Overcurrent Protection Circuit Using Shunt Transistor for Current Limiting

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Conventional overcurrent protection circuits fail to prevent damage when a user-set current exceeds safe limits due to unintentional shorting or incorrect resistor values, leading to high currents that can damage integrated circuit components and external circuits.

Innovation Solution

An overcurrent protection circuit that senses a mirrored current and limits the user-selectable current to a safe level by using a current limiting circuit, which activates a shunt transistor to restrict the base current into the series transistor when the voltage drop across a sense resistor exceeds a threshold, preventing excessive current flow.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If the user connects a resistor with too low a value or shorts pin 12 to ground, then the Iset current and A*Iset current increase to very high levels, but this causes damage to transistors and other components on the chip

Engineering Contradiction:

Improveuser ability to set currentVSAvoidovercurrent damage to components

Core Design Contradiction:

Adaptability or versatilityVSObject-affected harmful factors

Solution Approach 1:

The patent introduces an intermediary protection circuit between the current-setting resistor and the current-generating transistors. This protection circuit includes a sense resistor that monitors the current and a protection transistor that activates to limit the current when overcurrent conditions are detected, thereby mediating between user control and component protection

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent implements a feedback mechanism where the voltage drop across the sense resistor is continuously monitored and fed back to control the protection transistor. When the voltage drop exceeds a threshold (indicating overcurrent), the feedback signal activates the protection transistor to reduce the base current to Q4, thereby limiting the Iset and A*Iset currents to safe levels

Inventive Principle:

Principle #23Feedback

2Productivity

If the A*Iset current is used to control circuits external to the package, then the current can perform useful functions, but overcurrents may damage such external circuits

Engineering Contradiction:

Improveexternal circuit control capabilityVSAvoidovercurrent damage to external circuits

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

The protection circuit acts as an intermediary between the internal current-generating circuitry and the external circuits. By monitoring the A*Iset current through the sense resistor and activating the protection transistor when necessary, it mediates to prevent harmful overcurrents from reaching external circuits while allowing normal operation

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The protection circuit applies preliminary anti-action by being positioned upstream in the current path. It preemptively detects potential overcurrent conditions through the sense resistor and activates the protection transistor before excessive current can damage external circuits, counteracting the harmful effect before it occurs

Inventive Principle:

Principle #9Preliminary anti-action

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 solution effectively limits the current to prevent damage by maintaining it below a maximum allowable level, even when the user-set resistor value is too low, thereby ensuring component safety and optimizing circuit operation.

Implementation Method 1

The A1*Iset current flows through a sense resistor internal to the chip. If the voltage drop across the sense resistor exceeds a threshold...

Methodology Applied

Scientific EffectOhm's Law: Ohm's Law

Implementation Method 2

the voltage drop turns on a shunt transistor that limits the base current into the Iset series transistor to thus limit the Iset current to a certain current level

Methodology Applied

Scientific EffectTransistor current control:

Data Source

PatentUS7881031B2Overcurrent protection circuit when setting current using a package control pin

Publication Date: 2011.02.01 MICREL INC

AI summary

An overcurrent protection circuit for a current setting circuit is disclosed herein that prevents a user-selectable current from exceeding a current limit when an incorrect current selecting component (or current selecting circuit) is connected to an external control pin of a package by the user, or when the control pin is inadvertently grounded. The protection circuit senses a current (A1*Iset) mirrored from the user-set current (Iset). If the mirrored current is above a threshold, the protection circuit limits the Iset current to be at or below a current limit level. In one embodiment, the protection circuit comprises a transistor that turns on when the mirrored current exceeds a threshold, and the transistor shunts control current from a series transistor generating the user-set current Iset.