Cascode Power Switch for Normally ON GaN Transient Protection
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Solution Overview
Problem
Group III-V high electron mobility transistors (HEMTs), such as GaN transistors, are normally ON and require constant control to prevent large current transients that can damage the power switch, load, and/or power source, especially when the controller malfunctions or is not properly connected.
Innovation Solution
A power switch design incorporating a cascode configuration with a normally ON depletion mode transistor in series with a normally OFF enhancement mode transistor, a safety diode, and a controller that dynamically controls the transistors to prevent large current transients, using a floating ground and controller power supply to maintain the normally ON transistor OFF when necessary.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a normally ON depletion mode transistor is used in power switching circuitry, then high breakdown voltage and low ON-state resistance are achieved, but large current transients may be generated that damage the power switch, load, and/or power source when the controller malfunctions or is not properly connected
Solution Approach 1:
The patent segments the single normally ON transistor into a cascode configuration with two transistors: a normally ON depletion mode transistor and a normally OFF enhancement mode transistor. This segmentation allows the depletion mode transistor to provide high breakdown voltage and low ON-state resistance while the enhancement mode transistor blocks current transients when the controller malfunctions, thus resolving the contradiction between transistor performance and protection against harmful current transients.
Solution Approach 2:
The enhancement mode transistor acts as an intermediary protective element between the power source and the depletion mode transistor. When the controller malfunctions or is not properly connected, the enhancement mode transistor remains OFF and prevents large current transients from reaching the depletion mode transistor, load, and power source, while still allowing the depletion mode transistor to function normally when controlled properly.
2Object-affected harmful factors
If constant control is applied to maintain the normally ON transistor OFF, then protection from current transients is provided, but controller complexity and power consumption increase
Solution Approach 1:
The control function is segmented between two transistors with different default states. The enhancement mode transistor provides passive protection by remaining OFF without requiring constant active control, while the depletion mode transistor is controlled only when power delivery is needed. This segmentation reduces controller complexity compared to constantly controlling a single normally ON transistor.
Solution Approach 2:
The enhancement mode transistor is configured to be normally OFF as a preliminary protective action before any switching operation. This preliminary state provides automatic protection against current transients without requiring continuous controller intervention, reducing the ongoing control burden and simplifying the controller's role to only initiating power delivery when needed.
3Reliability
If the controller is disconnected or malfunctions, then the power switch should be protected, but the normally ON transistor would naturally conduct causing damage
Solution Approach 1:
The cascode configuration segments the current path into two independently controllable transistor stages. The enhancement mode transistor segment provides automatic safety by remaining OFF when no control signal is present, preventing current flow even if the controller is disconnected or malfunctioning. This segmentation creates inherent automatic protection without requiring additional safety circuits.
Solution Approach 2:
Instead of using a single normally ON transistor that requires active control to prevent conduction, the patent inverts the approach by using a normally OFF enhancement mode transistor that passively blocks current. This inversion changes the default state from conductive to non-conductive, providing automatic safety when the controller is disconnected or malfunctioning, while still enabling power delivery when needed.
Data Source
AI summary
A power switch comprising: a cascode having a normally ON transistor connected in series with a normally OFF transistor at a cascode node; a safety switch between a gate of the normally ON transistor and a first ground common to the cascode and the switch; and a controller comprising: a controller power supply having a power supply output connected to the cascode node at which output the controller power supply provides a voltage relative to a second ground that is floating relative to the first ground; a first controller output connected to the gate of the normally ON transistor at which the controller generates a first voltage relative to the second ground; wherein the controller power supply is connected to the safety switch and voltage provided by the controller power supply is coupled to and operates to maintain the safety switch OFF and nonconducting and in the absence of the voltage the safety switch is turned ON and conducting.