Bootstrap Power Converter Control for Duty-Cycle Threshold Charging
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Solution Overview
Problem
In power converters with bootstrap driving, special operating conditions like no-load cause the bootstrap capacitor voltage to drop, leading to simultaneous turn-on of upper and lower switching tubes, risking circuit damage, and result in overshoot of inductor current and excessive output voltage.
Innovation Solution
Adjust the duty cycle of the first switch to a preset threshold when it decreases, ensuring the bootstrap capacitor is charged, and control the second switch's turn-on timing to stabilize voltage and prevent excessive output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the lower switching tube is turned off during no-load operation, then power consumption is reduced, but the bootstrap capacitor voltage cannot be effectively supplemented and continues to drop
Solution Approach 1:
The control unit detects when the duty cycle of the lower switching tube approaches a threshold value and proactively generates a forced conduction signal to turn on the lower switching tube before the bootstrap capacitor voltage drops to a dangerous level. This preliminary action prevents the voltage drop issue from occurring in the first place, while still allowing the lower switching tube to remain off during normal no-load operation to save power.
Solution Approach 2:
The control unit continuously monitors the duty cycle of the lower switching tube and uses this feedback information to determine when to generate the forced conduction signal. When the duty cycle reaches the threshold value, the feedback mechanism triggers the forced conduction mode, creating a closed-loop control system that maintains bootstrap capacitor voltage stability while minimizing power consumption.
2Productivity
If the lower switching tube is turned on after being drive-off, then normal operation is restored, but the upper switching tube generates a voltage jump that cannot be absorbed causing simultaneous turn-on of both switches
Solution Approach 1:
Before restoring normal operation by turning on the lower switching tube, the control unit first generates a forced conduction signal to ensure the bootstrap capacitor is adequately charged. This preliminary charging action prepares the system in advance, so that when the lower switching tube is subsequently turned on for normal operation, the upper switching tube's voltage jump is properly absorbed and simultaneous turn-on is prevented.
Solution Approach 2:
The forced conduction mode acts as a cushioning mechanism that absorbs the potential voltage jump energy before normal operation is restored. By charging the bootstrap capacitor in advance during forced conduction, the system creates an energy buffer that prevents harmful voltage spikes when the lower switching tube is turned on, thereby protecting against simultaneous switch turn-on.
3Loss of energy
If the duty cycle of the first switch is continuously reduced to zero during no-load, then power loss is minimized, but the bootstrap capacitor voltage drops and causes circuit damage
Solution Approach 1:
The control unit implements periodic forced conduction by continuously monitoring the duty cycle and generating forced conduction signals at appropriate intervals. Instead of maintaining continuous conduction (which would increase power loss), the system uses periodic forced conduction events to recharge the bootstrap capacitor, thereby minimizing power loss while preventing circuit damage through regular voltage top-up.
Solution Approach 2:
The control unit detects when the duty cycle approaches the threshold value and proactively generates a forced conduction signal before the bootstrap capacitor voltage drops to a dangerous level. This preliminary action prevents circuit damage by ensuring the bootstrap capacitor is recharged in advance, while allowing the duty cycle to be reduced to zero during intervals between forced conduction events to minimize power loss.
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
Prevents circuit damage by stabilizing bootstrap capacitor voltage, avoiding simultaneous switch turn-on and excessive inductor current, while maintaining stable output voltage.
Implementation Method 1
a bootstrap capacitor configured to supply power to the second driving unit, when the first switch is turned on, the bootstrap capacitor is charged via the first switch
Data Source
AI summary
The present disclosure provides a method for controlling a power converter and a power converter. The power converter includes a first switch, a second switch, a first driving unit corresponding to the first switch and a second driving unit corresponding to the second switch, and a bootstrap capacitor configured to supply power to the second driving unit. When the first switch is turned on, the bootstrap capacitor is charged via the first switch. The method includes: when a duty cycle of the first switch is decreased to a first threshold, controlling the duty cycle of the first switch to be a preset duty cycle every time at least one switching cycle elapses, and the preset duty cycle is not less than the first threshold.