Neutral-Point-Clamped Three-Level Circuit for Balanced Switch Voltage
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Solution Overview
Problem
Neutral-point-clamped three-level circuits experience uneven voltage distribution between inner and outer switches during freewheeling oscillation, leading to potential overvoltage damage to the inner switch, which is typically addressed by using higher withstand voltage switches, increasing costs without solving the root cause of the voltage imbalance.
Innovation Solution
A control method that adjusts the driving timing of switches to change the direction of freewheeling current, ensuring even voltage distribution between inner and outer switches without altering the circuit structure or using higher withstand voltage switches, by controlling the turn-off timing of switches based on the polarity of the modulation wave and inductor current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If higher withstand voltage switches are used for inner switches, then reliability is improved, but cost increases
Solution Approach 1:
The patent changes the timing parameter of switch turn-off to resolve the voltage distribution problem. By adjusting the turn-off timing sequence and introducing a time delay for the inner switch, the patent achieves uniform voltage distribution across all switches, eliminating the need for higher withstand voltage inner switches and thus reducing cost while maintaining reliability
Solution Approach 2:
The patent applies preliminary action by pre-setting the turn-off timing sequence before the overvoltage condition occurs. The outer switch is turned off first, and the inner switch is turned off after a predetermined time delay, which proactively prevents voltage imbalance and potential overvoltage damage before it can occur
2Ease of operation
If inner switch is turned off after outer switch with fixed time delay, then ease of operation is improved, but voltage distribution uniformity worsens
Solution Approach 1:
The patent transitions from a static fixed time delay to a dynamic adaptive timing control. The turn-off timing is adjusted based on real-time detection of inductor current polarity and magnitude, allowing the control system to adapt to varying operating conditions and maintain uniform voltage distribution dynamically rather than relying on a fixed predetermined delay
3Device complexity
If conventional turn-off sequence is used, then device complexity is reduced, but harmful effects increase
Solution Approach 1:
The patent implements feedback control by continuously detecting the inductor current polarity and magnitude, and using this information to dynamically adjust the switch turn-off timing. This feedback mechanism enables the system to adapt to changing conditions and prevent overvoltage oscillation while maintaining relatively simple control logic
Solution Approach 2:
The patent converts the potentially harmful freewheeling oscillation current into a useful signal for control. By detecting the inductor current polarity and using it to determine the optimal turn-off timing, the patent transforms the oscillation phenomenon from a harmful effect into a basis for achieving uniform voltage distribution and preventing overvoltage damage
Data Source
Figure 1~2a
Figure 2b~2c
Figure 3~4
AI summary
The present application provides a neutral-point-clamped three-level circuit and a control method. By estimating a polarity of a modulation wave, when the modulation wave is in a positive half wave and a trigger signal is received, a first switch, a third switch and a fourth switch are controlled in off state, and a second switch is controlled to maintain on state, where the second switch is turned off after a preset duration and an inductor current is less than or equal to a preset current value; when the modulation wave is in a negative half wave and a trigger signal is received, a first switch, a second switch, and a fourth switch are controlled in off state, and a third switch is controlled to maintain on state, where the third switch is turned off after a preset duration and the inductor current is less than or equal to a preset current value. A problem is solved in the present application with regard to uneven voltage distribution between inner and outer switches after the neutral-point-clamped three-level circuit is shut down, so that switches having lower withstand voltages can be chosen for both the inner and outer switches.