its duration is only microseconds to milliseconds, but its peak value can reach 5 to 100 times the rated current. At best, it causes equipment restarts and performance degradation; at worst, it directly burns out components and causes safety accidents.

I. Three Core Causes of Inrush Current

The essence of inrush current is the transient energy change of energy storage components or a sudden change in load characteristics in a circuit. It mainly falls into three categories:

The most common is power-on surge. Electronic equipment typically has a filter capacitor at the power input. At the moment of power-on, the voltage across the capacitor is 0, equivalent to a momentary short circuit. The power supply voltage is directly applied across the capacitor, forming a large charging current. For example, the input filter capacitor of a household air conditioner can reach 1000μF, and the peak inrush current at power-on can reach tens of amperes, which is tens of times the rated current.

Secondly, there are load switching surges. When an inductive load (motor, transformer) is disconnected, the magnetic field stored in the inductor may not be released instantaneously, generating a reverse high voltage that triggers a current surge. When capacitive loads are connected in parallel, the added capacitor is equivalent to a momentary short circuit, causing a sharp increase in charging current. For example, when a server cluster starts up, the capacitors of multiple power modules charge simultaneously, and the surge current is amplified by the superposition of currents.

The third type is external interference surges. External factors such as power grid fluctuations, lightning strikes, and electromagnetic interference can all cause them. Lightning surges can reach peak values of over 10kA, lasting only on the order of microseconds, and are extremely destructive. In industrial power grids, the starting and stopping of welding machines and frequency converters can also cause surges in surrounding equipment.

II. The Four-Layer Harmful Chain of Surge Current

Although surge currents are short in duration, their destructive power spreads in layers:

At the device level, the rated current of semiconductor devices (PMIC, MOSFET) is far lower than the surge peak value. Instantaneous overcurrent can cause PN junction burnout and gate breakdown; electrolytic capacitors can bulge and leak due to instantaneous heating; thin wires and connectors may melt and oxidize due to Joule heating.

At the equipment performance level, surges cause a momentary drop in power supply voltage, triggering equipment restarts and resets; rapidly changing large currents generate strong electromagnetic radiation, interfering with sensitive circuits such as sensors and RF modules; long-term, repeated surge impacts accelerate device aging and shorten equipment lifespan.

At the system level, when data center server clusters are powered on simultaneously, the superimposed surge current may cause grid voltage collapse; in industrial control systems, surge-induced PLC malfunctions may cause production accidents; in automotive electronics, charging surges may damage the battery management system, affecting driving safety.

III. Scenario-Specific Surge Suppression Solutions

The core idea of surge suppression is to "increase the equivalent resistance, extend the rise time, and absorb surge energy." Different scenarios require specific solutions:

1. Low-Power Devices (Consumer Electronics, IoT Modules)

Recommended low-cost passive solution: PTC thermistor + small TVS diode. The PTC has low resistance at room temperature; when surge current passes through, it heats up, and the resistance increases sharply, limiting the current. The TVS diode has a response time of less than 1ns, which can quickly clamp overvoltage and protect sensitive chips. For example, for a 3.3V powered IoT module, a 0603 packaged PTC (R₂₅=10Ω) + 0402 current-limiting resistor (1Ω) + SMD0603 TVS diode (V_BR=5V) can be used, balancing suppression effectiveness with small size requirements.

2. High-Power Devices (Industrial Inverters, Server Power Supplies)

Active suppression solution required: soft-start circuit + common-mode inductor. The current rise is controlled slowly by a MOSFET and an RC delay circuit. During normal operation, the MOSFET is saturated and conducting, resulting in extremely low power consumption. The common-mode inductor simultaneously suppresses surges and electromagnetic interference. High-power AC equipment can also be equipped with a bidirectional thyristor, allowing the current to gradually conduct at the voltage zero-crossing point by controlling the firing angle.

3. Outdoor/High-Voltage Equipment (Base Stations, New Energy Vehicles)

An external protection solution is required: varistor + gas discharge tube + TVS diode. The varistor handles grid surges, the gas discharge tube absorbs large lightning currents, and the TVS diode precisely clamps the voltage to protect the core components. For automotive applications, AEC-Q200 certified automotive-grade PTC and TVS diodes must be selected, meeting the wide temperature range requirement of -40~125℃.

IV. Key Design Tips and Testing Verification

During PCB layout, surge suppression devices should be placed close to the power input, with lead lengths not exceeding 5mm to reduce parasitic parameters. The sampling resistor for active suppression circuits should be close to switching devices to ensure detection accuracy. A 0.1μF ceramic capacitor and a 1μF tantalum capacitor should be connected in parallel at the power input to achieve high- and low-frequency decoupling and keep them away from noise sources such as DC-DC converters.

Testing verification must comply with the IEC 61000-4-5 standard. A surge generator should be used to produce an 8/20μs standard waveform, and surge peak value, rise time, and other parameters should be measured using an oscilloscope and current probe. Different testing levels apply to different scenarios: Level 1 (0.5kV) for laboratory environments and Level 4 (4kV) for outdoor equipment.

Surge current suppression is a critical aspect of electronic equipment reliability design, with the core being "scenario matching + parameter quantification." Low-cost passive solutions are preferred for low-power scenarios, while active soft-start solutions are used for high-power scenarios. Outdoor equipment requires enhanced multiple protections. In actual design, it is necessary to first calculate the surge peak value and energy, then select devices with matching current capacity and energy absorption capacity, and if necessary, verify and optimize the scheme through testing.