Four-Level Power Converter With Split-Frequency Switch Topology
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Solution Overview
Problem
Four-level NPC power converters face increased device count and losses due to switches rated for different voltages, with high-frequency switched devices incurring unwanted losses.
Innovation Solution
A power converter design with a DC link and phase legs comprising semiconductor switches, where inner switches are bidirectional and rated for less than ⅔ of the DC voltage, switching at high frequencies for low switching losses, and outer switches switching at low frequencies for reduced conduction losses, maintaining a similar device count to three-level converters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If four-level NPC converter topology is used to increase voltage levels and reduce noise, then AC voltage quality is improved, but device count increases leading to higher complexity and losses
Solution Approach 1:
The converter is segmented into three distinct switch series (upper, inner, lower) with different voltage ratings and switching frequencies. The upper and lower switches are rated for VDC/2 and switch at low frequencies, while the inner switches are rated for VDC/3 and switch at high frequencies. This segmentation allows each component to be optimized for its specific function, achieving four-level output without proportionally increasing total device count.
2Strength
If switches with different voltage ratings are used in four-level converter, then voltage stress is reduced on individual devices, but switching losses increase due to high-frequency operation of VDC/3 rated switches
Solution Approach 1:
Different parts of the converter have different local qualities: the inner switch series operates at high frequency with VDC/3 voltage rating optimized for switching performance, while the upper and lower switch series operate at low frequency with VDC/2 voltage rating optimized for conduction performance. This local optimization allows each switch series to be tailored to its specific operational requirements, minimizing total losses.
3Ease of manufacture
If all devices are rated for the same voltage rating in three-level ANPC converter, then device selection is simplified, but switching losses increase for high-frequency switched devices
Solution Approach 1:
The invention changes the voltage rating parameter distribution across different switch series. Instead of uniform VDC/2 rating, the inner switches are rated for VDC/3 while upper and lower switches are rated for VDC/2. This parameter change enables the inner switches to operate in their optimal switching region with reduced switching losses, while the outer switches handle the higher voltage at low frequency where conduction losses dominate.
Data Source
AI summary
Power converter for converting between a DC voltage and a AC voltage. The power converter may include: a DC link with a series of three capacitors, the outer nodes of the series forming an upper and a lower DC terminal and connection points between the capacitors forming an upper and a lower intermediate voltage node; and one or more phase legs. Each phase leg includes: an upper switch series between the upper DC terminal and the lower intermediate voltage node, with two semiconductor switches; a lower switch series between the lower DC terminal and the upper intermediate voltage node, with two semiconductor switches; and an inner switch series between the midpoints of the upper and the lower switch series, the inner switch series comprising two semiconductor switches, the midpoint forming an AC terminal of the power converter, wherein the semiconductor switches of the inner switch series are bidirectional semiconductor switches.