Flying Capacitor 3-Level Power Conversion Voltage Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
FC-type three-level power conversion devices face challenges in charging both flying capacitors and filter capacitors without outputting electric current, which can lead to voltage imbalances and inefficiencies, such as excessive voltage application to semiconductor switching elements, waveform distortion, and overheating.
Innovation Solution
The device employs a configuration with semiconductor switching elements connected in series between the filter capacitors and a direct current voltage source, using filter reactors to charge the flying and filter capacitors based on voltage comparisons and command values, allowing current flow through specific paths to maintain optimal voltage levels without external current output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the device outputs electric current to charge flying capacitors and filter capacitors, then the capacitors can be charged to predetermined values, but the device cannot remain at rest without self-discharging and requires separate charging means
Solution Approach 1:
The semiconductor switching elements serve dual functions: they control power conversion during operation and enable capacitor charging during idle periods. The same switching elements that regulate output voltage are used to charge both flying capacitors and filter capacitors, eliminating the need for separate charging circuits and reducing overall system complexity.
Solution Approach 2:
The device charges its own capacitors using its internal power conversion circuitry without requiring external charging equipment. The control unit activates specific switching elements to create charging paths from the power source to the capacitors, allowing the system to self-maintain required voltage levels during idle periods.
2Device complexity
If the flying capacitor voltage deviates from half of DC voltage, then the device structure is simpler, but excessive voltage is applied to semiconductor switching elements causing ripple distortion and overheating
Solution Approach 1:
The control unit continuously monitors the voltage across flying capacitors and compares it against the reference value of half the DC voltage. Based on this feedback, the control unit adjusts the switching patterns of semiconductor elements to correct any voltage deviations, ensuring that switching elements operate within safe voltage limits and preventing ripple distortion and overheating.
3Ease of operation
If the filter capacitor is connected to remove switching ripples, then output voltage quality improves, but inrush current flows from battery to filter capacitor during startup
Solution Approach 1:
Before connecting the battery to the filter capacitor, the control unit activates semiconductor switching elements to pre-charge the filter capacitor to the appropriate voltage level. This preliminary charging action prevents the large inrush current that would otherwise occur when the battery is first connected, while still enabling the filter capacitor to effectively remove switching ripples during normal operation.
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 solution enables the charging of both flying and filter capacitors within the device, preventing inrush currents and maintaining efficient voltage control, thus reducing switching losses and improving the stability of the power conversion process.
Implementation Method 1
a flying capacitor connected between a node between the first and second semiconductor switching elements and a node between the third and fourth semiconductor switching elements; a filter reactor including a first end connected to a node between the second and third semiconductor switching elements; a lower voltage side filter capacitor connected between the negative electrode of the higher voltage side filter capacitor and a second end of the filter reactor
Data Source
AI summary
A flying capacitor (FC)-type 3-level power conversion device turns on or off first to fourth semiconductor switching elements based on comparison between a flying capacitor voltage and a half of higher-voltage side filter capacitor voltage, comparison between the higher-voltage side filter capacitor voltage and the flying capacitor voltage plus a lower-voltage side filter capacitor voltage, comparison between the flying capacitor voltage and the lower-voltage side filter capacitor voltage, and comparison between the lower-voltage side filter capacitor voltage or the higher-voltage side filter capacitor voltage and a filter capacitor voltage command value, so that an electric current flows along a path including a filter reactor L and charging a flying capacitor so as to charge a lower-voltage side filter capacitor or a higher-voltage side filter capacitor to predetermined values.