PSFB Frequency Modulation for Low-Voltage Output Regulation
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Solution Overview
Problem
Conventional phase-shift full bridge (PSFB) converters face inefficiencies during nominal AC line voltage conditions due to suboptimal duty cycles, leading to increased power dissipating losses and reduced efficiency, while they must maintain output voltage regulation during AC line voltage interruptions.
Innovation Solution
The PSFB converter employs a controller to switch the full-bridge at a nominal switching frequency under nominal AC line voltage conditions and reduces the switching frequency under reduced input voltage conditions to maintain regulation, achieving a more ideal duty cycle and minimizing freewheeling periods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large output capacitor is used to bridge AC line voltage interruptions, then output voltage regulation during interruptions is improved, but duty cycle becomes suboptimal and efficiency deteriorates
Solution Approach 1:
The patent implements dynamic switching frequency adjustment based on input voltage conditions. Under nominal input voltage, the converter operates at a higher switching frequency to optimize duty cycle and reduce losses. When input voltage drops during AC interruption, the switching frequency is reduced to maintain adequate freewheeling time and output voltage regulation. This dynamic adaptation resolves the contradiction by allowing optimal efficiency during normal operation while ensuring reliability during interruptions.
2Reliability
If duty cycle is increased to maintain regulation during AC voltage drop, then output voltage regulation is improved, but efficiency deteriorates due to extended freewheeling periods
Solution Approach 1:
The patent anticipates the problem of extended freewheeling periods by proactively reducing the switching frequency when input voltage drops. This preliminary action prevents the need to increase duty cycle excessively, thereby avoiding the associated circulation losses during freewheeling periods. The controller monitors input voltage and adjusts switching frequency in advance to maintain the balance between regulation and efficiency.
3Reliability
If switching frequency is reduced under reduced input voltage conditions, then adequate freewheeling time is achieved and regulation is maintained, but switching frequency deviates from nominal
Solution Approach 1:
The patent employs dynamic switching frequency control that adapts to input voltage conditions. The controller continuously monitors the DC link voltage and adjusts the switching frequency accordingly. Under nominal input voltage, the converter operates at nominal switching frequency for optimal efficiency. When input voltage drops, the switching frequency is dynamically reduced to provide adequate freewheeling time, ensuring continuous output voltage regulation. This dynamic adjustment resolves the contradiction between maintaining regulation and preserving nominal switching frequency.
Data Source
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AI summary
A phase shift full bridge (PSFB) converter includes: an isolation transformer (110); a full-bridge (112) having a first pair of switch devices (QA, QB) connected in series at a first node (A) coupled to a first terminal of the primary side of the isolation transformer (110), and a second pair of switch devices (QC, QD) connected in series at a second node (B) coupled to a second terminal of the primary side of the isolation transformer (110); a rectifier (114) coupled to the secondary side of the isolation transformer; and a controller (116) for switching the first and second pairs of switch devices out of phase with each other. Under nominal input voltage conditions for the PSFB, the controller switches the first and second pairs of switch devices at a nominal switching frequency. Under reduced input voltage conditions for the PSFB, the controller switches the first and second pairs of switch devices at a frequency lower than the nominal switching frequency.