High-Side NMOS Switch Active Clamping for Overcurrent Stability

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Solution Overview

Problem

Existing switching devices face challenges in managing overcurrent and overheating issues due to low on-resistance in NMOSFETs, particularly in high-side switches, which can lead to faults and reduced reliability.

Innovation Solution

Incorporating an active clamp circuit and a gate controller with a Zener diode to control the gate driving signal, limiting the output current and maintaining the gate-source voltage within a safe threshold, thereby preventing full shut-off during transitions and managing inductive loads.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If NMOSFET with low on-resistance is used for high-side switching, then switching performance is improved, but overcurrent and overheating issues occur reducing reliability

Engineering Contradiction:

Improveswitching device reliabilityVSAvoidovercurrent and overheating

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

An active clamp circuit is introduced as an intermediary component between the NMOSFET and the load. This circuit includes a clamp transistor, clamp resistor, and clamp capacitor that work together to limit current flow and dissipate excess energy, preventing overcurrent and overheating while allowing the NMOSFET to maintain its low on-resistance for good switching performance

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The active clamp circuit provides beforehand cushioning by pre-establishing current limiting and energy dissipation pathways before overcurrent conditions can develop. The clamp transistor and resistor are configured to activate when voltage exceeds safe thresholds, cushioning against potential damage before it occurs

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

2Speed

If gate driving signal is not controlled, then switching speed is improved, but full shut-off during transitions causes faults

Engineering Contradiction:

Improveswitching speedVSAvoidoperational stability

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

The gate controller implements feedback control by continuously monitoring the gate-source voltage and the state of the NMOSFET. Based on this feedback, the controller adjusts the gate driving signal to ensure the transistor transitions smoothly between states without entering a full shut-off condition during switching, thereby maintaining both speed and reliability

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The gate driving signal is made dynamic through the gate controller, which continuously adjusts the signal characteristics based on real-time operating conditions. This dynamic control prevents the gate voltage from dropping to levels that would cause full shut-off during transitions, while still allowing rapid switching when conditions are favorable

Inventive Principle:

Principle #15Dynamics

3Reliability

If active clamp circuit is added to limit voltage fluctuations, then reliability is improved, but device complexity increases

Engineering Contradiction:

Improveoperational stabilityVSAvoidcircuit complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The active clamp circuit components (clamp transistor, clamp resistor, clamp capacitor) are merged with the existing NMOSFET switching circuitry. The clamp transistor is positioned to share the same substrate and packaging as the main switching transistor, and the clamp resistor and capacitor are integrated into the same circuit board or module, reducing overall complexity despite the added functionality

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The active clamp circuit is designed to perform multiple functions: limiting voltage fluctuations, dissipating inductive kickback energy, preventing overcurrent, and protecting the NMOSFET during switching transitions. This multi-functionality justifies the added complexity by providing comprehensive protection and stability enhancement

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 active clamp circuit effectively limits voltage fluctuations and current spikes, enhancing the reliability and safety of the switching device by preventing full shut-off and maintaining operational stability.

Implementation Method 1

a Zener diode configured to limit the gate-source voltage of the NMOSFET to a predetermined value or less

Methodology Applied

Scientific EffectZener breakdown: Avalanche Breakdown

Implementation Method 2

an active clamp circuit configured to control the NMOSFET so as to keep the output voltage at an off transition of the NMOSFET equal to or higher than a lower limit voltage

Methodology Applied

Scientific EffectActive clamping:

Data Source

PatentUS12562732B2Switching device, electronic device, and vehicle

Publication Date: 2026.02.24 ROHM CO LTD

AI summary

A switching device, for example, includes a P-type semiconductor substrate configured to be fed with a ground voltage, a switching element connected between an application terminal for a supply voltage and an application terminal for an output voltage, a driver configured to turn on and off the switching element, and an active clamp circuit configured to control the switching element so as to keep the output voltage at an off transition of the switching element equal to or higher than a lower limit voltage lower than the ground voltage by an active clamp voltage.