DC-DC|Evaluation
Bootstrap
2016.02.15
Points of this article
・The Nch MOSFET, low in on-resistance, helps to improve efficiency and also provides a low-cost option.
・The use of the Nch MOSFET in the high-side transistor requires a bootstrap.
A bootstrap circuit is needed when an Nch MOSFET is used for the high-side transistor of the output switch. Nowadays a number of power supply ICs comes with a bootstrap circuit mounted, and thus an understanding of the operation of the bootstrap circuit in connection with the evaluation of power supply circuits may be helpful.
Due to the low on-resistance, the Nch MOSFET can be used as a power switch to improve efficiency. Given the same on-resistance, the Nch MOSFET can be a lower cost option compared with the Pch MOSFET. That said, turning Nch MOSFET completely on as a high-side switch requires a high enough VGS, that is, a voltage higher than the drain voltage. Normally, because the drain voltage is Vin (input voltage), representing the highest voltage in the circuit, any higher voltage must be supplied externally. To generate that high voltage, a bootstrap circuit is used.
Simple in structure, a bootstrap circuit is a step-up charge pump composed of a switch, a capacitor, and a diode, where a voltage equal to the switch voltage (Vin) plus the internal supply voltage is used as the gate drive for the high-side Nch MOSFET.
- Nch MOSFET, low in on-resistance, helps to improve efficiency and provides a low-cost option.
- Use of the high-side transistor as an Nch MOSFET requires a VGS higher than the drain voltage.
- The voltage from an internal supply for internal circuit may not be high enough to drive the Nch MOSFET.
- A step-up charge pump composed of a switch, a capacitor, and a diode generates the high voltage needed by the driver for the high-side Nch MOSFET.High-side gate driver power supply = Vin + Internal supply voltage – Diode VF
Lately, Nch MOSFETs have been mainly used for the output switching transistors in medium- or larger-power circuits, fueled by a trend that gives priority to efficiency at the expense of a modest increase in the number of components required. More recently, there has been an increase in the use of circuits incorporating external diodes as a way of cutting the number of required components.
Incidentally, for the same reasons as those mentioned above, bootstrap circuits are also used in nonsynchronous rectifying circuits. Beyond the Nch MOSFET, bootstrap circuits are also applied to reduce the saturation voltage in circuits that use bipolar NPN transistors.
【Download Documents】Characteristics and Evaluation Method of Switching Regulators
This handbook reviews the basics of switching regulators and explains how to understand and evaluate the characteristics of switching regulators necessary for design optimization, along with reading and understanding the datasheets of switching regulator ICs.
List of articles related to the「Bootstrap」
- Overview of Characteristics and Evaluation Method of Switching Regulators
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- Switching Regulator Basics
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- PWM & PFM
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Download Technical Documents
Basic of Linear Regulators and Switching Regulators
Basic studies for linear regulators and switching regulators as a DC-DC converter.
DC-DC
- Basic
- Design
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Evaluation
- Overview of Characteristics and Evaluation Method of Switching Regulators
- How to Read Power Supply IC Datasheets: Cover, Block Diagram, Absolute Maximum Ratings and Recommended Operating Conditions
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Introduction
- Definitions and Heat Generation
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- Controller IC Power Consumption Losses in a Synchronous Rectifying Step-Down Converter
- Gate Charge Losses in a Synchronous Rectifying Step-Down Converter
- Conduction Losses due to the Inductor DCR
- Example of Power Loss Calculation for a Power Supply IC
- Simplified Method of Loss Calculation
- Heat Calculation for Package Selection: Example 1
- Heat Calculation for Package Selection: Example 2
- Loss Factors
- Matters to Consider When Studying Miniaturization by Raising the Switching Frequency
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- Important Matters when Studying Large Output Currents Applications: Part 2
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