Bootstrap Circuit for DC-DC Converter Gate Drive
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Solution Overview
Problem
In buck converters with high input voltage, it is challenging to turn on N-type transistors due to the low voltage drop between the gate and source terminals, leading to poor performance as the gate terminal voltage cannot be elevated sufficiently without assistance.
Innovation Solution
A bootstrap circuit comprising a second transistor, a bootstrapping capacitor, and a clamping circuit is introduced, where the bootstrapping capacitor is charged to elevate the voltage level of the gate terminal of the first transistor above the reference voltage, ensuring the transistor can be turned on effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If N-type transistor is used to reduce gate-source voltage drop and save area, then area consumption is reduced and turned-on resistance is improved, but the transistor cannot be turned on when source terminal is charged to input voltage due to insufficient gate voltage level
Solution Approach 1:
The bootstrap capacitor is pre-charged to the input voltage level before the transistor switching operation. This preliminary charging action stores the necessary voltage energy in the capacitor, which is then used to elevate the gate voltage when needed, ensuring the transistor can be reliably turned on without requiring additional voltage generation circuitry during operation
Solution Approach 2:
The bootstrap capacitor acts as an intermediary energy storage element between the input voltage source and the transistor gate. It mediates the voltage level requirement by being charged to the input voltage and then providing the elevated voltage level to the gate terminal through the switching action of the second transistor, enabling the first transistor to turn on properly
2Reliability
If gate terminal voltage is elevated above input voltage to turn on transistor, then transistor turn-on performance is improved, but additional voltage generation assistance is required increasing circuit complexity
Solution Approach 1:
The bootstrap circuit uses the existing input voltage and the switching action of the second transistor to automatically charge the bootstrap capacitor and generate the elevated gate voltage. The circuit serves itself by utilizing its own operating voltages and switching elements, eliminating the need for external voltage boost converters or complex voltage generation circuitry
Solution Approach 2:
The second transistor serves multiple functions: it acts as a switching element in the main power conversion path and simultaneously functions as a voltage transfer switch for charging the bootstrap capacitor. This multi-functionality reduces the total component count and simplifies the overall circuit architecture while achieving the required gate voltage elevation
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 bootstrap circuit enhances the performance of the DC-to-DC converter by maintaining a stable voltage drop between the bootstrapping capacitor and the gate terminal, allowing the control logic to elevate the gate voltage level above the input voltage, thereby overcoming the limitations of prior art.
Implementation Method 1
the bootstrapping capacitor has a first terminal and a second terminal, and the first terminal is coupled to a source terminal of the second transistor
Data Source
AI summary
A bootstrap circuit applied to a first transistor of a direct-current (DC) to DC converter includes a second transistor, a bootstrapping capacitor and a clamping circuit, wherein the bootstrapping capacitor has a first terminal and a second terminal, and the first terminal is coupled to a source terminal of a transistor, and the source terminal of the second transistor is coupled to the first transistor; and the clamping circuit is coupled between a gate terminal of the second transistor and the second terminal of the bootstrapping capacitor, and is arranged to maintain a voltage drop between the second terminal of the bootstrapping capacitor and the gate terminal of the second transistor. A drain terminal of the second transistor is coupled to a first reference voltage, and a maximum of a voltage level of the gate terminal of the first transistor is greater than the first reference voltage.