Cascode Switching Device Charge Balancing Capacitor
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Solution Overview
Problem
Cascode switching devices experience internal switching losses and avalanche conditions due to charge imbalance caused by unbalanced parasitic capacitances, limiting their suitability for high frequency operations.
Innovation Solution
A charge balancing capacitor is connected in parallel with the gate-source parasitic capacitance of the high voltage, normally-on transistor and the drain-source parasitic capacitance of the control transistor to ensure that the voltage on the drain-source parasitic capacitance of the control transistor does not reach the avalanche breakdown threshold, allowing for unconditional zero voltage switching (ZVS) operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a cascode structure is used with high voltage normally-on transistors, then on-resistance and junction capacitance are reduced for high frequency operation, but internal switching losses occur due to charge imbalance between parasitic capacitances
Solution Approach 1:
The patent applies preliminary action by pre-charging the drain-source parasitic capacitance of the normally-on high voltage device through a dedicated charge pump circuit before the main switching operation. This ensures that the capacitance is already charged to the required voltage level, eliminating the need for it to discharge through the conduction channel during switching transitions, thereby preventing internal switching losses while enabling high frequency operation
2Loss of energy
If ZVS turn-on is implemented to reduce switching losses, then turn-on losses are reduced, but avalanche conditions occur when drain-source parasitic capacitance charge exceeds the sum of other parasitic capacitances
Solution Approach 1:
The patent implements preliminary action by pre-charging the drain-source parasitic capacitance of the normally-on device before ZVS turn-on occurs. The charge pump circuit activates in advance to ensure this capacitance is fully charged, so when ZVS turn-on happens, the capacitance does not need to discharge through the conduction channel, preventing avalanche breakdown while maintaining the benefits of reduced turn-on switching losses
Solution Approach 2:
The patent introduces an intermediary charge pump circuit that acts as a mediator between the power supply and the parasitic capacitances. This charge pump serves as an intermediate energy storage mechanism that balances the charge distribution among parasitic capacitances, preventing charge imbalance conditions that would otherwise lead to avalanche breakdown while enabling safe ZVS operation
3Loss of energy
If resonant current is used to discharge junction capacitance for ZVS, then turn-on losses are reduced, but full discharge cannot be achieved when capacitance is not fully discharged before turn-on
Solution Approach 1:
The patent applies preliminary action by using a charge pump circuit to pre-charge the drain-source parasitic capacitance of the normally-on device to the required voltage level before the main switching operation begins. This preliminary charging action ensures that when the device switches, the capacitance is already in the correct state, eliminating the need for incomplete discharge through resonant current and ensuring complete charge management without additional losses
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 solution enables cascode switches to operate unconditionally under ZVS conditions, avoiding avalanche conditions and reducing switching losses, making high voltage, normally-on transistors suitable for high frequency applications by ensuring full discharge of parasitic capacitances.
Implementation Method 1
a charge balancing capacitor connected in parallel with a parallel connection of the gate-source parasitic capacitance of the high voltage, normally-on transistor and the drain-source parasitic capacitance of the control transistor
Implementation Method 2
The fundamental principle of ZVS turn-on is to provide resonance between a circuit inductance and a (possibly parasitic) capacitance and use the resonant current to discharge the junction capacitance of the high voltage switching device to zero volts prior to the arrival or assertion of the driving signal
Data Source
AI summary
In a cascode switching device, avalanche breakdown of a control transistor and loss of soft switching or zero voltage switching in a high voltage normally-on depletion mode transistor having a negative switching threshold voltage and the corresponding losses are avoided by providing additional capacitance in parallel with a parallel connection of drain-source parasitic capacitance of the control transistor and gate-source parasitic capacitance of the high voltage, normally-on transistor to form a capacitive voltage divider with the drain-source parasitic capacitance of the high voltage, normally-on transistor such that the avalanche breakdown voltage of the control transistor cannot be reached. The increased capacitance also assures that the drain source parasitic capacitance of the high voltage, normally-on transistor is fully discharged before internal turn-on can occur.