Cascode Buck Output Stage for Lower MOSFET Voltage Stress
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Solution Overview
Problem
Conventional switching voltage regulators face performance limitations due to the high breakdown voltage requirements of MOSFETs, which lead to increased RDSon and gate charge, resulting in higher losses and reduced efficiency.
Innovation Solution
The use of a cascode output stage with a series connection of MOSFETs, where one switch has a lower breakdown voltage and the other has a higher breakdown voltage, effectively reduces the blocking requirements and utilizes relatively low-voltage switches in high-voltage applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If MOSFETs with high breakdown voltage are used in switching voltage regulators, then the blocking voltage requirement is met, but RDSon and gate charge increase leading to higher losses and reduced efficiency
Solution Approach 1:
The patent divides the high voltage blocking function into multiple MOSFETs connected in series. Each MOSFET handles a portion of the total voltage stress, allowing the use of lower-voltage devices with superior RDSon and gate charge characteristics while collectively achieving the required high breakdown voltage rating.
2Strength
If MOSFETs with high breakdown voltage are used, then the voltage blocking requirement is satisfied, but the gate charge increases exponentially due to larger gate area and thicker oxide
Solution Approach 1:
The total gate charge requirement is segmented across multiple MOSFETs in series. Each device contributes a fraction of the total gate charge, and since each MOSFET operates at lower voltage, their individual gate charges are significantly reduced compared to a single high-voltage device, resulting in lower overall gate charge.
3Strength
If MOSFETs with high breakdown voltage are used, then the voltage blocking capability is adequate, but the RDSon per unit area increases proportionally to the square of breakdown voltage
Solution Approach 1:
The patent segments the voltage blocking function across multiple MOSFETs in series. Each MOSFET experiences only a fraction of the total voltage, allowing the use of devices with lower RDSon per unit area. The combined on-resistance of the series connection is lower than that of a single high-voltage MOSFET, improving overall efficiency.
4Strength
If higher voltage MOSFETs are used, then the blocking voltage is sufficient, but the gate oxide thickness must increase which further increases gate charge
Solution Approach 1:
Instead of using a single MOSFET with thick gate oxide to achieve high breakdown voltage, the patent segments the voltage stress across multiple devices with thinner oxide. Each MOSFET in the series connection requires less gate oxide thickness, simplifying the device structure and reducing gate charge while collectively providing the required voltage blocking capability.
Data Source
AI summary
A buck voltage converter is disclosed. The buck voltage generator includes a controller configured to generate one or more pulse width modulation (PWM) signals, and a plurality of serially connected switches configured to receive the PWM signals and to generate an output voltage signal at an output terminal based on the received PWM signals. The output voltage signal has an average voltage corresponding with a duty cycle of the PWM signals, a first switch of the plurality of serially connected switches has a first breakdown voltage and a second switch of the plurality of serially connected switches has a second breakdown voltage, and the first breakdown voltage is less than the second breakdown voltage.