Cross-Coupled Bootstrap Charge Pump for High-Side MOSFET Drive
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Solution Overview
Problem
Conventional DC-DC converters face inefficiencies in driving high-side transistors due to the challenge of sourcing a sufficient gate voltage for n-channel MOSFETs, as their source terminal is not connected to ground, leading to voltage drops and inefficiencies when using internal diodes.
Innovation Solution
A bootstrap drive circuit utilizing a non-overlapping circuit, capacitors, and n-channel transistors synchronizes with PWM signals to efficiently drive high-side transistors, eliminating the need for clock signals or level shifters, and providing a boosted voltage to the gate terminal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If an internal diode is used in the bootstrap circuit, then the high-side transistor can be driven, but voltage drops occur and efficiency is reduced
Solution Approach 1:
The patent changes the electrical parameters of the bootstrap circuit by replacing the diode with an n-channel MOSFET switch. This allows the bootstrap capacitor to be charged to a higher voltage (Vcc plus diode drop) when using the MOSFET, eliminating the voltage drop issue while maintaining the driving capability for the high-side transistor gate.
Solution Approach 2:
The patent introduces an n-channel MOSFET as an intermediary active switch in the bootstrap circuit. This MOSFET acts as a controllable mediator between the regulated voltage source and the bootstrap capacitor, enabling efficient charge transfer without the inherent voltage drop of a diode, while being controlled by the PWM signal.
2Loss of energy
If an n-channel transistor is used as the bootstrap switch, then driving efficiency improves, but a voltage equal to or greater than the sum of the regulated voltage and threshold voltage is required
Solution Approach 1:
The patent applies preliminary action by charging the bootstrap capacitor to a higher voltage level (Vcc plus diode drop) during the low-side transistor ON state before the high-side transistor needs to be driven. This pre-charging ensures that when the high-side transistor is activated, sufficient gate voltage is already available, eliminating the need for additional voltage headroom beyond Vcc.
Solution Approach 2:
The patent changes the voltage parameter of the bootstrap capacitor by using an active MOSFET switch instead of a passive diode, enabling the capacitor to be charged to a higher voltage level. This parameter change allows the system to meet the voltage requirements of the high-side transistor gate drive without requiring the regulated voltage to be higher than Vcc.
3Reliability
If conventional bootstrap circuits are used, then the high-side transistor can be driven, but the circuit complexity increases with clock signals and level shifters
Solution Approach 1:
The patent extracts and removes unnecessary components from the conventional bootstrap circuit. By using the PWM signal directly to control the MOSFET switch and the bootstrap capacitor charging/discharging process, the patent eliminates the need for separate clock signals and level shifters, significantly reducing circuit complexity while maintaining reliable high-side transistor control.
Solution Approach 2:
The patent applies multi-functionality by using the PWM signal for dual purposes: controlling the low-side transistor and controlling the bootstrap switch (MOSFET). This universal use of the PWM signal eliminates the need for dedicated clock signals and level shifters, reducing component count while ensuring proper timing and voltage levels for high-side transistor drive.
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 reduces power consumption, minimizes complexity, and enhances efficiency by synchronizing with PWM duty cycles, ensuring effective control of high-side transistors with reduced voltage drops and no external filtering capacitors.
Implementation Method 1
charge a first capacitor of the bootstrap drive circuit to a regulated voltage during an ON phase of the DC-DC converter
Implementation Method 2
The internal diode 110 is forward biased and will charge the external bootstrap capacitor 114 to a voltage slightly below the regulated voltage (Vcc)
Data Source
AI summary
According to an embodiment, a bootstrap drive circuit for driving a bootstrap switch in a DC-DC converter is proposed. The bootstrap drive circuit includes a non-overlapping circuit, a first and a second capacitor, and a first and a second n-channel transistor. The non-overlapping circuit is configured to receive an input signal, the input signal being a function of a pulse width modulated (PWM) signal, wherein the input signal and the PWM signal are characterized by a first logic level, generate a first control signal from the input signal at a first output terminal of the non-overlapping circuit, the first control signal having the first logic level, and generate a second control signal from the input signal at a second output terminal of the non-overlapping circuit, the second control signal being characterized by a second logic level different from the first logic level.