Cascode Transistor Voltage Control via Intermediary Switch
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Solution Overview
Problem
Cascode transistors face issues with reliability due to high drain-source voltage exceeding the withstand voltage of the first switching element, leading to degradation and reduced lifetime, especially at high switching frequencies, and suffer from large switching losses and unstable switching operations due to parasitic inductances and resonance.
Innovation Solution
Incorporating a switch and capacitor in series between the connection node of a normally-off-type and a normally-on-type transistor, allowing external control to stabilize the voltage and reduce capacitance effects, thereby preventing excessive voltage application and minimizing switching losses and oscillations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a cascode transistor uses a normally-off-type transistor and a normally-on-type transistor in combination, then switching performance and operation frequency are improved, but the drain-source voltage may exceed the withstand voltage of the first switching element leading to reliability degradation
Solution Approach 1:
A third switching element is introduced as an intermediary component between the first and second switching elements. This third switch controls the connection between them, preventing excessive voltage from being applied to the first switching element while maintaining the cascode configuration's high-frequency switching capability. The third switch acts as a mediator that protects the first switch from voltage overload.
2Loss of energy
If high switching frequencies are used to reduce power consumption and switching loss, then efficiency is improved, but voltage spikes and resonance due to parasitic inductances cause unstable switching operations
Solution Approach 1:
The third switching element serves as a mediator that stabilizes the voltage at the connection node between the first and second switching elements. By controlling this intermediate node, the third switch suppresses voltage spikes and resonance caused by parasitic inductances, enabling stable high-frequency switching operations with reduced energy loss.
Solution Approach 2:
The control circuit monitors the switching state and adjusts the third switching element accordingly to maintain stable operation. The control circuit receives signals indicating switching status and uses feedback to regulate the third switch, preventing oscillations and ensuring stable high-frequency operation while minimizing switching losses.
3Ease of operation
If the cascode transistor configuration is used with enhancement mode FET and depression mode GaN-HEMT, then normally-off operation is achieved, but high drain-source voltage exceeds the withstand voltage of the enhancement mode FET
Solution Approach 1:
The third switching element is positioned as an intermediary between the enhancement mode FET (first switch) and the depression mode GaN-HEMT (second switch). It protects the enhancement mode FET from excessive drain-source voltage by controlling the voltage distribution, allowing the cascode configuration to maintain normally-off operation while preventing FET breakdown.
Solution Approach 2:
The third switching element provides beforehand protection by preemptively controlling the voltage at the connection node before excessive voltage can damage the first switching element. The control circuit anticipates voltage spikes and uses the third switch to cushion against them, preventing withstand voltage exceedance before it occurs.
Data Source
AI summary
A cascode transistor includes: a first switch; a second switch that has a withstand voltage higher than that of the first switch and is cascade coupled to a drain of the first switch; and a circuit in which a third switch and a capacitor are coupled in series with each other and that is provided between a connection node and a source of the first switch, the connection node being a node at which the first switch and the second switch are coupled to each other.