Parallel AC Switch Units for Three-Level Power Converter Surge Reduction
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Solution Overview
Problem
Three-level power converters face challenges in reducing turnoff surge voltages, which can lead to IGBT breakage and increased losses, especially in high-efficiency applications like solar power conditioners and UPS systems, due to high inductance components in commutation loops.
Innovation Solution
The semiconductor device incorporates first and second AC switch units in parallel, reducing the inductance of each commutation loop and eliminating the need for snubber circuits or high-withstand voltage switching elements, by connecting switching elements and AC switch units in one module, thereby minimizing turnoff surge voltages and losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If one uses a conventional three-level power converter configuration (single AC switch unit), then the device complexity is reduced, but the inductance component of the commutation loop increases, causing high turnoff surge voltages that can break IGBTs
Solution Approach 1:
The AC switch unit is segmented into two parallel AC switch units (first AC switch unit with Q3-Q4 and second AC switch unit with Q5-Q6), each handling different commutation paths. This segmentation divides the current path into multiple parallel routes, reducing the effective inductance in each commutation loop and thereby reducing turnoff surge voltages on individual IGBTs
2Reliability
If one adds a snubber circuit or film capacitor to bypass high frequency current, then the IGBT breakage risk is reduced, but the loss at switching increases
Solution Approach 1:
The harmful inductance component is extracted and eliminated by redesigning the commutation path through parallel AC switch units. By taking out the excessive inductance from the system through architectural redesign rather than adding protective components, the solution avoids the switching losses associated with snubber circuits and capacitors while still protecting against IGBT breakage
3Reliability
If one uses a switching element with high withstand voltage to avoid breakage, then the reliability improves, but the price and conduction loss increase
Solution Approach 1:
The approach changes the system-level parameter (commutation loop inductance) rather than changing the component parameter (IGBT withstand voltage). By reducing the inductance parameter through parallel AC switch unit configuration, the solution allows use of standard-voltage IGBTs with lower conduction loss while maintaining reliability through reduced surge voltage stress
4Reliability
If one reduces the switching speed of the IGBT to limit turnoff surge voltage, then the turnoff surge voltage is reduced, but the loss in the switching element increases
Solution Approach 1:
The inductance reduction is performed in advance through the parallel AC switch unit configuration before switching occurs. By preliminarily reducing the commutation loop inductance, the system naturally limits turnoff surge voltages during normal switching operations without requiring slowed switching speeds, thereby maintaining high switching speed and low switching losses
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
This configuration effectively reduces turnoff surge voltages and associated losses, enhancing conversion efficiency without requiring additional snubber circuits or high-withstand voltage elements, thus improving the reliability and efficiency of power converters.
Implementation Method 1
a high frequency current is caused by switching to flow through a commutation loop extending from the direct-current voltage source and returning to the direct-current voltage source via the switching element, thereby generating a high di/dt. -L.di/dt generated by the inductance component of this commutation loop is applied as a turnoff surge voltage to an IGBT in the switching element
Data Source
AI summary
First and second external terminals are connected to high-voltage and low-voltage terminals, respectively, of a direct-current voltage source circuit in which first and second direct-current voltage sources are connected in series. A third external terminal is connected to a connecting point between the first and second direct-current voltage sources. A first switching element is connected between the first and fourth external terminals. A second switching element is connected between the fourth and second external terminals. A first AC switch unit includes third and fourth switching elements connected in inverse series between the third and fourth external terminals. A second AC switch unit includes fifth and sixth switching elements connected in inverse series between the third and fourth external terminals. The first and second AC switch units are connected in parallel. The first and second switching elements and the first and second AC switch units are incorporated in one module.