Cascoded Transistor Switch Biasing for High-Voltage SOA Compliance
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Solution Overview
Problem
Low voltage devices used in high voltage operations face Safe Operating Area (SOA) violations and increased leakage currents, making low power design challenging.
Innovation Solution
A cascoded transistor switch design using three low voltage NMOS and PMOS devices in series, with biasing mechanisms to extend operating voltage and reduce leakage currents, ensuring well-defined logic states and improved reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If low voltage devices are used in high voltage operations, then device cost and availability are improved, but Safe Operating Area violations occur and device reliability deteriorates
Solution Approach 1:
The patent divides a single high-voltage switching function into multiple low-voltage transistor stages connected in series (cascoded configuration). Each transistor operates within its safe voltage range, collectively handling the total high voltage through segmentation of the voltage stress across individual devices.
Solution Approach 2:
The patent introduces intermediate voltage tapping points and biasing circuits between the low-voltage transistors in the cascoded stack. These intermediaries control the voltage distribution across each transistor stage, ensuring none exceeds its maximum rating while collectively blocking the full high voltage.
2Adaptability or versatility
If low voltage devices are used in high voltage operations, then device cost and availability are improved, but leakage current increases and power consumption worsens
Solution Approach 1:
The patent employs feedback mechanisms through biasing circuits that monitor the voltage at intermediate tapping points and adjust the gate voltages of individual transistors accordingly. This feedback ensures optimal switching behavior and minimizes leakage currents in each stage of the cascoded structure.
Solution Approach 2:
The patent dynamically adjusts the gate-source voltage parameters of each transistor in the cascoded stack based on the operating conditions and voltage distribution. By optimizing these parameters, the transistors achieve low leakage in the off-state while maintaining adequate voltage blocking capability.
3Reliability
If cascoded transistor structure is implemented, then voltage handling capability and reliability are improved, but device complexity increases
Solution Approach 1:
The patent designs the cascoded transistor stack where each transistor serves multiple functions: voltage blocking, current switching, and leakage control. The intermediate tapping points serve dual purposes of voltage distribution and biasing reference, reducing the need for separate control circuits.
Solution Approach 2:
The patent combines the voltage blocking function, switching function, and biasing function into a single integrated cascoded transistor structure. Multiple transistors are merged in series with shared control signals and common substrate connections, achieving high voltage capability without proportionally increasing control complexity.
Data Source
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AI summary
A power switch includes a first transistor having a first current electrode corresponding to an output of the switch, a second transistor having a first current electrode coupled to a second current electrode of the first transistor, and a third transistor having a first current electrode coupled to a second current electrode of the second transistor and a second current electrode coupled to a first power supply voltage terminal which provides a first power supply voltage. The second current electrodes of each of the first, second, and third transistors are body-tied. An analog switch is either couples the second current electrode of the second transistor or a first reference voltage to a control electrode of the second transistor based on a control signal. A voltage selector circuit either couples the second current electrode of the first transistor or the first reference voltage to a control electrode of the first transistor.