Bootstrap Capacitor Sharing for Full Gate Drive in Multilevel DC-DC Converters
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Solution Overview
Problem
Multilevel DC-DC converters face challenges in maintaining full gate drive for high-side primary switches, particularly when operating in full duty cycle mode, leading to channel resistance degradation due to inadequate voltage supply from bootstrap capacitors.
Innovation Solution
The method involves alternately sharing and transferring the bootstrap capacitor voltage between the control gates of high-side primary switches through secondary switching circuitry, ensuring full gate drive and avoiding channel resistance degradation by using high and low drive outputs to drive the switches in various duty cycle modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a bootstrap capacitor is used to drive high-side primary switches, then the switches can be operated, but channel resistance degradation occurs due to inadequate voltage supply in full duty cycle mode
Solution Approach 1:
The patent divides the single bootstrap capacitor into multiple separate bootstrap capacitors, each dedicated to driving a specific high-side primary switch. This segmentation ensures that each switch receives adequate voltage supply independently, preventing channel resistance degradation even in full duty cycle mode where the duty cycle exceeds 50%.
Solution Approach 2:
The patent introduces intermediate coupling capacitors that transfer voltage between different parts of the circuit. These coupling capacitors act as intermediaries to ensure proper voltage levels are maintained at the gates of high-side switches, resolving the voltage supply inadequacy without requiring complex additional capacitor networks.
2Reliability
If additional capacitors are added to provide full gate drive, then channel resistance degradation is prevented, but device complexity and cost increase
Solution Approach 1:
The patent designs the bootstrap capacitor network and coupling capacitors to serve multiple functions simultaneously. The same capacitor structures provide both voltage storage and voltage transfer functions, eliminating the need for separate dedicated components and reducing overall circuit complexity while ensuring full gate drive for all high-side switches.
3Productivity
If duty cycle exceeds 50%, then converter operates in full duty cycle mode, but bootstrap capacitor cannot maintain adequate voltage for gate drive
Solution Approach 1:
The patent implements dynamic voltage management through the coupling capacitors that actively transfer charge between bootstrap capacitor sections during different phases of the switching cycle. This dynamic charge redistribution ensures that each high-side switch receives adequate gate drive voltage regardless of the duty cycle percentage, enabling reliable operation from 0% to 100% duty cycle.
Data Source
AI summary
Methods and apparatus for bootstrap capacitor sharing in multilevel DC-DC converters are disclosed. In one example, a bootstrap capacitor voltage of the bootstrap capacitor can be alternately shared between respective control gates of a first high side primary switch and a central high side primary switch of the multilevel DC-DC converter at different times during a duty cycle of the multilevel DC-DC converter. In another example, the bootstrap capacitor voltage can be transferred to drive respective control gates of the first and central high side primary switches and can ensure full gate drive of the first and central high side primary switches to avoid channel resistance degradation thereof, even when the multilevel DC-DC converter is operated in a substantially full duty cycle mode.