Technical Information Site of Power Supply Design

2016.05.12 AC/DC

Designing Isolated Flyback Converter Circuits: Transformer Design (Structural Design) - 1

Design Method of PWM AC/DC Flyback Converters


In succession to the calculation of numerical values described in the preceding section, we now take up the design of a transformer T1 structure. For persons who are normally engaged exclusively in the design of electronic components, the design of a transformer that involves combinations of cores, bobbins, and winding wires and that requires experience-based skills may seem foreign. That said, since transformers are critical components for power supply design, and especially for AC/DC converters and isolated converters, this section is intended to provide you with at least an understanding of the procedure involved and required review processes.

The design of a transformer T1 structure proceeds in the following sequence of events:

(1) Selecting a bobbin
(2) Verifying the effective winding frame
(3) Determining a winding wire configuration
(4) Creepage distance and barrier tape
(5) Selecting wiring materials
(6) Connection diagram, Layer construction, Wire specifications
(7) Determining transformer specifications

As [Part 1], this section describes Steps (1) to (4); in the next section,
as [Part 2] we will explain Steps (5) to (7).

Core JFE MB3 EER28.5A or compatible
Lp 249μH
Np 30 turns
Ns 6 turns
Nd 8 turns

(1) Selecting a bobbin

As shown in the figure, there are vertical and horizontal bobbins (depending on the core size, there may be exclusively vertical or horizontal bobbins). A selection must be made in consideration of height and mounting area.

The required number of pins must also be considered. The numbers shown in the table represent the number of turns that were calculated in the section on numerical calculations. A bobbin must be selected that has enough pins on which these wires can be wound.


(2) Verifying the effective winding frame

Next, from bobbin specifications we determine the effective winding frame. The shaded area indicated by the red arrow in the figure represents the area in which wires can actually be wound. On the drawing for the bobbin to be used, accurately verify this area, which differs bobbin to bobbin.


The photo illustrates an actual bobbin, in which the part indicated by the red arrow represents the effective winding frame. In the case of the particular core selected, JFE EER28.5, the area measures J = 16.6mm and H = 4mm.

(3) Determining a winding wire configuration

The configuration of winding wires has a significant impact on the properties of the resulting transformer. In this section, we describe two configurations.


<Simple configuration>

  • Small number of layers → Cost effective
  • Inferior degree of coupling → Elevated surge voltage and increased losses
  • Number of bobbin pins → Can be decreased

<Sandwich winding configuration >

  • Large number of layers → Note the thickness of each winding wire
  • Excellent degree of coupling → Reduced surge voltage and minimized losses
  • Number of bobbin pins → Increased

The left side has the simplest configuration. To the extent that the number of layers is small, the left side yields cost benefits. However, because each winding wire is only one layer deep, the Np wire that has as many as 34 turns cannot be wound in one column, and when the wire is wound in two or three columns, the level of coupling diminishes.

In terms of the number of pins, four pins on one side suffice. This configuration is favored in situations where output power is small or the number of pins on a bobbin must be limited to four pins on one side.

The right side represents a configuration called sandwich winding. In this configuration, primary coils Np1 and Np2 are used to hold the other coil between them to increase the degree of coupling between the primary coils and the other coil. The attendant increase in the number of layers, however, increases the thickness of the spool, resulting in a situation where in terms of the number of pins on a bobbin, at least five pins per side are required.

With regard to coil configuration, the above method does not necessarily represent the correct solution. In order to achieve the best possible properties, we need to create several prototypes, time consuming as it may be, configure a well-organized circuit by combining other components on an actual board layout, and shape it into optimal specifications by checking the properties.

(4) Creepage distance and barrier tape

For compliance with safety standards, we need to ensure adequate insulation in terms of a creepage distance between the primary and secondary coils on the transformer. The creepage distance is determined by considering the operating voltage, the extent of contamination of the operating environment, and the specific materials that are used. Barrier tape is used as a means of securing an adequate creepage distance.

When transformer T1 is to comply with Safety Standard IEC60950, achieving the following limits or less, a creepage distance must be determined:

  • Operating voltage: 300V
  • Level of pollution: 2
  • Materials: Ⅲa(CTI<400)
  • Required minimum creepage distance based on IEC60950:
    Basic insulation: 3.2 mm
    Reinforced insulation: 6.4 mm(basic insulation ×2)
    ↑ In the present design, we use reinforced insulation
※ In this design, since the input voltage is approximately
   270 V, a linear interpolation can be made from 250 V and
   300 V, which represent standards.

From 250 V: 2.5 mm and 300 V: 3.2 mm, at
270 V the value is 2.78 mm, which is rounded to 3 mm.
For reinforced insulation, the actual value is doubled, which is 6 mm.

* In the case of a vertical bobbin, since the top part
  does not have an extraction line,
  the creepage distance can be halved, or set to 3 mm.


We now provide approximate definitions of the several technical terms on standards mentioned above. For specific details, the standards document must be consulted.

The creepage distance is determined as a pollution degree and CTI.

  1. The degree of pollution is classified in four levels, 1to 4, according to the level of contamination due to dust particles and other factors in the air in which the device is used.
    • Level 1: This level represents absence of any contamination or a condition in which only dry and non-conductive contamination occurs; this is a condition in which the contamination has no impact or the device is in clean air, as in the case of a clean room.
    • Level 2: Normally, this level represents a condition in which exclusively conductive contamination occurs. Temporary conductivity due to condensation may be anticipated. Examples: Electric devices on a control panel or an operating environment for appliances and office equipment.
    • Level 3: This level represents a condition in which conductive contamination may occur or dry, non-conductive contamination may occur that can turn into conductive contamination due to anticipated condensation, such as the environment that prevails in factories.
    • Level 4: This level represents a condition in which the contamination can result in sustained conductive contamination due to the presence of conductive dust particles or rain or snow, such as the outdoor environment.
  2. CTI(Comparative Tracking Index)
    • The CTI value refers to the maximum voltage that does not produce tracking when a 0.1% solution of ammonium chloride is titrated at a rate of 1 drop every 30 seconds for a total of 50 drops.
    • Classification of molding material by CTI value (IEC 60664-1)
      Material group Ⅰ: a minimum CTI level of 600
      Material group Ⅱ: A CTI level greater than or equal to 400 and less than 600
      Material group Ⅲa: A CTI level greater than or equal to 175 and less than 400
      Material group Ⅲb: A CTI level greater than or equal to 100 and less than 175

    * The bobbin material in Class Ⅲa is general-purpose PM9820/Sumitomo Bakelite (Phenol),
       with a CTI level < 400

We now move on to the section on [Designing Isolated Flyback Converter Circuits: Transformer Design (Structural design) - 2].

Key Points:

・After the calculation of numerical values, the work proceeds to the design of a concrete transformer structure.

・When a rough structural design is fixed in addition to the calculation of numerical values, it is possible to speed up the finalization step with the assistance available from manufacturers of transformers.

Design Example for PWM Flyback Converter

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