Transistor Power Switch Current Sensing With Feedback Sense FET
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Solution Overview
Problem
Conventional current sensing circuits for power transistors, particularly in vertical FET devices, face accuracy issues when monitoring load current due to mismatched threshold voltages and require additional components like charge pumps, which are inefficient and costly.
Innovation Solution
A current sensing circuit that uses a differential amplifier and feedback transistors with a common drain configuration, coupled with a resistor network to accurately sense load current through a sense transistor proportional to the load current, without the need for charge pumps, ensuring high accuracy and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional current sensing circuits are used for vertical FET devices, then current monitoring can be achieved, but measurement precision deteriorates due to mismatched threshold voltages
Solution Approach 1:
The patent introduces a sense transistor as an intermediary device that copies the current through the power transistor. This sense transistor, being identical to the power transistor, experiences the same threshold voltage conditions, thereby eliminating threshold voltage mismatch errors in the current measurement.
Solution Approach 2:
The patent creates a copy of the power transistor (the sense transistor) that replicates the same electrical characteristics and threshold voltage. By measuring the current through this identical copy under the same gate voltage conditions, accurate current sensing is achieved without threshold voltage mismatch.
2Measurement precision
If charge pumps are added to improve current sensing accuracy, then measurement precision improves, but device complexity and cost increase
Solution Approach 1:
The patent makes the sense transistor self-serving by directly using the power transistor's gate voltage to control the sense transistor. This eliminates the need for external charge pumps or additional voltage generation circuits, achieving accurate current sensing through the inherent characteristics of the transistor pair.
3Measurement precision
If a sense transistor identical to the power transistor is used, then measurement precision improves, but loss of energy increases due to continuous operation
Solution Approach 1:
The patent applies partial action by using a sense transistor with the same structure as the power transistor but operating at a scaled-down level. The sense transistor processes only the measurement signal rather than the full power current, achieving accurate sensing with reduced energy consumption compared to a full-power identical transistor.
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 proposed solution provides accurate and efficient current sensing and limiting capabilities, even in vertical FET devices, by maintaining high gate-to-source voltages for both sense and blocking transistors, ensuring reliable protection against overcurrent conditions without the inefficiencies of charge pumps.
Implementation Method 1
a differential amplifier having a first input coupled to one of the first and second current conduction terminals of the first power transistor and having a second input coupled to the other one of the first and second current conduction terminals, and having an output signal responsive to a voltage difference between the first input and the second input
Implementation Method 2
a feedback transistor having another current conduction path coupled in series between the current sense transistor and a monitor node, having a feedback transistor gate terminal coupled to the output of the differential amplifier
Implementation Method 3
a resistor coupled between the monitor node and ground, the sense current flowing through the resistor, the sense current being proportional to the load current flowing through the second power transistor
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
In described examples, an apparatus (600) includes: a first power transistor (B-FET) having a first current conduction path coupled between an input (VIN) for receiving a supply voltage and a node (VMID) and a first gate terminal coupled to a first gate control signal (BGATE); a second power transistor (HS-FET) having a second current conduction path coupled between the node (VMID) and an output terminal (VOUT) for supplying a load current (IL) to a load; and a second gate terminal (HGATE) coupled to a second gate control signal; and a current sense transistor (SENSE FET) having a third gate terminal coupled to the first gate control signal (BGATE)), and outputting a sense current (Isense). The apparatus further includes: a differential amplifier (607) having an output signal, a feedback transistor (FB-FET) having a gate terminal coupled to the output signal of the differential amplifier; and a resistor (RMON) coupled between a monitor node (VMON) and ground.