Cascode Power Switch for GaN Transient Current 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 power switches, loads, and power sources in power switching circuitry, especially when the controller malfunctions or is not properly connected.
Innovation Solution
A power switch configuration using a cascode of a normally ON depletion mode transistor in series with a normally OFF enhancement mode transistor, along with a safety diode and a controller powered by a separate power supply, dynamically controls the ON/OFF states to prevent transient currents, ensuring the normally ON transistor remains OFF unless intentionally turned ON, and includes a floating ground with an offset voltage to manage the transistors' states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a normally ON depletion mode transistor is used in power switching circuitry, then high breakdown voltage and high current density are achieved, but large current transients may be generated that damage the power switch, load, and power source when the controller malfunctions or is not properly connected
Solution Approach 1:
A safety diode is introduced as an intermediary component between the controller power supply output and the cascode node. This diode acts as a protective mediator that prevents harmful current transients from reaching the power switch, load, and power source when the controller fails or is improperly connected, while allowing normal operation when the controller functions correctly.
Solution Approach 2:
The safety diode is pre-configured in the circuit to provide protective cushioning against potential current transient damage. By being in place before any malfunction occurs, it creates a safety mechanism that automatically activates to prevent damage during aberrant situations, such as when the controller is not properly connected or malfunctions.
2Reliability
If constant control is applied to maintain the normally ON transistor OFF, then large current transients are prevented, but the controller must be in constant control and connected to the power source before the load
Solution Approach 1:
The safety diode enables the power switching circuitry to protect itself automatically without requiring constant active control from the controller. When the controller is absent or malfunctioning, the diode self-activates to prevent current transients, allowing the system to protect itself in aberrant situations without human intervention or complex connection sequencing.
3Object-affected harmful factors
If the controller power supply voltage has an absolute value greater than the threshold voltage of the normally ON transistor, then the safety diode can turn OFF the normally ON transistor to protect against transients, but the controller must manage floating ground voltages
Solution Approach 1:
The ground system is segmented into two separate grounds: a first ground connected to the power source and load, and a second floating ground connected to the controller power supply output. This segmentation isolates the controller's voltage references from the power circuit grounds, allowing the safety diode to effectively manage transistor gating without creating ground loops or voltage conflicts.
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 first diode connected between a gate of the normally ON transistor and a first ground common to the cascode and the diode; 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; and 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.