Low-Side MOSFET Current Sensing for Compact Power Tool Inverters

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Solution Overview

Problem

Existing power tool devices rely on shunt resistors for current sensing, which increase the size of the electronic package, lead to heat loss, and are costly, while also limiting the selection of power MOSFETs due to the need for lower internal resistance.

Innovation Solution

Implementing low-side field effect transistors (FETs) as current sensing power MOSFETs within the inverter bridge, eliminating shunt resistors and using bias resistors to provide voltage signals to the electronic processor for current determination, allowing for smaller packages, improved efficiency, and cost savings by enabling the use of MOSFETs with higher internal resistance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If shunt resistors are used for current sensing, then current measurement is achieved, but the electronic package size increases and heat loss occurs

Engineering Contradiction:

Improvecurrent sensing capabilityVSAvoidelectronic package size

Core Design Contradiction:

Measurement precisionVSVolume of stationary object

Solution Approach 1:

The patent extracts the current sensing function from the traditional shunt resistor approach and integrates it into the power MOSFET itself. By utilizing the inherent source resistance of the MOSFET as the sensing element and removing the external shunt resistor, the electronic package size is reduced while maintaining current measurement capability through voltage sensing at the source terminal.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent merges the power switching function and current sensing function into a single component - the power MOSFET. The source terminal of the MOSFET serves dual purposes: controlling the power flow and providing the sensing point for current measurement, thereby eliminating the need for separate shunt resistors and reducing overall package size.

Inventive Principle:

Principle #5Merging (Combining)

2Measurement precision

If shunt resistors are used for current sensing, then current measurement is achieved, but heat loss increases

Engineering Contradiction:

Improvecurrent sensing capabilityVSAvoidheat loss

Core Design Contradiction:

Measurement precisionVSLoss of energy

Solution Approach 1:

By removing the external shunt resistor and using only the intrinsic source resistance of the MOSFET for sensing, the patent eliminates the additional heat-generating element. The sensing is performed through voltage measurement at the source terminal without requiring a separate resistive path that would dissipate additional power.

Inventive Principle:

Principle #2Taking out (Extraction)

3Measurement precision

If shunt resistors are used for current sensing, then current measurement is achieved, but the selection of power MOSFETs is limited due to the need for lower internal resistance

Engineering Contradiction:

Improvecurrent sensing capabilityVSAvoidMOSFET selection flexibility

Core Design Contradiction:

Measurement precisionVSAdaptability or versatility

Solution Approach 1:

Instead of requiring the MOSFET to have low internal resistance (as would be needed if using a shunt resistor in parallel), the patent inverts the approach by utilizing the MOSFET's source resistance as the sensing element itself. This allows selection of MOSFETs with higher internal resistance, expanding the range of available power devices.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The MOSFET's own source resistance serves the dual function of power switching and current sensing. By making the MOSFET self-sufficient for both functions, the patent eliminates the constraint of needing low internal resistance and allows the device to serve its own sensing needs without external components.

Inventive Principle:

Principle #25Self-service

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 the size of the electronic package, enhances efficiency, and avoids heat loss, while providing cost-effective current sensing capabilities in power tools by using bias resistors and high-side FETs as regular power MOSFETs.

Implementation Method 1

The electronic processor is configured to measure a voltage at a terminal of the field effect transistor and determine the current flowing through the field effect transistor based on the voltage

Methodology Applied

Scientific EffectField effect transistor voltage measurement: Ohm's Law

Implementation Method 2

A connection point between the current sense terminal and the bias resistor is used for measuring the voltage

Methodology Applied

Scientific EffectVoltage division: Ohm's Law

Data Source

PatentUS12105149B2Current sensing in power tool devices using a field effect transistor

Publication Date: 2024.10.01 MILWAUKEE ELECTRIC TOOL CORP

AI summary

Power tool devices described herein include a housing, a power source interface, a field effect transistor within the housing connected between the power source interface and a load of the power tool device, and an electronic processor coupled to the field effect transistor. The electronic processor is configured to control the field effect transistor to drive the load and measure a voltage at a terminal of the field effect transistor. The electronic processor is also configured to determine the current flowing through the field effect transistor based on the voltage without using a shunt resistor.