Multilevel DC-DC Converter for Balanced DC Link Voltages
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Solution Overview
Problem
Multi-level converters in systems like UPS systems face challenges in regulating uniform DC voltages across DC links, leading to circulating currents and imbalances in capacitors, particularly in applications with high crest factor loads or half-wave rectified loads.
Innovation Solution
A DC-DC converter apparatus with a switching circuit and control circuit that selectively charges capacitors using inductors and transistors with varying voltage ratings, allowing for balanced voltage regulation across capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If multiple multi-level inverters are connected in parallel to meet load demand, then load capacity is improved, but circulating currents occur due to non-uniform DC voltages
Solution Approach 1:
A DC-DC converter is introduced as an intermediary device between the energy storage device and the multi-level inverter. This converter actively regulates the DC link voltage to match the common DC bus voltage, preventing circulating currents while enabling parallel operation of multiple inverters to meet high load demands.
Solution Approach 2:
The system dynamically adjusts the DC link voltage parameter through the DC-DC converter to maintain uniformity with the common DC bus voltage. By changing this voltage parameter in real-time, the system eliminates the conditions that cause circulating currents while preserving the ability to handle high load demands.
2Stability of the object's composition
If DC link voltage is increased to reduce circulating currents, then voltage uniformity is improved, but voltage stress on switching devices increases
Solution Approach 1:
The voltage regulation function is segmented between two components: the DC-DC converter handles the voltage matching and uniformity maintenance, while the multi-level inverter structure handles the voltage stress distribution across multiple switching devices. This segmentation allows voltage uniformity to be achieved without increasing stress on individual switching devices.
Solution Approach 2:
The DC-DC converter acts as an intermediary that decouples the voltage uniformity requirement from the switching device stress. It provides the necessary voltage regulation to achieve uniformity while the multi-level inverter topology independently manages the voltage stress distribution across its switching devices.
3Stability of the object's composition
If a balancing circuit is used to equalize DC links, then voltage uniformity is improved, but device complexity increases
Solution Approach 1:
The DC-DC converter is designed with multi-functionality: it performs both the primary function of energy transfer from the energy storage device and the secondary function of DC link voltage regulation. This eliminates the need for a separate balancing circuit, reducing overall device complexity while maintaining voltage uniformity.
Solution Approach 2:
The voltage balancing function is merged into the DC-DC converter itself rather than being implemented as a separate balancing circuit. By combining these functions, the system achieves voltage uniformity without the additional complexity of dedicated balancing circuitry.
4Stability of the object's composition
If selective capacitor charging is implemented, then capacitor voltage equality is improved, but switching circuit complexity increases
Solution Approach 1:
The control circuit implements periodic switching actions to selectively charge capacitors based on their voltage levels. By using periodic control pulses rather than continuous complex switching, the system achieves capacitor voltage equality while keeping the switching circuit relatively simple.
Solution Approach 2:
The control circuit uses feedback from capacitor voltage measurements to determine which capacitors need charging. This feedback mechanism enables selective capacitor charging with simple logic: if a capacitor voltage is below the threshold, it receives a charging pulse; otherwise, it remains untouched, achieving voltage equality without complex switching arrangements.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The apparatus effectively balances DC link voltages, reducing circulating currents and enhancing capacitor voltage equality, particularly in multi-level inverter applications.
Implementation Method 1
at least one inductor configured to be coupled in series with the second port of the switching circuit and the energy storage device
Implementation Method 2
a string of capacitors including at least two capacitors coupled in series
Data Source
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AI summary
An apparatus includes a string of capacitors including at least two capacitors coupled in series and a switching circuit including a first port having first and second terminals connected to a first node and a second end node, respectively, of the string of capacitors, and a second port configured to be coupled to an energy storage device. The switching circuit is configured to selectively connect first and second terminals of the second port to a first end node, a second end node, and at least one interconnection node of the string of capacitors. The apparatus also includes at least one inductor configured to be coupled in series with the second port of the switching circuit and the energy storage device and a charging switch configured to directly connect the first terminal of the second port to the second terminal of the second port. The apparatus further includes a control circuit configured to close the charging switch to charge the inductor from the energy storage device in first intervals and to selectively close at least two switches of the switching circuit in second intervals to selectively charge the capacitors of the string of capacitors from the charged at least one inductor.