Bootstrap Circuit for Power Converter Voltage Stability
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Solution Overview
Problem
Bootstrap capacitors in power converters often fail to recharge in time, leading to a decrease in bootstrap voltage, which can prevent the high side switch from turning on properly, especially under light load conditions or high duty cycles, resulting in output voltage spikes when the voltage resumes to its desired value.
Innovation Solution
A bootstrap circuit with a first charging circuit, a comparing circuit, and a boost circuit that charges the bootstrap capacitor when the voltage difference between the input and output voltages is below a threshold, ensuring the capacitor is replenished even during short on-times of the low side switch, preventing voltage spikes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the low side switch on-time is short (light load or high duty cycle), then the converter efficiency is improved, but the bootstrap capacitor cannot be charged in time leading to voltage drop
Solution Approach 1:
The patent applies preliminary action by charging the bootstrap capacitor to a voltage higher than the input voltage before the high side switch needs to turn on. The charging circuit proactively raises the bootstrap capacitor voltage above Vin using a charge pump mechanism, ensuring sufficient voltage headroom is available before the switching event occurs, thus preventing voltage drop even when low side switch on-time is short
Solution Approach 2:
The patent introduces an intermediary charge pump circuit between the input voltage source and the bootstrap capacitor. This intermediary circuit actively transfers energy to the bootstrap capacitor, mediating the charging process and ensuring the capacitor reaches the required voltage level regardless of the low side switch on-time duration
2Reliability
If the bootstrap capacitor voltage drops, then the high side switch cannot turn on properly, but increasing the low side switch on-time to recharge the capacitor causes output voltage spikes
Solution Approach 1:
The charging circuit performs preliminary charging of the bootstrap capacitor to a voltage higher than Vin before the high side switch needs to operate. This advance preparation ensures the capacitor maintains sufficient voltage without requiring extended low side switch on-time, thereby preventing output voltage spikes that would result from prolonged switching
Solution Approach 2:
The patent changes the voltage parameter of the bootstrap capacitor by charging it to a voltage higher than the input voltage Vin. This parameter change allows the high side switch to turn on properly even when the low side switch on-time is short, eliminating the need to extend switching duration and thus preventing output voltage spikes
3Device complexity
If a diode is used to charge the bootstrap capacitor from input voltage, then the circuit is simple, but the capacitor voltage cannot exceed the input voltage
Solution Approach 1:
The patent introduces a charge pump circuit as an intermediary between the input voltage source and the bootstrap capacitor. This intermediary actively transfers and elevates voltage, enabling the bootstrap capacitor to charge to a voltage higher than Vin, thus resolving the limitation of simple diode-based charging while maintaining reasonable circuit complexity
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
The solution ensures a stable bootstrap voltage, preventing the high side switch from malfunctioning and reducing large output voltage spikes, thereby maintaining converter efficiency and preventing damage to the converter and load.
Implementation Method 1
a bootstrap capacitor CB, having a first terminal and a second terminal, is configured to provide a bootstrap voltage
Implementation Method 2
a first diode D1, having a first terminal and a second terminal, wherein the first terminal is configured to receive the input voltage Vin, and wherein the second terminal is coupled to the first terminal of the bootstrap capacitor CB
Implementation Method 3
a boost circuit is configured to generate a voltage higher than the input voltage Vin
Data Source
AI summary
A power converter with bootstrap circuit, the power converter has a high side switch, a low side switch, a bootstrap circuit and a bootstrap capacitor for providing a bootstrap voltage to supply a high side driver of the high side switch. The power converter receives an input voltage and provides an output voltage based on driving the high side switch and the low side switch to switch on and off. The bootstrap circuit has a first comparing circuit, a first comparing circuit, a boost circuit and a second charging circuit. The second charging circuit charges the bootstrap capacitor when a voltage difference between the input voltage and the output voltage is smaller than a voltage threshold.