Three-Level Rectifier Switching Branch Voltage Balancing
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Solution Overview
Problem
Three-level rectifiers face an uneven distribution of voltage across components in a reverse blocking state due to impedance differences, leading to undesirable voltage exposure for some diodes, which is not effectively addressed by existing solutions that require additional components like resistors, resulting in increased costs and losses.
Innovation Solution
Controlling each controllable switch in the switching branch to a conductive state during its reverse blocking state, ensuring the voltage stress across diodes is limited to half the total DC intermediate circuit voltage without additional components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional resistors are connected in parallel with series-connected diodes to balance voltage distribution, then voltage balance across diodes is improved, but device complexity and power losses increase
Solution Approach 1:
The invention extracts the voltage balancing function from external additional components (resistors) and transfers it to the existing controllable switches S1 and S2. By controlling these switches to be in conducting state during specific periods, the voltage balance across diodes D1-D4 is achieved without adding extra components to the circuit.
Solution Approach 2:
The existing switches S1 and S2 in the circuit are made to serve dual functions: their primary switching function for rectification and an additional voltage balancing function. During specific periods when diodes are in reverse blocking state, these switches are controlled to conduct, thereby automatically balancing the voltage across the diodes without requiring external assistance.
2Reliability
If additional resistors are connected in parallel with series-connected diodes to balance voltage distribution, then voltage balance across diodes is improved, but power losses increase
Solution Approach 1:
The voltage balancing function is extracted from external resistive components and implemented using the existing controllable switches. This eliminates the need for additional resistors that would continuously consume power, thereby reducing overall system power losses while maintaining voltage balance.
Solution Approach 2:
The switches S1 and S2 are controlled to be in conducting state periodically during specific periods when diodes are in reverse blocking state. This periodic activation achieves voltage balancing only when needed, rather than continuous operation of resistors, thereby minimizing energy losses.
3Device complexity
If diodes are subjected to total DC voltage in reverse blocking state, then device simplicity is maintained, but diode voltage stress increases leading to potential failure
Solution Approach 1:
The invention provides prior protection to the diodes by controlling switches S1 and S2 to be in conducting state during the reverse blocking periods of the diodes. This beforehand action limits the voltage across each diode to at most half the total DC voltage, preventing excessive voltage stress and potential diode failure before it can occur.
Solution Approach 2:
The controllable switches S1 and S2 act as intermediary elements between the DC voltage source and the diodes. By controlling these switches, the full DC voltage is not directly applied to the diodes in reverse blocking state, but rather the voltage is mediated and limited to half the total voltage, thereby protecting the diodes from excessive stress.
Data Source
Figure 1~2
AI summary
A method for controlling a switching branch for a three-level rectifier, and a switching branch comprising a first diode (D1) and a second diode (D2) connected in series, a third diode (D3) and a fourth diode (D4) connected in series, a first controllable switch (S1) connected between a neutral DC output pole (NP) and a connection point between the first and the second diode, and a second controllable switch (S2) connected between the neutral DC output pole (NP) and a connection point between the third and the fourth diode, means (10) for controlling the first controllable switch (S1) to be in a conductive state during a reverse blocking state of the first diode (D1) and the second diode (D2), and means (10) for controlling the second controllable switch (S2) to be in a conductive state during a reverse blocking state of the third diode (D3) and the fourth diode (D4).