Cascode Compound Switch Coupling Circuitry for dV/dt Control
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Solution Overview
Problem
Cascode compound switches in high-voltage switching applications experience rapid transitions, leading to excessive electromagnetic interference (EMI) and potential damage to motor windings, due to uncontrolled slew rates, which violate electromagnetic emission regulations and compromise insulation integrity.
Innovation Solution
Incorporating additional slew rate control circuitry, including diode or transistor coupling, to independently control the cascode and switching transistors, allowing for controlled slew rate management during both 'on' to 'off' and 'off' to 'on' transitions, thereby reducing EMI and protecting motor windings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If cascode compound switches are used for high-voltage switching applications, then the switch can conduct large current in on state with high voltage capability, but the switching transitions become excessively rapid causing electromagnetic interference (EMI)
Solution Approach 1:
The patent introduces a driver circuit as an intermediary between the control signal and the cascode compound switch. This driver circuit includes a first stage that controls the low-voltage transistor and a second stage that controls the high-voltage transistor, mediating the switching action to prevent excessive slew rates. The driver circuit acts as a buffer that decouples the control logic from the power switching, allowing controlled transition rates.
Solution Approach 2:
The patent segments the control of the cascode compound switch into two independent stages: a first driver stage controlling the low-voltage transistor and a second driver stage controlling the high-voltage transistor. This segmentation allows independent optimization of each switching event, enabling controlled slew rates while maintaining high-voltage capability. The segmented approach prevents the simultaneous uncontrolled switching that causes EMI.
2Productivity
If switching transitions are made rapid for high power delivery, then power conversion efficiency improves, but displacement currents through parasitic capacitances increase causing EMI
Solution Approach 1:
The patent implements dynamic control of the switching transitions by using a two-stage driver circuit that can independently adjust the switching characteristics of each transistor stage. The driver circuit dynamically manages the gate voltages to achieve optimal switching speed while controlling the rate of change of voltage (dV/dt) across the switch, thereby limiting displacement currents through parasitic capacitances.
Solution Approach 2:
The patent changes the voltage parameters applied to the gate terminals of the transistors in a controlled sequence. The first driver stage changes the voltage to turn on/off the low-voltage transistor, and the second driver stage changes the voltage to turn on/off the high-voltage transistor. This parameter change approach allows control over the switching speed and dV/dt, reducing displacement currents while maintaining power conversion efficiency.
3Speed
If slew rate is increased for faster switching, then switching speed improves, but EMI radiation increases violating electromagnetic emission regulations
Solution Approach 1:
The driver circuit serves as an intermediary that decouples the control signal from the power switching nodes. It provides controlled voltage transitions to the gate terminals, mediating between the fast control logic and the power switch to achieve moderate switching speeds that comply with EMI regulations. The driver circuit's output impedance and timing characteristics act as a buffer that prevents excessive slew rates.
4Strength
If cascode compound switch uses wide-bandgap semiconductor for HVT, then voltage withstanding capability improves, but cost increases compared to silicon-based transistors
Solution Approach 1:
The patent applies local quality by using wide-bandgap semiconductor material specifically for the high-voltage transistor where high voltage capability is required, while the low-voltage transistor can use conventional silicon-based materials. This localized application of advanced materials optimizes performance only where necessary, reducing overall cost compared to using wide-bandgap materials for the entire cascode structure.
Data Source
Figure 1A~1B
Figure 2
Figure 3A~3B
AI summary
A high-voltage (HV) compound switch can include coupling circuitry to help provide better slew rate (dV/dt) control, such as to limit electromagnetic energy radiation during switching, which can cause undesirable EMI. Further, efficiency and on- state resistance can be improved by controllably forward¬ biasing the "normally on" JFET when the compound switch is in an "on" state. In such an on- state, the JFET temperature can be monitored, such as by monitoring the gate- source junction voltage or the gate current of the JFET. Such temperature information can be used for control or other purposes.