Bidirectional MOSFET Switch with Galvanic Isolation for Multiplexers
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Solution Overview
Problem
Existing bidirectional MOSFET switch circuits face challenges with high-voltage signal switching due to galvanic coupling of control and signal voltages, requiring large control currents, and are complex to implement in high-density multiplexers, especially for safety and insulation requirements in testing and measurement technology.
Innovation Solution
A bidirectional MOSFET switch with a junction FET transistor for galvanic isolation and a potential-free voltage source to generate a control current, allowing low control current requirements and efficient switching with reduced power losses, using a depletion layer FET transistor to create a high-impedance state for switching and a flip-flop for state storage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If photovoltaic control is used for galvanic isolation, then control current is prevented from being superimposed on signal current, but a relatively large control current is required to drive the gates of T1 and T2
Solution Approach 1:
A capacitor is introduced as an intermediary energy storage element between the photovoltaic control circuit and the MOSFET gates. The capacitor accumulates energy from the photovoltaic source and releases it rapidly to charge the MOSFET gates, eliminating the need for continuous large control current while maintaining galvanic isolation.
Solution Approach 2:
The capacitor is pre-charged during the period when the switching element is non-conductive, storing energy in advance. When switching is required, the pre-stored energy is immediately released to quickly charge the MOSFET gates, enabling fast switching without requiring continuous high control current.
2Reliability
If individual electrically isolated control is implemented for each pair of transistors in a multiplexer, then galvanic isolation is maintained, but the structure becomes technically complex and packing density is reduced
Solution Approach 1:
A single shared capacitor is designed to serve multiple MOSFET pairs in the multiplexer. The capacitor can be selectively connected to different MOSFET gate pairs through switching mechanisms, providing galvanic isolation for all channels while using only one isolation component instead of one per channel.
Solution Approach 2:
Multiple individual isolation capacitors that would traditionally be required for each MOSFET pair are merged into a single shared capacitor. This consolidation reduces component count, simplifies the multiplexer structure, and improves packing density while maintaining the necessary galvanic isolation through selective connection.
3Reliability
If photovoltaic control is used, then galvanic isolation is achieved, but power losses increase due to losses in the coupling path between LED and receiver diode
Solution Approach 1:
The capacitor stores energy during periods when switching is not required, accumulating energy in advance. When switching is needed, the pre-stored energy is released rapidly, eliminating the need for continuous power transmission through the lossy photovoltaic coupling path and reducing overall power losses.
4Ease of operation
If control voltage is galvanically coupled to signal voltage, then the control voltage must be within the transistor specification range, but this is technically difficult with high-voltage signals
Solution Approach 1:
The capacitor acts as an intermediary that decouples the control voltage from the signal voltage. It allows the control circuit to operate at low voltages within transistor specifications while still being able to control MOSFETs switching high-voltage signals, by transferring energy rather than direct voltage coupling.
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
Enables quick switching of MOSFET transistors with reduced power losses and simplified circuit design, improving scalability and packing density in multiplexers by minimizing the need for extensive insulation and reducing control current requirements.
Implementation Method 1
The depletion layer FET transistor is thus set up to convert the control current into a gate-source voltage by means of current saturation
Implementation Method 2
a control input that is galvanically isolated by means of potential isolation and is connected to a control unit that is set up to switch a control current for a junction FET transistor via a further MOSFET transistor
Data Source
Figure 1~3
Figure 4~6
AI summary
The invention relates to a bidirectional MOSFET switch comprising an input (A) and an output (B) and two MOSFET transistors (T1, T2) connected to each other at their source and gate terminals, wherein the input (A) and the output (B) are each connected to a drain terminal of the two MOSFET transistors (T1, T2), a control input (D) galvanically isolated by means of a potential isolation (I1) which is connected to a control unit (C1) which is configured to switch a control current for a FET transistor (T3) via a further MOSFET transistor (T4), which is configured to generate a gate voltage Vgs between the gate (G) and source (S) of the two MOSFET transistors (T1, T2) for switching them, and a potential-free voltage source (V1) which is galvanically connected to the input (A) and is configured to supply a gate control current for the two to generate MOSFET transistors (T1, T2).