Isolated Flyback Converters: Performance Evaluation and Check Points
Critical checkpoint: Transformer saturation
- Magnetic field intensity
- Saturation flux density
- Saturation magnetization
- Transformer design
- Transformer saturation
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".
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.
- 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.
- Input voltage: Minimum, maximum values (at power supply startup, during stead-state operation)
- Load current: Maximum value
- Ambient temperature: Upper- and lower-limit temperatures
・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.