Flying Capacitor Converter Start-Up with Capacitive Pre-Charging
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Solution Overview
Problem
Multi-level flying capacitor (ML-FC) converters face higher conduction losses during start-up due to switches being exposed to full input voltage, preventing the use of lower blocking voltage components with smaller on-resistance, which are more cost-effective and efficient.
Innovation Solution
Implementing a power converter with a commutation cell that includes a start-up capacitor connected between the input capacitor and the flying capacitor, forming a capacitor divider, and using a high current diode to prevent reverse current flow during steady state, along with a pull-down switch to maintain the voltage of the center node at approximately V_in/2.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If ML-FC converters use switches with lower blocking voltage to reduce conduction losses during regular operation, then conduction losses are reduced, but during start-up the switches are exposed to full input voltage which prevents using lower voltage rating components
Solution Approach 1:
The patent pre-charges the flying capacitor to a fraction of the input voltage (e.g., V_in/2) before the main power switch is activated. This preliminary voltage establishment on the flying capacitor creates a voltage divider effect that limits the voltage stress on the main switch during start-up, enabling the use of lower voltage rating switches with smaller on-resistances and reduced conduction losses.
Solution Approach 2:
The patent introduces a start-up capacitor connected in series with the flying capacitor to form a capacitive voltage divider. This intermediary capacitor network mediates the voltage distribution during start-up, ensuring that the main switch experiences only a fraction of the full input voltage stress, thereby enabling the use of lower voltage rating switches.
2Ease of manufacture
If ML-FC converters use lower voltage silicon components, then cost and figure of merit improve, but start-up conditions expose switches to full input voltage preventing utilization of these components
Solution Approach 1:
The flying capacitor is pre-charged to a fraction of the input voltage before the main power switch operates. This preliminary voltage establishment enables the use of lower voltage rating silicon components during regular operation, improving cost-effectiveness and figure of merit, while the start-up capacitor ensures these components are not exposed to full input voltage stress during initialization.
3Reliability
If a capacitor divider is implemented to reduce switch voltage stress during start-up, then lower voltage rating transistors can be used, but additional components (start-up capacitor, high current diode, pull-down switch) are added to the circuit
Solution Approach 1:
The start-up capacitor is designed to serve dual purposes: during start-up, it forms a voltage divider with the flying capacitor to limit switch voltage stress; during regular operation, it can be disconnected or bypassed. The high current diode and pull-down switch work together to enable this transitional operation, allowing the same circuit topology to benefit from reduced voltage stress during start-up without permanently increasing component count in the main power path.
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
Reduces switch voltage stress to (N-1) times lower than conventional buck converters, allowing the use of lower voltage rating transistors with smaller on-resistances, reducing conduction losses and enabling on-chip implantation of power stages with cost-effective silicon components.
Implementation Method 1
a start-up capacitor, C div , of comparable size to a flying capacitor that is connected between an input capacitor and a top of the flying capacitor, forming a capacitor divider
Implementation Method 2
a high current capacitor diode D in adapted to prevent reverse current flow during steady state operation
Implementation Method 3
a pull-down switch configured to pull a diode anode to ground after start-up, and maintain the diode in a reverse biased state of operation during a state of normal operation
Data Source
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AI summary
Embodiments described herein introduce two sets of methods for limiting the voltage stress across switches in multi-level flying capacitor step-down de-de converters during the start-up sequence.Systems, devices, apparatuses, controllers, control methods, and processors are contemplated for an improved power converter topology.The embodiments are designed to aid in reducing the deficiencies noted in respect of ML-FCs, among others.