Current-Limiting MOSFET Structure for Solid State Relays
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Solution Overview
Problem
Current limiting circuits in power MOSFET devices require a current-sensing resistor across the base to emitter junction of a bipolar transistor, leading to increased power dissipation due to the resistor's resistance and the need for additional sense resistors in bi-directional switches.
Innovation Solution
A current-limiting circuit that eliminates the use of current-sensing resistors by utilizing the ON-resistance of one MOSFET device to sense current for another, with a gate driver controlling both devices to manage current flow, allowing the bipolar transistor to function as both a vertical and a reverse-conducting device.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a current-sensing resistor is used across the base to emitter junction of a bipolar transistor for current limiting, then the current can be limited to protect the MOSFET device, but the resistance of the sense resistor increases power dissipation during conduction
Solution Approach 1:
The invention extracts and eliminates the current-sensing resistor from the circuit by using the intrinsic ON-resistance of the MOSFET channel instead. The bipolar transistor senses the voltage drop across the channel resistance to detect overcurrent conditions, removing the need for an external sense resistor and its associated power losses.
Solution Approach 2:
The MOSFET channel resistance serves dual functions: it acts as both the conduction path for normal operation and as the current-sensing element for protection. This multi-functionality eliminates the need for separate sense resistors in both forward and reverse conducting devices, reducing overall power dissipation.
2Reliability
If two sense resistors are used in a bi-directional current-limiting switch, then current can be limited in both directions, but the power dissipation increases further
Solution Approach 1:
Each MOSFET in the bi-directional switch uses its own channel ON-resistance for current sensing in both forward and reverse directions. This eliminates the need for separate sense resistors for each direction, as the channel resistance itself provides the sensing function bidirectionally.
Solution Approach 2:
The invention merges the current-sensing function with the power conduction path by using the channel resistance for both purposes simultaneously. This consolidation eliminates redundant sense resistors and reduces the total number of components contributing to power losses.
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 configuration reduces power dissipation by eliminating separate sense resistors and increases area efficiency, effectively limiting current without additional power loss, while maintaining protection against high drain currents.
Implementation Method 1
When the voltage drop on the sense-resistor 12 reaches a voltage value VBE of the bipolar transistor of about 0.7 to 0.8 volt, the bipolar transistor becomes conductive
Implementation Method 2
a channel of each of the devices simultaneously conducts a current
Implementation Method 3
a gate driver connected to the gates of the forward- and reverse-conducting devices for controlling the devices
Data Source
AI summary
A current-limiting circuit for limiting rising of a current above a predetermined level. The circuit including forward- and reverse-conducting devices, each device including a MOS and a bipolar transistor, wherein ON-resistance of one of the devices is used instead of a current-sensing resistance for another of the devices; and a gate driver connected to the gates of the forward- and reverse-conducting devices for controlling the devices such that a channel of each of the devices simultaneously conducts a current.