AC-DC|Design
Selecting Critical Components: MOSFET Q1
2018.10.11
Points of this article
・As the MOSFET Q1, a SiC MOSFET, which is a recurring theme of these design examples, is used.
・When selecting the MOSFET, the maximum Vds, peak current, loss due to ON-resistance, maximum allowable power dissipation for the package, and other factors are considered.
・The ID rating is selected to be roughly Ippk×2.
・Vds is calculated according to an equation.
The discussion of transformer design has been concluded, and so we begin discussion of component selection, centered on peripheral components of the BD7682FJ-LB power supply IC. We present excerpts of the peripheral circuits of components being explained, but the entire circuit must be studied as well, and when doing so this resource may be used.
Selecting Critical Components:MOSFET Q1
The MOSFET Q1 is the transistor that drives the primary side of the transformer, and is a “SiC MOSFET”, one recurring theme of this design.
In MOSFET selection, the maximum drain-source voltage, peak current, loss due to the ON-resistance (Ron), maximum allowable power dissipation for the package, and other factors must be considered.
As the input voltage becomes lower, the MOSFET ON-time becomes longer and heat generation due to Ron losses becomes greater. A low Ron is one feature of SiC MOSFETs, and this conduction loss is also small. However, the mounting board and the state in which the product is incorporated must always be checked, and a heat sink or other means of heat dissipation should be provided when necessary.
The rating for ID is selected with Ippk×2 as a rough guide. Here Ippk was determined to be 0.66 A in ② of the section on transformer design.
Vds is calculated using the following equation.
Vspike is hard to calculate numerically. Hence on the basis of an empirical rule, addition of a snubber circuit is assumed, and a MOSFET with a Vds of 1700 V is selected. In this calculation example, a ROHM SiC MOSFET, the SCT2H12NY (1700 V, 1.15 Ω, 4 A, 44 W), or the SCT2H12NZ (1700 V, 1.15 Ω, 3.7 A, 35 W), is selected.
Below, the maximum ratings of the SCT2H12NY are shown as an example. Please refer to the relevant data sheet for other parameters and further details.
List of articles related to the「Selecting Critical Components: MOSFET Q1」
- Introduction
- Power Supply ICs Used in Design: Optimized for SiC MOSFETs
- Design Example Circuit
- Transformer T1 Design – 1
- Transformer T1 Design – 2
- Selecting Critical Components: Input Capacitor and Balancing Resistor
- Selecting Critical Components: Switch Setting Resistors for Overload Protection Points
- Selecting Critical Components: VCC-Related Components of Power Supply ICs
- Selecting Critical Components: Components Related to Power Supply IC BO (Brownout) Pins
- Selecting Critical Components: Components Related to Snubber Circuits
- Selecting Critical Components: MOSFET Gate Drive Adjustment Circuit
- Selecting Critical Components: Output Rectifying Diode
- Selecting Critical Components: Output Capacitors, Output Setting and Control Components
- Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins
- Selecting Critical Components: Components for Dealing with EMI and Output Noise
- PCB Layout Example
- Example Circuit and Component List
- Evaluation Results: Efficiency and Switching Waveform
- Summary
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Basic of AC-DC Conversion
Basic studies to understand AC-DC converters and to go designing.
AC-DC
- Basic
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Overview of Design Method of PWM AC-DC Flyback Converters
- Want are Isolated Flyhback Convertors?
- Isolated Flyback Converter Basics: What is Switching AC-DC Conversion?
- Isolated Flyback Converter Basics: What are Characteristics of Flyback Converter?
- Isolated Flyback Converter Basics: Flyback Converter Operation and Snubber
- Isolated Flyback Converter Basics: What are Discontinuous Mode and Continuous Mode?
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- Determining Power Supply Specifications
- Choosing an IC for Design
- Designing Isolated Flyback Converter Circuits
- Designing Isolated Flyback Converter Circuits: Transformer Design (Calculating numerical values)
- Designing Isolated Flyback Converter Circuits: Transformer Design (Structural Design) – 1
- Designing Isolated Flyback Converter Circuits: Transformer Design (Structural Design) – 2
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? MOSFET related – 1
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Introduction
- Design Procedure
- IC Used in Design
- Power Supply Specifications and Replacement Circuit
- Synchronous Rectifying Circuit Section: Selection of Synchronous Rectifying MOSFET
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- Troubleshooting ①: Case When Secondary-Side MOSFET Suddenly Turns OFF
- Troubleshooting ②: Case When Secondary-Side MOSFET Turns On Due to Resonance Under Light Loading
- Troubleshooting ③: Case When, Due to Surge, VDS2 Rises to Above Secondary-Side MOSFET VDS Voltage
- Comparison of Efficiency of Diode Rectification and Synchronous Rectification
- Points to Note Relating to PCB Layout
- Summary
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Introduction
- Power Supply ICs Used in Design: Optimized for SiC MOSFETs
- Design Example Circuit
- Transformer T1 Design – 1
- Transformer T1 Design – 2
- Selecting Critical Components: MOSFET Q1
- Selecting Critical Components: Input Capacitor and Balancing Resistor
- Selecting Critical Components: Switch Setting Resistors for Overload Protection Points
- Selecting Critical Components: VCC-Related Components of Power Supply ICs
- Selecting Critical Components: Components Related to Power Supply IC BO (Brownout) Pins
- Selecting Critical Components: Components Related to Snubber Circuits
- Selecting Critical Components: MOSFET Gate Drive Adjustment Circuit
- Selecting Critical Components: Output Rectifying Diode
- Selecting Critical Components: Output Capacitors, Output Setting and Control Components
- Selecting Critical Components: Current Sense Resistors and Components Related to Detection Pins
- Selecting Critical Components: Components for Dealing with EMI and Output Noise
- PCB Layout Example
- Example Circuit and Component List
- Evaluation Results: Efficiency and Switching Waveform
- Summary
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Overview of Design Method of PWM AC-DC Flyback Converters
- Evaluation
- Product Information
- FAQ