Compound Switch Reduces Power Loss in High Voltage Transistors
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Solution Overview
Problem
High voltage switching transistors with normally-on characteristics face challenges in being turned off effectively, leading to potential short circuits and increased power losses due to parasitic capacitance charging and voltage stress, especially when used in cascode configurations with normally-off transistors.
Innovation Solution
A method involving a compound switch configuration where a normally-on transistor is coupled in series with a normally-off transistor, allowing simultaneous shutdown of both transistors during reverse conduction, reducing power loss by charging the gate-source capacitance of the normally-on transistor with a negative supply voltage and using a dual-drive approach to minimize voltage stress and switching losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a normally-on transistor is used for high voltage switching, then the device can operate at very high voltages without breakdown, but the device cannot be turned off effectively leading to potential short circuits and increased power losses
Solution Approach 1:
The patent divides the single normally-on transistor into two separate transistors: a normally-on transistor for voltage blocking and a normally-off transistor for reliable switching. This segmentation allows each transistor to specialize in one function, resolving the contradiction between voltage withstanding capability and switch-off reliability.
Solution Approach 2:
The normally-off transistor acts as an intermediary between the control signal and the normally-on transistor. It mediates the switching action, allowing the normally-on transistor to maintain its voltage blocking capability while the normally-off transistor provides reliable turn-off control.
2Reliability
If a negative voltage is generated to turn off the normally-on transistor, then the transistor can be turned off, but the circuit complexity increases due to additional voltage generation requirements
Solution Approach 1:
The normally-off transistor self-regulates the switching control without requiring external negative voltage generation circuits. It naturally turns off when the gate-source voltage drops below its threshold, eliminating the need for complex negative voltage generation and reducing overall circuit complexity.
3Reliability
If the normally-on transistor is operated in series with a normally-off transistor, then overall normally-off behavior is achieved, but power losses increase due to parasitic capacitance charging and voltage stress
Solution Approach 1:
The patent applies preliminary action by pre-charging the gate-source capacitance of the normally-on transistor to the negative supply voltage before switching operations. This preliminary charging reduces the capacitance charging/discharging losses during normal switching, thereby reducing power loss while maintaining the series configuration benefits.
4Loss of energy
If a dual-drive approach is used to minimize voltage stress, then switching losses are reduced, but the control circuit complexity increases
Solution Approach 1:
The patent implements partial dual-drive action by applying negative voltage to the gate of the normally-on transistor only during specific critical switching transitions when voltage stress is highest. During normal operation, the normally-off transistor handles control independently. This partial application of dual-drive reduces switching losses during critical moments while avoiding the full complexity of continuous dual-drive control.
Data Source
AI summary
In accordance with an embodiment, a method includes conducting a reverse current through a first switch that includes a normally-on transistor coupled in series with a normally-off transistor between a first switch node and a second switch node. While conducting the reverse current, the first switch is turned-off by turning-off the normally-off transistor via a control node of the normally-off transistor and reducing a drive voltage of the normally-on transistor by decreasing a voltage between the control node of the normally-on transistor and a reference node of the normally-on transistor. After turning-off the first switch, a second switch coupled to the first switch is turned on.