Back-to-Back MOSFET Isolation Circuit for Controlled Node Switching
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Solution Overview
Problem
Existing electrical isolation techniques, such as opto-couplers and transformers, are either costly, consume significant space, or do not provide adequate control over the isolation and connection of circuit nodes, particularly for power supply reference nodes and system ground in varying operational phases.
Innovation Solution
A circuit configuration using a pair of N-channel MOSFETs in a back-to-back connection, driven by a PNP bipolar junction transistor, allows for controlled isolation or connection of a power supply reference node to system ground through a control signal, enabling selective isolation or connection based on logic levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If opto-couplers or transformers are used for electrical isolation, then isolation reliability is improved, but device cost and space requirements increase
Solution Approach 1:
The patent replaces traditional mechanical/electromagnetic isolation devices (opto-couplers, transformers) with a solid-state electronic isolation circuit using MOSFETs and BJTs. This substitution maintains isolation functionality while eliminating the need for bulky, expensive components, achieving both reliability and compactness.
Solution Approach 2:
The patent changes the isolation mechanism from electromagnetic coupling (transformers) or optical coupling (opto-couplers) to field-effect transistor control. By using MOSFETs with high input impedance and controlled gate voltages, the circuit achieves electrical isolation through parameter control rather than physical separation, reducing component size and cost.
2Reliability
If traditional isolation devices are used, then isolation is provided, but control flexibility over isolation and connection states is reduced
Solution Approach 1:
The patent implements dynamic control of isolation states through voltage-controlled MOSFETs. The isolation circuit can be rapidly switched between connected and isolated states by changing the control voltage applied to the MOSFET gates, providing real-time adaptability that static isolation devices cannot achieve.
Solution Approach 2:
The patent introduces control voltage signals as intermediaries between the control logic and the isolation circuit. These voltage signals mediate the transition between isolation and connection states, enabling precise and flexible control over the isolation behavior without requiring mechanical switches or complex relay circuits.
3Device complexity
If MOSFETs are used for isolation control, then device cost and space are reduced, but control precision and reliability may be compromised
Solution Approach 1:
The patent uses a complementary pair of MOSFETs (one N-channel, one P-channel) that mirror each other's function. This redundancy ensures that if one transistor fails or exhibits variability, the other can compensate, maintaining isolation reliability while keeping the circuit compact and cost-effective.
Solution Approach 2:
The patent applies different MOSFET types (N-channel and P-channel) in different positions within the circuit to optimize local characteristics. The N-channel MOSFET handles one direction of control while the P-channel handles the opposite direction, ensuring reliable isolation control in both states while maintaining small device footprint.
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 solution provides efficient and flexible control over the isolation and connection of circuit nodes, reducing costs and space requirements while maintaining effective isolation and current flow management.
Implementation Method 1
An isolation circuit includes a pair of N-channel metal oxide semiconductor field effect transistors (MOSFETs) in a back-to-back connection. The N-channel MOSFETS may be driven by a PNP bipolar junction transistor (BJT). A control signal applied to the BJT emitter controls the operation of the pair of MOSFETs.
Data Source
AI summary
A circuit to isolate a first circuit node from second circuit node at certain times yet connect the first circuit node and second circuit node at other times. For example, the isolation circuit may isolate a reference node from a system ground during certain phases of operation, but temporarily connect the reference node to the system ground during other phases. An isolation circuit of this disclosure may include a pair of MOSFETs in a back-to-back connection. The MOSFETs may be placed between the two nodes to be isolated. The MOSFETS may be driven by a bipolar junction transistor (BJT). A control signal applied to the BJT emitter controls the operation of the pair of MOSFETs. The isolation or connection from the power supply reference node to system ground may be controlled by applying a HIGH or LOW logic signal to the PNP transistor emitter.