3L-ANPC Converter Shutdown Sequence for SiC Switch Voltage Stress
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Solution Overview
Problem
Traditional three-level active neutral point clamped (3L-ANPC) converters using silicon carbide (SiC) MOSFETs face increased voltage stress and reduced lifetime due to large commutation loops, leading to potential damage during shutdown events and poor reliability, as the faster switching of SiC MOSFETs results in higher voltage stress and the MOSFET body diodes are not designed to handle high currents for extended periods.
Innovation Solution
A method for shutting down a phase-leg of a 3L-ANPC converter that involves determining if a switch fault has occurred, turning off specific switches in a predetermined sequence to avoid large commutation loops, thereby reducing voltage stress and extending the lifespan of the switches by ensuring current flows through body diodes for a shorter duration, forming small commutation loops and reducing overall loop inductance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If SiC MOSFETs are used for faster switching, then switching losses are reduced, but voltage stress across the switches increases due to commutation loop inductance
Solution Approach 1:
The commutation loop is segmented into a small commutation loop and a large commutation loop. The small commutation loop includes only the essential components (switch, capacitor, and connected busbars) necessary for normal operation, while the large commutation loop includes additional components (unconnected busbars, inductor, capacitor) that are bypassed during normal switching. This segmentation allows the system to use the low-inductance small loop for fast switching while avoiding the high voltage stress that would occur if current commutated through the entire large loop.
2Ease of operation
If large commutation loops are formed, then current can be commutated, but loop inductance increases resulting in higher voltage stress and reduced switch lifetime
Solution Approach 1:
The commutation path is divided into two segments: a small commutation loop for normal operation and a large commutation loop for fault conditions. The small loop provides a low-inductance path that maintains switch reliability during normal commutation, while the large loop remains available but is not used during normal operation, thus avoiding its high inductance from degrading switch lifetime.
Solution Approach 2:
The circuit is pre-configured with both small and large commutation loops before operation begins. The small commutation loop is established as the default path for all normal switching operations, eliminating the need to form or activate the large loop during routine commutation. This preliminary configuration ensures that low-inductance commutation is always the default behavior, protecting switch lifetime while maintaining full commutation capability when needed.
3Ease of manufacture
If MOSFET body diodes conduct current for extended periods, then cost and space are reduced, but thermal and reliability constraints are violated due to inability to handle high current
Solution Approach 1:
The control system is pre-programmed with a shutdown sequence that proactively manages body diode current conduction. Upon detecting a shutdown condition, the controller executes a specific sequence: turns off switches S1 and S4, waits for a predetermined time, turns on switches S2 and S3, waits another predetermined time, then turns off switches S5 and S6, and finally turns off switches S2 and S3. This preliminary action ensures body diodes only conduct for brief intervals during the shutdown transition, preventing thermal overload while maintaining the cost-effective body diode architecture.
Solution Approach 2:
The shutdown sequence implements periodic action by using predetermined wait times between switching operations. These time intervals allow current to naturally decay and body diodes to recover between conduction events, preventing continuous high-current stress. The periodic nature of the switching sequence ensures that body diodes operate within their thermal and current ratings while still achieving complete shutdown.
4Productivity
If all devices are turned off simultaneously during trip event, then shutdown is achieved, but large commutation loop is used causing overvoltage and potential damage
Solution Approach 1:
The shutdown process is segmented into multiple sequential stages rather than a single simultaneous switch-off event. The sequence divides the six switches into groups that are turned off at different times: first group (S1, S4), second group (S2, S3 after intermediate step), third group (S5, S6), and final group (S2, S3). This segmentation ensures that current commutation always occurs through the small commutation loop with low inductance, preventing overvoltage generation even during rapid shutdown.
Solution Approach 2:
The control system prepares and executes a predetermined shutdown sequence that is activated upon trip detection. This preliminary action establishes a safe commutation path before any switches are turned off, ensuring that current always has a low-inductance path through the small commutation loop. The sequence proactively prevents the formation of large commutation loops during shutdown by controlling the timing and order of switch transitions, thereby eliminating overvoltage risk while achieving rapid shutdown.
Data Source
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AI summary
A method for shutting down a phase-leg of a three-level active neutral point clamped converter (100) is provided. The method includes the following steps. A determining step determines if a switch fault has occurred, and if a switch fault has occurred then each switch of the plurality of switches are turned off. If a switch fault has not occurred and a shutdown is requested, then an operating step operates the plurality of switches to turn off a first switch and a fourth switch. A waiting step waits for a first predetermined time period. An operating step operates the plurality of switches to turn on a second switch and a third switch. A waiting step is repeated. An operating step operates the plurality of switches to turn off a fifth switch and a sixth switch. A waiting step is repeated. An operating step turns off the second switch and the third switch.