Embedded Power Cord Inrush Limiter Using MCU-Controlled NTC Shunting

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Solution Overview

Problem

High-capacity power supply unit (PSU) loads experience inrush currents exceeding 200 A, leading to relay failure and potential damage due to terminal blowout, and existing power distribution units (PDUs) lack active overcurrent protection, especially under poor power quality conditions.

Innovation Solution

An embedded in-cord inrush current limiter device with a housing containing a switch, NTC thermistor, voltage and current sensors, and a microcontroller unit (MCU) that engages the NTC thermistor to limit inrush current when thresholds are reached, using a bistable power relay or solid-state switching devices.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If active overcurrent protection devices are added to every rPDU outlet, then inrush current protection is improved, but device complexity and cost increase

Engineering Contradiction:

Improveinrush current protectionVSAvoiddevice complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The power cord is divided into functional segments: a connector portion and an embedded limiter portion. The inrush current limiter is embedded only in the cord rather than being integrated into the entire PDU system, segmenting the protection function to where it is most needed while keeping the rest of the system simple.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The power cord acts as an intermediary component between the PDU and end devices. By embedding the inrush current limiter in the cord, the protection function is introduced through this intermediate element rather than requiring modification of the PDU or end devices themselves.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If inrush current limiter is embedded in power cord, then protection is provided without significant cost or space impact, but device complexity increases

Engineering Contradiction:

Improveovercurrent protectionVSAvoiddevice complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The inrush current limiter circuitry is nested within the existing power cord structure. The housing containing the NTC thermistor, switch, and control circuitry is embedded within the cord insulation, creating a nested configuration where the protection device is housed inside the already-present cord.

Inventive Principle:

Principle #7Nested doll (Nesting)

Solution Approach 2:

The NTC thermistor provides automatic inrush current limiting based on its temperature-dependent resistance characteristics without requiring external control. The thermal effect of the NTC thermistor itself provides the protection mechanism, making the system self-regulating.

Inventive Principle:

Principle #25Self-service

3Reliability

If NTC thermistor is used for inrush current limiting, then current protection is achieved, but voltage drop occurs affecting power quality

Engineering Contradiction:

Improveinrush current limitationVSAvoidvoltage drop

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The switch periodically shunts the NTC thermistor to allow it to cool down between inrush current events. This periodic resetting of the NTC thermistor's temperature ensures it returns to its low-resistance state, ready to provide protection for the next inrush event while minimizing continuous voltage drop.

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

The system dynamically switches between two operational states: normal operation where the NTC thermistor is shunted (low resistance path), and protection mode where the NTC thermistor is engaged (high resistance path during inrush). This dynamic switching optimizes both protection effectiveness and voltage quality.

Inventive Principle:

Principle #15Dynamics

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The device effectively mitigates inrush currents, preventing relay failure and damage to PDUs and end devices, while providing compact overcurrent protection without significant cost or space impact.

Implementation Method 1

a switch and negative temperature coefficient (NTC) thermistor serially connected to a first conductor

Methodology Applied

Scientific EffectNegative temperature coefficient (NTC) thermistor: Thermistor

Implementation Method 2

engage the NTC thermistor to limit inrush current

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Data Source

PatentUS20250330012A1Embedded in-cord active inrush current limiter device for power supply unit

Publication Date: 2025.10.23 VERTIV CORP

AI summary

An apparatus and method for limiting inrush current to an end device provides an alternating current (AC) to the end device via a power cord including first and second current carrying conductors. A voltage sensor enclosed by a housing attached to a power cord and connected to the conductors senses an input voltage. A current sensor within the housing senses an inrush current via the first conductor. A microcontroller unit (MCU) within the housing receives the sensed input voltage and inrush current. Based on the sensed input voltage, the MCU determines a voltage drop across a negative temperature coefficient (NTC) thermistor serially connected (with a switching device) to the first conductor. When the sensed inrush current and/or the sensed input voltage reaches a threshold level, the MCU engages the NTC thermistor to limit inrush current by opening the switching device.