AC-DC|Evaluation
Critical checkpoint: Transformer saturation
2017.05.11
Points of this article
・A transformer must not be allowed to reach saturation.
・Use an oscilloscope with current probe, etc. to observe the primary-side current waveform.
・If a transformer reaches saturation, an excessive current flows, possibly causing destruction of a MOSFET or other device.
In the previous section, we explained “MOSFET VDS and IDS, and rated voltage of output rectifier diode” listed below, that should be verified apart from specifications, relating to “Critical checkpoint”. This time, we explain “Transformer saturation”.
Transformer Saturation
The saturation of a transformer T1 explained here relates to the primary windings and secondary windings which govern flyback operation. Third windings (pins 4, 5) which generate a power supply VCC for the power supply IC are added to T1, and where the tertiary windings are concerned, VCC is checked separately to ensure that it is being generated according to design.
Initially, we review transformer saturation. The magnetic material used in a transformer (iron, ferrite, etc.) has a characteristic called the saturation flux density. When the current flowing in the primary windings of the transformer is increased, the magnetic field intensity increases, but the magnetic flux density does not increase indefinitely, and there is a limit to the increase in current beyond which the flux density increases hardly at all. This state is called the saturation magnetization, and the flux density in this state is the saturation flux density.
Exceeding this limit to enter the saturation magnetization state is called saturation of the transformer. This is the same as what happens in an inductor. Transformer saturation not only means the flux density cannot increase; it also has the unfortunate consequence of a sudden decrease in inductance.
When the inductance decreases, the resistance component with respect to the current becomes only the resistance of the transformer windings. That is, when the transformer reaches saturation, a large current flows. This is the reason when transformer saturation is a problem in power supply design. The same is true of inductors as well.
The waveform data on the right is the Ids of an internal MOSFET that switches the primary side of a transformer; the green line represents normal operation, that is, a state in which the transformer is not saturated. On the other hand, the dashed red line is a representative waveform in a case of transformer saturation.
As explained above, when a transformer reaches the saturated state, a high current flows, and so a sudden increase in current that could be characterized as a current spike appears in the Ids. If this current is excessive, destruction of the MOSFET could result.
When designing a transformer, the maximum primary-side current Ippk should be calculated, and the transformer should be designed appropriately; but if an Ids current waveform like that in the above waveform data is observed, the transformer design must be reexamined. Please refer to this link regarding transformer design.
Below, we summarize checkpoints and condition settings relating to transformer saturation.
<Checkpoints relating to transformer saturation>
- Use an oscilloscope with current probe and other instruments to observe the waveform of the drain current Ids.
- When a transformer has reached saturation, the slope of the rising Ids curve changes, and the Ids rises rapidly.
- This increase in current could destroy a MOSFET or other device.
- When transformer saturation has been confirmed, check the actual transfer state pertaining to Ippk and the like.
- In some cases, the transformer design must be reexamined.
<Condition settings when checking transformer performance>
- Input voltage: Minimum, maximum values (at power supply startup, during stead-state operation)
- Load current: Maximum value
- Ambient temperature: Upper- and lower-limit temperatures
【Download Documents】Isolated Flyback Converters: Performance Evaluation and Checkpoints
This handbook explains how to evaluate the performance of isolated flyback type AC-DC converters using power supply ICs, with examples of actual measurement data. Important checkpoints are also explained.
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
Design
- 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?
- Design Procedure
- 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
- 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
- Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components-C1, R3 at MAX_TON Pin, and VCC Pin
- Synchronous Rectification Circuit Section: Selection of Peripheral Circuit Components-D1, R1, R2 at DRAIN Pin
- Shunt Regulator Circuit Section: Selection of Peripheral Circuit Components
- 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
- 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
- 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: Transformer saturation
- Critical checkpoint: MOSFET VDS and IDS, and rated voltage of output rectifier diode
- Critical checkpoint: Vcc voltage
- Critical checkpoint: Measuring temperature and loss
- Critical checkpoint: Aluminum electrolytic capacitors
- Summary
Product Information
FAQ