Cascode Amplifier Turn-Off Bias Control for Lower Switching Loss
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Solution Overview
Problem
Modern power supply designs experience significant power loss due to switching losses when transitioning between ON and OFF states, primarily caused by voltage changes while current is flowing, which current technologies have not effectively mitigated.
Innovation Solution
An apparatus comprising a cascode amplifier with a common-gate transistor and a common-source transistor, coupled with a feedback circuit and a bias circuit, that senses the rising drain-voltage and adjusts the gate-voltage to maintain the common-gate transistor in an ON state until a second gate-voltage is applied, allowing for faster switching to an OFF state, thereby reducing switching losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the common-source transistor is switched to OFF state, then the drain-voltage increases, but the common-gate transistor may remain in ON state causing extended switching loss
Solution Approach 1:
The feedback circuit monitors the drain-voltage of the common-source transistor and generates a feedback signal that is processed by the bias circuit to dynamically adjust the gate-voltage of the common-gate transistor. This closed-loop feedback mechanism ensures the common-gate transistor switches to OFF state promptly when drain-voltage increases, minimizing the time period where both transistors are in transition state and reducing switching loss.
Solution Approach 2:
The bias circuit prepares the gate-voltage of the common-gate transistor in advance by continuously monitoring the drain-voltage through the feedback circuit. When the drain-voltage begins to increase (indicating the common-source transistor is turning OFF), the bias circuit proactively adjusts the gate-voltage to ensure the common-gate transistor switches OFF before significant switching loss occurs, rather than waiting for the loss to accumulate.
2Reliability
If the gate-voltage is maintained to keep common-gate transistor in ON state, then the amplifier operates normally, but switching loss increases during transition
Solution Approach 1:
The gate-voltage of the common-gate transistor is dynamically adjusted rather than maintained at a fixed level. The bias circuit continuously modulates the gate-voltage based on real-time feedback from the drain-voltage monitoring, allowing the transistor to transition smoothly between ON and OFF states while minimizing the time spent in the high-loss transition region, thus reducing overall switching loss while maintaining operational reliability.
3Loss of energy
If the common-gate transistor switches to OFF state quickly, then switching loss is reduced, but the capacitance charging time is extended
Solution Approach 1:
The feedback-controlled bias circuit rapidly adjusts the gate-voltage to force the common-gate transistor through the transition to OFF state as quickly as possible, effectively 'rushing through' the capacitance charging phase. This minimizes the time the transistor spends in the transition state where switching loss occurs, accepting that the total charging time may be slightly extended but ensuring the high-loss period is minimized.
Data Source
AI summary
An apparatus for turning off a cascode amplifier having a common-gate transistor and a common-source transistor is disclosed that includes the cascode amplifier, a feedback circuit, and a bias circuit. The feedback circuit is configured to receive a drain-voltage from the drain of the common-source transistor when the common-source transistor is switched to a first OFF state and produce a first feedback signal. The drain-voltage is equal to a source voltage of the common-gate transistor and the drain-voltage increases in response to switching the common-source transistor to the first OFF state. The bias circuit is configured to receive the first feedback signal and produce a bias-voltage. A first gate-voltage is produced from the bias-voltage. The cascode amplifier is configured to receive the first gate-voltage and a second gate-voltage. The common-gate transistor is configured to switch to a second OFF state in response to receiving the second gate-voltage.