AC-DC｜Design
Troubleshooting ①： Case When Secondary-Side MOSFET Suddenly Turns OFF
2020.04.08
Points of this article
・The secondary side of a conventional isolated flyback converter is replaced, and therefore it is extremely important that actual operation be confirmed.
・If malfunctioning of the secondary-side MOSFET due to noise occurs, a ferrite bead can be added to the drain line and the resistance value of the filter resistance can be increased.
By the end of the previous article, selection of the required components and calculation of component values were completed. As the next step, we begin mounting the selected components on a printed circuit board and begin the task of checking the various characteristics and confirming that the design specifications have been met. From this point, we shall explain a number of problems that may be anticipated when checking characteristics, and measures to address these problems.
In “Design of a Secondary-Side Synchronous Rectification Circuit to Improve the Efficiency of an AC-DC Converter”, the aim is to boost efficiency while maintaining the existing power supply specifications by the use of synchronous rectification on the secondary side of an isolated PWM flyback AC-DC converter circuit. Here, rather than using a new design, we opt for what is in a sense a modified design, with the primary side left as-is and the secondary side replaced with a circuit that uses a controller IC called the BM1R00147F. Hence it is extremely important to verify that the circuit as a whole operates properly as an AC-DC converter.
Problem ①：Case When Secondary-Side MOSFET Suddenly Turns OFF
There are cases in which a malfunction that turns the secondary-side MOSFET off occurs due to noise appearing in the voltage of the drain pin of the power supply IC. The diagram below shows operation in which VGS2, which normally should be on longer (indicated by the broken-line waveform), turns off after a short time due to noise.
Countermeasure ①-1: Insert a ferrite bead B1, increase the drain pin connection resistance R1
In response to Problem ①, “Countermeasure ①-1″* involves inserting a ferrite bead B1 for surge absorption and increasing the value of the filter resistor R1 to prevent malfunctions caused by noise. As the ferrite bead B1, a device with high impedance at low frequencies, for example the MPZ1608S102AT manufactured by TDK, is effective. The diagram below on the left shows the position of bead insertion and indicates the resistor R1 for adjustment and reference values. The diagram on the right shows the operation waveform with the countermeasure implemented. (*The designation “Countermeasure ①-1” is used to mean a first countermeasure to Problem ①.)
Points to be Noted
If the impedances of B1 and R1 are set too high, under light loading the secondary-side MOSFET may turn on due to resonance operation. Hence operation under light loading must be checked. Details of this phenomenon and countermeasures will be explained in the next article.
AC-DC
Basic
- AC-DC Basics
- DC-DC Conversion (Regulated) System after Smoothing
- Design Procedure for AC-DC Conversion Circuits (Overview)
- Issues and considerations in AC-DC Conversion Circuit Design
- Summary
- Extra Plus Basic Knowledge
Design
-
Overview of Design Method of PWM AC-DC Flyback Converters
- Isolated Flyback Converter Basics: Flyback Converter Operation and Snubber
- Isolated Flyback Converter Basics: What are Discontinuous Mode and Continuous Mode?
- Want are Isolated Flyhback Convertors?
- Design Procedure
- Isolated Flyback Converter Basics: What is Switching AC-DC Conversion?
- Determining Power Supply Specifications
- Designing Isolated Flyback Converter Circuits
- Isolated Flyback Converter Basics: What are Characteristics of Flyback Converter?
- Designing Isolated Flyback Converter Circuits: Transformer Design (Calculating numerical values)
- Choosing an IC for Design
- 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
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? MOSFET related – 2
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? CIN and Snubber
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? Output Rectifier and Cout
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components ? VCC of IC
- Designing Isolated Flyback Converter Circuits: Selecting Critical Components – IC Settings Etc.
- Designing Isolated Flyback Converter Circuits: Addressing EMI and Output Noise
- Example Board Layout
- Summary
-
Overview of Design Examples of AC-DC Non-isolated Buck Converters
- What are Buck Converters? – Basic Operation and Discontinuous Mode vs. Continuous Mode
- Selection of Power Supply ICs and Design Examples
- Selecting Critical Components: Input Capacitor C1 and VCC Capacitor C2
- Selecting Critical Components: Inductor L1
- Selecting Critical Components: Current Sense Resistor R1
- Selecting Critical Components: Output Capacitor C5
- Selecting Critical Components: Output Rectifying Diode D4
- EMI Countermeasures
- Board Layout and Summary
-
Introduction
- Design Procedure
- IC Used in Design
- Power Supply Specifications and Replacement Circuit
- Synchronous Rectifying Circuit Section: Selection of Synchronous Rectifying MOSFET
- Synchronous Rectification Circuit Section: Power Supply IC Selection
- Troubleshooting ①： Case When Secondary-Side MOSFET Suddenly Turns OFF
- Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components－C1, R3 at MAX_TON Pin, and VCC Pin
- 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
- Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components－D1, R1, R2 at DRAIN Pin
- Shunt Regulator Circuit Section: Selection of Peripheral Circuit Components
-
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
Evaluation
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What are Isolated Flyback Converters Performance Evaluation and Checkpoints?
- Overview and important features of a power supply IC used in example performance evaluation
- Design goals and circuits in performance evaluation
- Performance evaluation using an evaluation board: Measurement method and results
- Critical checkpoint: Output transient response and rising output voltage waveform
- Critical checkpoint: Measuring temperature and loss
- Critical checkpoint: Aluminum electrolytic capacitors
- Summary
- Critical checkpoint: Transformer saturation
- Critical checkpoint: MOSFET VDS and IDS, and rated voltage of output rectifier diode
- Critical checkpoint: Vcc voltage
Product Information
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