Multi-Level Converter Startup Circuit for Boot and Fly Capacitor Precharge
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Solution Overview
Problem
Multi-level power converters face challenges in efficiently charging boot and fly capacitors at startup, implementing a shut-down mode for quick re-start, and maintaining proper capacitor voltage levels during operation.
Innovation Solution
The circuit includes switchable current sources and diode ladders to charge and discharge capacitors efficiently, a startup charger circuit for initial boot capacitor charging, and a diode ladder for periodic recharging, along with PFETs to parallel diodes for improved voltage distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional capacitor charging circuits are used at startup, then the circuit structure is simple, but the charging efficiency is low and startup time is long
Solution Approach 1:
The patent applies preliminary action by pre-charging boot capacitors and fly capacitors through dedicated charging circuits before the main power converter operation begins. The boot capacitor charging circuit charges capacitors C1-C4 through a voltage divider network before power switches are activated, and fly capacitors are pre-charged to specific voltage levels through controlled current sources, enabling faster startup without requiring ground-based charging during operation.
Solution Approach 2:
The patent segments the capacitor charging function into separate dedicated circuits: boot capacitor charging circuits for each power switch, fly capacitor charging circuits for each fly capacitor, and shutdown mode charging circuits. This segmentation allows independent optimization of each charging path and enables parallel charging operations, improving overall startup speed while maintaining manageable circuit complexity through modular design.
2Loss of energy
If capacitors are charged from ground state during normal operation, then charging is possible, but converter efficiency decreases and performance is reduced
Solution Approach 1:
The patent implements beforehand cushioning by maintaining charged standby capacitors (C1-C4 boot capacitors and fly capacitors) at defined voltage levels before shutdown occurs. During shutdown mode, the circuit preserves these charge levels through dedicated charging circuits that prevent complete discharge to ground, providing a voltage cushion that enables rapid re-start without requiring full ground-based recharging, thus minimizing energy loss during shutdown-restart cycles.
3Reliability
If boot capacitors and fly capacitors are not pre-charged, then circuit design is simpler, but quick re-start capability is lost
Solution Approach 1:
The patent applies universality by designing charging circuits that serve multiple functions: they charge capacitors during normal startup, maintain voltage levels during shutdown mode, and enable rapid re-start by preserving charge. The same boot capacitor charging circuits and fly capacitor charging circuits handle both initial charging and shutdown-mode maintenance, reducing the need for separate dedicated circuits for each function while improving reliability.
4Stability of the object's composition
If capacitor voltage levels are not maintained during operation, then circuit operation is simpler, but voltage stability deteriorates
Solution Approach 1:
The patent implements feedback through voltage detection circuits that continuously monitor the voltage levels across boot capacitors (C1-C4) and fly capacitors. These detection circuits provide feedback signals to control logic that adjusts the charging current sources to maintain voltages within specified ranges. For example, the controller monitors fly capacitor voltages and activates charging current sources when voltages drop below threshold levels, ensuring stable operation without requiring complex continuous regulation circuits.
Data Source
AI summary
A circuit suitable for use with a multi-level power converter cell that (1) charges boot capacitors at startup to a sufficient level to power level shifters and drivers that control the power switches within the cell, (2) pre-charges each fly capacitor to a target voltage, (3) provides a shut-down and/or a standby mode of operation that enables a quick re-start of operation, and (4) balances fly capacitor voltages when the fly capacitor(s) is/are not actively charge-balanced. One embodiment includes a first switchable current source coupled between a fly capacitor and an input voltage; a second switchable current source coupled between the fly capacitor and a reference potential; and a third switchable current source coupled in parallel with the fly capacitor; wherein the switchable current sources are configured to charge the fly capacitor in a first mode of operation, and to discharge the fly capacitor in a second mode of operation.