GaN Cascode Transistor Gate Protection Circuit
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Solution Overview
Problem
Gallium nitride (GaN) power transistors, being depletion mode and normally-on, can experience destructive gate current flow, necessitating additional circuitry like antiparallel diodes, which is disadvantageous and limits interchangeability with silicon MOSFETs, and attempts to make them normally-off result in device performance penalties.
Innovation Solution
A cascode transistor circuit integrating a gallium nitride or silicon carbide field effect transistor with a silicon MOSFET, including an active control circuit and storage capacitor, provides protection against negative current and other faults by using a comparator and OR gate to control the MOSFET, allowing for intelligent protection mechanisms within an integrated circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If additional layers are introduced in the layer stack to make the device normally-off, then interchangeability with MOSFETs is improved, but device performance deteriorates
Solution Approach 1:
The device is segmented into two separate transistors: a GaN HEMT for high-voltage switching and a silicon MOSFET for protection and control. This segmentation allows each transistor to operate in its optimal performance range without compromising the other, resolving the contradiction between interchangeability and performance by distributing functions across different technology nodes.
Solution Approach 2:
The silicon MOSFET acts as an intermediary between the gate driver and the GaN HEMT. It provides the normally-off characteristic needed for safe operation while allowing the GaN HEMT to maintain its superior switching performance. The MOSFET mediates the control signals and protects the GaN gate from destructive currents.
2Reliability
If an antiparallel diode is added to prevent destructive gate current, then gate protection is improved, but device complexity increases
Solution Approach 1:
The protection function is merged with the control function by using the silicon MOSFET's body diode and threshold voltage characteristics to provide both protection against negative gate current and normal switching control. This eliminates the need for separate antiparallel diodes and complex protection circuits.
Solution Approach 2:
The silicon MOSFET provides self-protection through its inherent characteristics: the body diode blocks reverse current and the threshold voltage prevents gate overcurrent. The device structure itself provides the protection mechanism without requiring additional external components.
3Reliability
If the GaN transistor is used as normally-on device, then device performance is maintained, but gate current protection becomes problematic
Solution Approach 1:
The silicon MOSFET is configured to automatically activate and prevent negative gate current before it can reach destructive levels. The MOSFET's threshold voltage creates a preliminary barrier that stops harmful current flow before it affects the GaN HEMT gate.
Solution Approach 2:
The circuit provides automatic feedback protection where the MOSFET continuously monitors the gate voltage conditions and adjusts its state to prevent harmful currents. When negative voltage appears at the gate, the MOSFET automatically turns on to clamp the voltage and protect the GaN device.
Data Source
AI summary
The invention provides a cascode transistor circuit with a main power transistor and a cascode MOSFET formed as an integrated circuit, packaged to form the cascode transistor circuit. A control and protection circuit is integrated into the integrated circuit together and a storage capacitor provides an energy source to drive the control and protection circuit. A charging circuit is also integrated into the integrated circuit for charging the storage capacitor.