Flying-Capacitor Multilevel Converter Voltage Balance Control
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Solution Overview
Problem
Conventional voltage balance control methods for flying-capacitor multilevel converters face challenges in accurately controlling voltage balance, especially near AC zero-crossing points or under light load conditions, due to ripple currents and measurement errors, which limit their application.
Innovation Solution
A voltage balance control device and method that includes a control signal processor, voltage/current detecting unit, capacitor voltage balance controller, current direction forecasting unit, and computing unit, which generate control signals to maintain anticipated voltage values by adjusting duty cycles based on current direction forecasting and feedback signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional voltage balance control methods are used, then the control system is simple, but voltage balance accuracy deteriorates near AC zero-crossing points or under light load conditions due to ripple currents and measurement errors
Solution Approach 1:
The patent applies preliminary action by forecasting the current direction before it actually flows, using past current direction information and voltage change amounts to predict future current direction. This allows the control system to prepare appropriate duty cycle adjustments in advance, ensuring accurate voltage balance control even during transitional periods like AC zero-crossing points or light load conditions where conventional methods fail due to ripple currents and measurement errors.
Solution Approach 2:
The patent implements feedback by continuously monitoring the voltage change amounts of flying capacitors and using this information to adjust duty cycles of switch elements. The control system feeds back the voltage balance status and current direction information to dynamically adjust switching duties, ensuring that voltage balance accuracy is maintained under all operating conditions including challenging scenarios where conventional methods deteriorate.
2Measurement precision
If duty cycle adjustments are made frequently to maintain voltage balance, then voltage balance is improved, but switching losses increase
Solution Approach 1:
The patent applies partial action by making duty cycle adjustments only when and where needed based on forecasted current direction and voltage balance status. Rather than continuously adjusting all switch elements, the system selectively adjusts only those switches that will have the most impact on voltage balance during the predicted current direction, thereby maintaining precise voltage balance control while minimizing unnecessary switching operations and reducing switching losses.
3Measurement precision
If the control system uses complex algorithms to improve voltage balance accuracy, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent introduces an intermediary element - the current direction forecasting unit - that simplifies the control logic by providing predicted current direction information. This intermediary allows the control system to make informed duty cycle adjustments without requiring complex real-time analysis of ripple currents and measurement errors, thereby achieving high voltage balance measurement accuracy while keeping the overall control system complexity manageable through modular functional units.
Data Source
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AI summary
ABSTRACT OF THE DISCLOSURE A voltage balance control device (1a) and a voltage balance control method for a flying-capacitor multilevel converter (1) are provided. A current direction forecasting unit (12) acquires a voltage change amount of any selected flying capacitor (Cm) of the flying-capacitor multilevel converter (1), and receives a feedback signal of two adjacent switch elements (Qm and Qm+1) corresponding to the selected flying capacitor (Cm). A computing result is generated according to an average value or a cumulative value of the feedback signal in the adjusting period. After multiplication and/or division is performed on the voltage change amount and the computing result, the current direction can be forecasted according to the obtained sign. Consequently, the voltage balance of the flying capacitor (C1 to Cp) of the flying-capacitor multilevel converter (1) can be achieved.