Inverter Circuit DC Voltage Balancing via Zero-Crossing Switching
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Solution Overview
Problem
Existing NPC inverter circuits face challenges in balancing the DC voltage intermediate circuit, leading to asymmetry and subsequent saturation of AC voltage output inductances, which affects the generation of ideal sinusoidal AC voltage.
Innovation Solution
The method involves detecting asymmetry between intermediate circuit capacitors and adjusting the clocking of the freewheeling circuit's switching elements around zero crossings of AC voltage and current, allowing for energy transfer between capacitor halves to balance the circuit without additional components or amplitude variations in AC voltage half-waves.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If different amounts of energy are removed from the intermediate circuit halves to balance asymmetry, then the DC voltage intermediate circuit is balanced, but a direct current flows that leads to saturation of the inductances connected to the AC voltage output
Solution Approach 1:
The patent applies periodic action by switching off the clocking of bridge branch switching elements around zero crossings of AC voltage and/or current. This periodic interruption occurs at specific moments in the AC cycle (at zero crossings) to prevent harmful DC current accumulation while maintaining the balancing effect on the intermediate circuit capacitors. The switching elements are activated only during portions of the AC cycle where they contribute to balancing without creating harmful DC components.
Solution Approach 2:
The patent converts the potentially harmful effect of energy feedback to the DC circuit into a beneficial balancing mechanism. By detecting asymmetry between intermediate circuit capacitors and selectively switching off clocking during specific portions of the AC cycle, the system uses the natural AC waveform to transfer energy from the capacitor with higher charge to the one with lower charge, thereby balancing the DC link without requiring additional active balancing components.
2Stability of the object's composition
If the clocking of switching elements is adjusted to balance the intermediate circuit, then asymmetry between capacitors is reduced, but energy feedback to the DC circuit increases
Solution Approach 1:
The patent implements periodic action by restricting the switching off of clocking to specific intervals around zero crossings of the AC voltage and/or current. During these brief intervals, energy feedback to the DC circuit is allowed to occur for balancing purposes. During the remainder of the AC cycle, normal clocking continues without excessive energy feedback, thus maintaining a favorable energy efficiency while still achieving the balancing effect when needed.
3Reliability
If switching elements are opened around zero crossings, then direct current is prevented that would saturate inductances, but the complexity of control increases
Solution Approach 1:
The patent employs feedback by detecting the asymmetry between the intermediate circuit capacitors and using this information to control the switching of the bridge branch elements. The control system monitors the voltage levels across the capacitors and adjusts the clocking accordingly, switching off around zero crossings when asymmetry is detected. This feedback mechanism enables automatic adaptation to maintain reliability while managing control complexity through intelligent decision-making based on real-time circuit state.
Data Source
Figure 1~2
Figure 3~4
AI summary
The method involves detecting direct current (DC) voltages or loads applied to intermediate circuit capacitors (C1, C2) of a direct voltage intermediate circuit (2). Timing of switching elements (S1, S2) i.e. half bridge switches, of a bridge branch (10) is changed to zero crossing of alternating voltage and/or current. Timing of switching elements (S3, S4), a bidirectional switch, and diodes (D3, D4) of a free-wheeling circuit (14) is changed when an asymmetry is detected between the detected DC voltages or the loads. An independent claim is also included for an inverter circuit for converting DC voltage into alternating current (AC) voltage.