Cascode FET Configuration for Drain Slew Rate and EMI Control
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Solution Overview
Problem
Conventional cascode switch circuits lack control over the slew rate of drain voltage, leading to excessive ringing and high electromagnetic interference, and experience high voltage spikes at the drain terminal of the normally-off FET during switching transitions, resulting in increased power dissipation and instability.
Innovation Solution
A composite device with a wide-bandgap normally-on FET and a normally-off FET connected in a cascode configuration, featuring a capacitive path between the gate and drain terminals, where the current flowing through the gate terminal controls the slew rate of the drain voltage, and a shielding terminal is used to minimize transient voltage spikes at the drain terminal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional cascode configuration is used, then the circuit operates as a normally-off FET device, but the slew rate of drain voltage cannot be controlled resulting in excessive ringing and high electromagnetic interference
Solution Approach 1:
The patent introduces a shielding terminal as an intermediary element connected to the drain of the normally-on FET. This shielding terminal acts as a mediator that provides a controlled capacitive path for voltage transitions, thereby controlling the slew rate of drain voltage and reducing electromagnetic interference without affecting the normal cascode operation.
Solution Approach 2:
The patent modifies the conventional cascode configuration by adding a shielding terminal that changes the capacitive parameters of the circuit. By controlling the capacitance at the shielding terminal, the slew rate of the drain voltage is controlled, which reduces ringing and electromagnetic interference while maintaining the normally-off operation.
2Loss of energy
If conventional cascode configuration is used, then the circuit structure is simple, but high voltage spikes occur at the drain terminal during switching transitions causing high power dissipation
Solution Approach 1:
The shielding terminal serves as an intermediary that absorbs and controls voltage spikes during switching transitions. By providing a dedicated capacitive path through the shielding terminal, the voltage stress on the normally-off FET is reduced, thereby lowering power dissipation during switching events.
Solution Approach 2:
The patent implements beforehand cushioning by pre-configuring the shielding terminal with appropriate capacitance values. This cushioning effect is prepared in advance to absorb voltage spikes during switching transitions, preventing excessive voltage stress and reducing power dissipation before the harmful effects can occur.
3Object-generated harmful factors
If additional circuit components are added to control slew rate, then electromagnetic interference is reduced, but device complexity increases
Solution Approach 1:
The shielding terminal performs multiple functions simultaneously: it controls the slew rate of drain voltage, reduces electromagnetic interference, and provides voltage stress protection during switching transitions. By making this single element multi-functional, the patent avoids adding multiple separate components to control electromagnetic interference.
Solution Approach 2:
The patent merges the electromagnetic interference control function with the existing cascode structure by integrating the shielding terminal into the drain path. This merging approach allows slew rate control and EMI reduction to be achieved without adding independent control circuits or components.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively controls the slew rate of the drain voltage, reduces electromagnetic interference, and minimizes the risk of voltage spikes at the drain terminal, enhancing the stability and reliability of the composite device during switching transitions.
Implementation Method 1
providing a capacitive path across a drain terminal and a gate terminal of the composite device such that current flowing through the gate terminal controls a slew rate of a drain voltage appearing at the drain terminal
Data Source
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AI summary
The present disclosure describes a composite device including first field effect transistor (FET) device and second FET device. First FET device includes first drain, first source, first gate and shielding terminal. First FET is made of wide-bandgap semiconductor material. Second FET device includes second drain, second source, and second gate. First and second FET devices are electrically connected in cascode configuration for providing a capacitive path between drain and gate terminals of composite device such that current flowing through gate terminal controls slew rate of drain voltage appearing at drain terminal. Cascode configuration includes an electrical connection of first drain to drain terminal, an electrical connection of first source to second drain, an electrical connection of second gate to first gate and gate terminal, an electrical connection of shielding terminal to second source, and an electrical connection of second source to source terminal of composite device.