Asymmetric Two-Way Switching Element for Battery Protection
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Solution Overview
Problem
Conventional two-way switching elements for secondary batteries are limited in miniaturization and cost reduction due to the need for two MOSFETs of the same size connected in series, which increases internal resistance and power loss.
Innovation Solution
A switching device comprising a first, second, and third MOSFET, where the first MOSFET is the main switch, and the second and third MOSFETs are used to form current paths in two directions through parasitic diodes, allowing reduced chip size and lower on-resistance, with the option to maintain conventional ON resistance for further miniaturization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If two MOSFETs of the same size are connected in series to form a two-way switching element, then the switching element can switch current paths in two directions, but the chip size increases and manufacturing costs increase
Solution Approach 1:
The patent applies asymmetry by using MOSFETs of different sizes (first MOSFET and second MOSFET with different channel widths) in the two-way switching element. The first MOSFET has a larger size to handle higher current during normal operation, while the second MOSFET has a smaller size since it only needs to conduct during reverse polarity protection. This asymmetric design reduces the total chip area compared to using two identical MOSFETs, while still achieving full two-way switching capability.
2Adaptability or versatility
If two MOSFETs of the same size are connected in series to form a two-way switching element, then the switching element can switch current paths in two directions, but the manufacturing cost increases
Solution Approach 1:
The patent reduces manufacturing cost through asymmetric MOSFET sizing. By making the second MOSFET smaller (narrower channel width) than the first MOSFET, the patent uses less semiconductor material and reduces fabrication complexity. The control circuit is also optimized to drive the smaller second MOSFET only when reverse polarity detection occurs, further reducing power consumption and operational costs.
3Reliability
If two MOSFETs of the same size are connected in series, then the switching element can prevent overcharge and overdischarge, but the internal resistance increases and power loss increases
Solution Approach 1:
The patent minimizes power loss through asymmetric MOSFET design. The first MOSFET is sized to have low on-resistance for normal high-current bidirectional operation. The second MOSFET is sized smaller since it only conducts during reverse polarity events, which are transient and lower current. This optimization ensures that during normal charging and discharging operations, the current flows through the optimally sized first MOSFET, maintaining low power loss while still providing reverse polarity protection capability.
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 a significantly smaller two-way switching element with reduced manufacturing costs, maintaining or reducing ON resistance, and allowing efficient charging and discharging of secondary batteries without full current interruption.
Implementation Method 1
The MOSFETs have parasitic diodes and, therefore, even when the MOSFETs are in OFF state, a current path can be formed in a desired direction by use of the parasitic diodes
Data Source
AI summary
Provided is a switching element including: first switching element primarily used for formation of a two-way current path; a second switching element that forms, at the time when the first switching element is turned off, a current path by switching a parasitic diode from another; and a third switching element. The second and third switching elements may be of smaller chip size because they allow a current to flow through them only while the current path of the first switching element is being switched. This contributes miniaturization of the switching element as well as reduction in the ON resistance. Moreover, adoption of the switching element to a protection circuit realizes miniaturization of the protection circuit.