Bootstrap Bias Circuit for Stable PFM High-Impedance Operation
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Solution Overview
Problem
The PFM mode in power converters leads to instability in the bootstrap capacitor voltage due to insufficient replenishment during high-impedance states, causing unnecessary switching losses and voltage instability on the bootstrap voltage bus.
Innovation Solution
A switched capacitor bootstrap circuit is employed in high-impedance PFM mode, while an active bootstrap circuit operates in other modes, using flying capacitors and auxiliary switches to maintain capacitor voltage without intermittent switch activation, reducing switching losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the low-side switch and high-side switch are kept in high-impedance state for a long time in PFM mode, then switching losses are reduced, but the bootstrap capacitor voltage becomes unstable
Solution Approach 1:
The bootstrap capacitor is charged in advance during the on-state of the low-side switch before entering the high-impedance PFM mode. This preliminary charging action ensures that the bootstrap capacitor has sufficient voltage to maintain stable operation during the subsequent high-impedance period without requiring intermittent switch activation.
Solution Approach 2:
The low-side switch is activated periodically at specific intervals during PFM mode to replenish the bootstrap capacitor charge. This periodic activation maintains the bootstrap voltage within a stable range while minimizing the frequency of switch operations, thereby reducing overall switching losses compared to continuous or frequent activation.
2Stability of the object's composition
If the low-side switch is periodically turned on to replenish the bootstrap capacitor, then the bootstrap voltage becomes stable, but unnecessary switching losses occur
Solution Approach 1:
The bootstrap circuit serves itself by utilizing the natural on-state of the low-side switch during normal power converter operation to recharge the bootstrap capacitor. This self-service mechanism eliminates the need for additional dedicated recharge cycles, maintaining voltage stability without incurring extra switching losses from unnecessary switch activations.
Solution Approach 2:
The control method dynamically adjusts the bootstrap capacitor recharge timing based on the operating mode. In PFM mode, recharge occurs during the on-state of the low-side switch without requiring extended or frequent activation, thereby optimizing the balance between voltage stability and switching loss reduction through parameter optimization.
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 approach maintains stable bootstrap voltage without additional switching losses, improving efficiency by eliminating the need for intermittent low-side switch activation.
Implementation Method 1
a bootstrap capacitor Cbst connected between a bootstrap voltage bus BST, and a common node of the high-side switch Q1 and the low-side switch Q2
Implementation Method 2
The diode D1 and the bootstrap capacitor Cbst form a bootstrap circuit configured to provide bias power for the first driver 101 and the auxiliary circuits 110
Data Source
AI summary
An apparatus includes a first switch and a second switch connected in series between an input voltage bus and ground, a bootstrap capacitor connected between a bootstrap voltage bus, and a common node of the first switch and the second switch, and a bias power control circuit configured to function as a switched capacitor bootstrap circuit when the apparatus is configured to operate in a high impedance PFM mode, and configured to function as an active bootstrap circuit when the apparatus is configured to operate in another operating mode.