Cascode Switch Voltage Clamp for Midpoint Drift Control
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Solution Overview
Problem
High breakdown voltage switch devices using a cascode configuration of lower-rated transistors suffer from midpoint voltage drift under static DC blocking conditions, leading to the normally-on transistor turning on and potentially damaging the normally-off transistor.
Innovation Solution
Incorporating a voltage clamp circuit with a normally-off clamp transistor and diodes between the gate and drain of the normally-off power transistor, along with capacitors and pulldown devices, to stabilize the midpoint voltage and prevent false turn-on events.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a cascode configuration with normally-on and normally-off power transistors is used to achieve high breakdown voltage, then the voltage rating is improved, but the midpoint voltage stability deteriorates under static DC blocking conditions
Solution Approach 1:
A voltage clamp circuit is introduced as an intermediary component between the normally-on and normally-off transistors. This clamp circuit, comprising a clamp transistor and diode, actively regulates the midpoint voltage by providing a stable reference potential, preventing voltage drift that would otherwise cause the normally-on transistor to turn on unintentionally.
Solution Approach 2:
The patent modifies the electrical parameters at the midpoint node by introducing the voltage clamp circuit that maintains the midpoint voltage within a specific range. The clamp circuit changes the voltage parameter dynamically to prevent it from drifting into the turn-on region of the normally-on transistor, thereby stabilizing the cascode device operation.
2Reliability
If the midpoint voltage drifts under static DC blocking conditions, then the normally-on transistor turns on, but this causes the entire blocking voltage to transfer to the normally-off transistor which is not rated for such high voltage
Solution Approach 1:
The voltage clamp circuit performs preliminary anti-action by preemptively clamping the midpoint voltage before it can drift into the turn-on region of the normally-on transistor. The clamp transistor and diode are configured to activate beforehand and prevent the harmful voltage transfer condition from occurring in the first place.
Solution Approach 2:
The clamp circuit provides beforehand cushioning by creating a voltage buffer zone at the midpoint. The diode and clamp transistor form a protective barrier that absorbs voltage fluctuations and prevents them from propagating to the normally-off transistor, cushioning it against voltage overload damage.
3Reliability
If a voltage clamp circuit is added to stabilize midpoint voltage, then the reliability is improved, but the device complexity increases
Solution Approach 1:
The voltage clamp circuit is segmented into distinct functional components: a clamp transistor for active voltage regulation and diodes for voltage reference and protection. This segmentation allows each component to perform its specific function efficiently while maintaining modularity, making the added complexity manageable and beneficial for reliability.
Data Source
AI summary
A switch device includes: a first power transistor die including a normally-on power transistor; a second power transistor die including a normally-off power transistor having a drain electrically connected to a source of the normally-on power transistor to form a cascode device; and a capacitor electrically connected between a gate of the normally-on power transistor and a source of the normally-off power transistor. Both power transistors have at most half a maximum rated drain-to-source voltage as the switch device. The second power transistor die further includes a voltage clamp circuit having a normally-off clamp transistor with a drain electrically connected to the gate of the normally-on power transistor and a source electrically connected to the source of the normally-off power transistor, and diode(s) between the drain and a gate of the normally-off clamp transistor. Additional switch devices embodiments are described.