Scaled Transistor Current Sensing for High-Side FETs
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current current sensing methods for high-side FETs in boost converters, such as p-channel FETs, face challenges in accurately monitoring current due to low switch-on resistance and significant voltage drops, which can lead to inefficiencies and electrical losses, especially when operating in various modes like step-up and down-conversion.
Innovation Solution
A sensor system comprising a FET and a sensor circuit that includes a scaled version of the power FET, an operational amplifier, a variable current source, and a current mirror, which generates a signal indicative of the current flowing through the FET, allowing for precise monitoring even under low voltage drops and in different operational modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional current sensing methods are used for high-side FETs, then current monitoring is achieved, but electrical losses increase due to significant voltage drops across the sensing path
Solution Approach 1:
The patent uses a scaled-down replica FET (third FET) that copies the electrical characteristics of the power FET but operates at lower current levels. This replica FET is driven by the same gate signal as the power FET, creating a proportional current relationship. The sensing circuit measures current through this low-current replica instead of the high-current power FET, eliminating significant voltage drops and electrical losses while maintaining measurement accuracy through the known current scaling ratio.
2Measurement precision
If a sensing circuit is added to monitor current through high-side FETs, then current measurement capability is improved, but device complexity increases
Solution Approach 1:
Instead of adding complex high-side current sensing circuits that can accurately measure current through the power FET, the patent creates a simplified copy system using a scaled replica FET. The replica FET naturally reproduces the gate voltage and current characteristics of the power FET at a reduced scale, allowing standard low-side sensing techniques to be used. This copying approach transforms a complex high-voltage sensing problem into a simple low-voltage measurement while maintaining proportional accuracy.
3Loss of energy
If the FET switch-on resistance is reduced to improve efficiency, then electrical losses decrease, but current measurement becomes more difficult due to smaller voltage drops
Solution Approach 1:
The patent decouples the measurement function from the power switching function by creating a dedicated replica FET for sensing purposes. Even when the power FET has very low on-resistance making voltage drop measurement difficult, the replica FET provides a measurable current signal through its higher effective resistance. The current mirror and scaling circuitry amplify this replica current to a measurable level, enabling accurate current detection regardless of how low the power FET's on-resistance is optimized.
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 enables accurate and efficient monitoring of current through high-side FETs, reducing electrical losses and improving converter efficiency by providing a proportional current measurement that can be used for control and regulation, even in low-power applications and during down-mode operations.
Implementation Method 1
an operational amplifier, a variable current source, and a current mirror, which generates a signal indicative of the current flowing through the FET
Implementation Method 2
a current mirror, which generates a signal indicative of the current flowing through the FET
Data Source
AI summary
A sensor comprises a first transistor comprising a first control terminal, a second transistor that is a scaled version of and connected to the first transistor and comprising a second control terminal, an operational amplifier connected to both the first and second transistors and configured to generate an intermediate signal at an output terminal, a variable current source, a current mirror, a measurement circuit, and a chopper circuit. The first and second control terminals are configured to receive a drive signal. The variable current source is configured to generate a first variable current as a function of the intermediate signal. The current mirror configured to apply a second variable current proportional to the first variable current to the second transistor. The measurement circuit is configured to generate a measurement signal indicative of current through the first transistor. The chopper circuit is configured to shift an offset of the operation amplifier.