Parallel MOSFET Load Switch Circuit for Thermal Runaway Prevention
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Solution Overview
Problem
Conventional load switch circuits in portable electronic devices are prone to thermal runaway due to prolonged operation in the saturation region, leading to potential device failure, as the MOSFET transistors heat up during power management, especially when using slow ramp control voltage signals.
Innovation Solution
A load switch circuit utilizing a pair of parallel-connected MOSFET devices with different threshold voltage characteristics and transistor sizing, where a smaller area MOSFET with lower threshold voltage is turned on first to conduct current, followed by a larger area MOSFET with higher threshold voltage, ensuring operation above the drain current crossover point to prevent thermal runaway.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a slow ramp control voltage signal is used to turn on the load switch, then the load does not experience current surge, but the transistor dwells in the saturation region for a long time causing excessive heating and thermal runaway
Solution Approach 1:
The patent divides the single MOSFET switch into two parallel MOSFET switches (first and second MOSFETs) with different threshold voltages. This segmentation allows the switching process to be divided into two phases: the first MOSFET handles the initial turn-on with lower threshold voltage, and the second MOSFET takes over with higher threshold voltage, preventing prolonged saturation region operation and reducing thermal runaway risk.
Solution Approach 2:
The patent applies different threshold voltage characteristics to different MOSFETs in the parallel configuration. The first MOSFET has a lower threshold voltage optimized for initial turn-on with slow ramp control, while the second MOSFET has a higher threshold voltage that activates later, creating localized functional differences that solve the thermal management problem.
2Device complexity
If a single MOSFET is used for load switching, then the circuit is simple, but the transistor operates in saturation region causing thermal runaway
Solution Approach 1:
The patent segments the switching function across two parallel MOSFETs with different threshold voltages. This segmentation prevents either MOSFET from dwelling in the saturation region for extended periods, thereby improving reliability and preventing thermal runaway while maintaining relatively simple circuit topology.
Solution Approach 2:
The first MOSFET acts as an intermediary during the turn-on transition. It initially conducts current with its lower threshold voltage, serving as a mediator that protects the second MOSFET from immediate saturation region stress, thereby improving overall device reliability.
3Object-affected harmful factors
If the control voltage ramps up slowly to prevent current surge, then load protection is improved, but power dissipation increases causing thermal issues
Solution Approach 1:
The patent segments the power dissipation burden across two MOSFETs. The first MOSFET handles initial power dissipation during slow ramp-up, while the second MOSFET activates later with higher threshold voltage, reducing the total time any single device dissipates high power and thereby reducing overall thermal issues.
Solution Approach 2:
The first MOSFET performs preliminary action by initially conducting the load current during the slow ramp-up phase. This preliminary conduction prevents current surge to the load while the second MOSFET remains off, avoiding its saturation region operation and reducing overall power dissipation.
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 ensures thermal stability and avoids thermal runaway by distributing power dissipation evenly across the semiconductor chip, allowing the load switch to operate at low on-resistance while maintaining performance and extending device lifespan.
Implementation Method 1
the first MOSFET device with low threshold voltage and small area is turned on first to conduct current
Implementation Method 2
distributing power dissipation evenly across the semiconductor chip, allowing the load switch to operate at low on-resistance
Data Source
AI summary
A switch circuit includes a first MOS transistor and a second MOS transistor of a same conductivity type connected in parallel between a first terminal and a second terminal. The first and second MOS transistors have respective gate terminals coupled to the control terminal to receive a control signal to turn the switch circuit on or off where the control signal transitions from a first voltage level to a second voltage level at a slow rate of change. The first MOS transistor has a first threshold voltage and the second MOS transistor has a second threshold voltage where the first threshold voltage is less than the second threshold voltage.