Inrush Current Control Circuit Using Transistor Bypass

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Solution Overview

Problem

Conventional inrush current control circuits in power supplies are power-consuming and often impractical, especially when meeting energy efficiency standards, and they can damage devices due to high inrush currents during hot plugging.

Innovation Solution

A low-component inrush current control circuit that uses a bypass transistor, resistors, and a diode to selectively short-circuit an inrush current limiting resistor, driven by a pulse width modulated signal, allowing for efficient bypassing of the resistor during power supply turn-on and maintaining the bypass state during operation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a relay is used in parallel with a resistor for inrush current control, then the inrush current can be limited, but the relay control winding consumes 10 to 20 mA which increases power consumption

Engineering Contradiction:

Improveinrush current protectionVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The patent replaces the mechanical relay system with an electronic transistor-based switch (Q3) controlled by a voltage threshold circuit. The transistor is turned on when the voltage across capacitor C1 exceeds a threshold determined by resistors R4 and R5, eliminating the need for a relay control winding and reducing power consumption while maintaining inrush current protection functionality

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The circuit uses the existing voltage across capacitor C1 to automatically trigger the bypass transistor Q3 when the threshold is reached. The circuit self-regulates by using its own operating parameters (voltage, current) to control the bypass operation without requiring external control signals or additional power-consuming control components

Inventive Principle:

Principle #25Self-service

2Loss of energy

If a TRIAC is used in parallel with a resistor for inrush current control, then power loss is reduced, but a floating bias gate drive source is required which is not always practical

Engineering Contradiction:

Improvepower lossVSAvoidgate drive implementation

Core Design Contradiction:

Loss of energyVSEase of operation

Solution Approach 1:

The patent introduces an intermediary voltage threshold circuit using resistors R4 and R5 that references the control voltage to ground. This intermediary circuit converts the high-voltage switching requirement into a manageable base-emitter voltage threshold for the transistor, eliminating the need for floating gate drive while maintaining effective bypass control

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent replaces the TRIAC with a bipolar junction transistor (Q3) that can be controlled by a simple voltage threshold circuit. The transistor's base-emitter junction provides a natural voltage threshold (approximately 0.7V) that eliminates the need for complex floating gate drive circuits required by TRIACs

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Reliability

If an inrush current limiting resistor is used, then inrush current is limited, but the resistor remains in the circuit during operation causing continuous power loss

Engineering Contradiction:

Improveinrush current limitationVSAvoidcontinuous power loss

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

The patent implements a dynamic bypass system where transistor Q3 is initially off during power-up, allowing the inrush limiting resistor R1 to function. When the voltage across C1 reaches the threshold set by R4 and R5, Q3 turns on and creates a low-impedance bypass path around R1, eliminating continuous power loss while maintaining protection during the critical inrush period

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

This solution reduces power consumption and prevents device damage by efficiently managing inrush currents, meeting energy efficiency standards and eliminating arcing issues in hot-pluggable power supplies.

Implementation Method 1

The capacitor (C2) has a first capacitor terminal connected to the control terminal (gate or base) of the bypass transistor (Q3), and has a second capacitor terminal connected to the first current path terminal (source or emitter) of the bypass transistor (Q3)... The turn-on time constant is dependent on the second resistor (R2) and the capacitor (C2)

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

The diode (D3) has a cathode terminal connected to the second resistor terminal of the second resistor (R2), and has an anode terminal connected to one of the terminals of the switching transistor (Q1) of the power supply... The diode (D3) keeps the voltage at the control terminal (gate or base) of the bypass transistor (Q3) sufficiently high to keep the bypass transistor (Q3) turned on

Methodology Applied

Scientific EffectDiode: Diode

Data Source

PatentUS8422179B2Inrush current control

Publication Date: 2013.04.16 INTERSIL AMERICAS INC

AI summary

An inrush current control circuit selectively short-circuit bypasses an inrush current limiting resistor (R1) of a power supply that includes a switching transistor (Q1) having a control terminal (gate or base) driven in dependence on a pulse width modulated (PWM) drive signal. The inrush current control circuit includes a bypass transistor (Q3), a first resistor (R3), a capacitor (C2), a second resistor (R2) and a diode (D3), wherein an anode terminal of the diode (D3) is connected to one of the terminals of the switching transistor (Q1) of the power supply.