Flying Capacitor Boost Converter Pre-Charging for Diode Overvoltage
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Solution Overview
Problem
In photovoltaic systems, the flying capacitor in three-level Boost circuits with lower input voltage is not adequately pre-charged, leading to excessive reverse voltage on diodes, risking over-voltage failure and potential damage.
Innovation Solution
A controller is used to monitor and control the DC-DC converter's operation by adjusting the duty cycles of switching transistors and reducing the direct-current bus voltage when the difference between the bus voltage and flying capacitor voltage exceeds the diode's withstand voltage, ensuring safe operation by synchronously charging the flying capacitor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multiple DCDC converters are connected in parallel with different input voltages, then the system can handle varying input conditions, but the flying capacitor in converters with lower input voltage is not adequately pre-charged, causing excessive reverse voltage on diodes
Solution Approach 1:
The controller pre-charges the flying capacitor before enabling the DCDC converter operation. By detecting that the flying capacitor voltage is below a threshold (e.g., half of output voltage), the controller extends the pre-charge period through continued operation of the pre-charge circuit, ensuring the flying capacitor reaches adequate voltage level before the converter switches are activated, thus preventing diode over-voltage failure
Solution Approach 2:
The controller continuously monitors the flying capacitor voltage and uses this feedback to control the pre-charge circuit operation. When the flying capacitor voltage reaches the threshold level, the controller stops the pre-charge circuit; otherwise, it continues charging. This closed-loop feedback mechanism ensures reliable pre-charging under varying input voltage conditions while preventing over-voltage stress on diodes
2Reliability
If the pre-charge circuit operates for an extended period to ensure flying capacitor voltage, then the flying capacitor is adequately pre-charged, but the system startup time is increased
Solution Approach 1:
The pre-charge circuit automatically regulates its own operation duration based on the flying capacitor voltage feedback. The controller enables the pre-charge circuit at startup and automatically disables it when the flying capacitor voltage reaches the threshold level, eliminating the need for manual timing adjustments or external intervention. This self-regulating mechanism ensures adequate pre-charging while minimizing unnecessary extended operation time
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
Ensures the safety of diodes by preventing excessive voltage stress and allows the converter to operate safely under varying input conditions, enhancing power generation efficiency and stability.
Implementation Method 1
a flying capacitor Cf... A second terminal of the flying capacitor Cf is connected to the midpoint of the direct-current bus through the third diode D3
Implementation Method 2
a first diode D1, a second diode D2, a third diode D3... A cathode of the first diode, an anode of the second diode, and a first terminal of the flying capacitor are all connected to a second node
Data Source
AI summary
A three-level flying capacitor DCDC converter, a photovoltaic system and a method for controlling the three-level flying capacitor DCDC converter are provided. The three-level flying capacitor DCDC converter includes an inductor, a first switching transistor, a second switching transistor, a first diode, a second diode, a third diode, a flying capacitor and a controller. A first terminal of the inductor is connected to a positive input terminal of the DCDC converter. A second terminal of the first switching transistor is connected to a negative input terminal of the DCDC converter through the second switching transistor. Operation of the DCDC converter is stopped and a direct-current bus voltage is reduced in response to a difference between the direct-current bus voltage and a voltage of the flying capacitor greater than or equal to a withstand voltage of the second diode.