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Switchmode Transformer Reference Design

The Precision Factor

Switchmode Transformer designs are used extensively in high frequency electronic applications, usually within a switchmode power supply.  Common transformer configurations include Flyback Transformers, High Frequency Transformers and Push-Pull Transformers.  Precision, Inc, a specialist in custom and standard magnetic design and manufacture, has been involved in developing Switchmode Transformers and other magnetic components for its customers since 1964.  During that time, Precision, Inc has customized many designs, specifically to our customer’s specific need.

Which High-Frequency Power Switch Mode Transformer is right for your application?

Related Precision Products

Selecting the right High Frequency Transformer depends on many application variables:

  1. Calculate the sizing factor.
  2. Choose the transformer frequency.
  3. Intersect these two numbers on the sizing chart (click on the chart to the right)
  4. Note the color of the curves your point lies between or on. Follow the curves to the right of the chart to quickly identify the transformer families that meet your specific criteria.

Calculating the Sizing Factor

  1. Add output WDC, output rectifier losses, and output inductor losses.
  2. Multiply the result from Step 1 by one of these topology factors:
    • 1.00 for push-pull center-tapped primary, full-wave center-tapped secondary
    • 0.86 for bridge input primary, full-wave center-tapped secondary
      (use 0.72 for full-wave bridge output)
    • 0.93 for current-fed bridge input primary, full-wave center-tapped secondary (use 0.77 for full-wave bridge output)
    • 2.05 for forward converter
    • 2.24 for fly back converter
  3. Multiply the result from Step 2 by the temperature rise factor (assumes 50°C ambient):
    • 1.00 for 40°C rise
    • 0.73 for 60°C rise
  4. IEC spacing requirements will increase the transformer sizing factor. Note that certain families are more appropriate for IEC spacing (they require a smaller increase in the sizing factor.) The following table gives the range of factors for two recommended families, using standard spacing technique:

    IEC Spacing






    1.11 - 1.50




    1.06 - 1.12

    1.11 - 1.26

    1.17 - 1.45

    The smallest factor is for the largest size in the family; and the largest factor is for the smallest size. The asterisk * indicates that standard spacing techniques cannot be used on the smallest sizes in the EFD family.

    Special techniques can be used to reduce the factor on all families to approximately 1.10. Contact Precision engineering for more information.
  5. Multiply the result from Step 3 by the standard or special spacing factor from Step 4.
  6. Your final result is the “Sizing Factor” for use in the Product Family Sizing Chart.
Enter required output power in Watts DC:
Select circuit topology:
Push-pull input, full wave center tapped secondary (1.00)
Bridge input, full wave center tapped secondary (0.86)
Bridge input, bridge output (0.72)
Current fed bridge input, full wave center tapped secondary (0.93)
Current fed bridge input, bridge output (0.77)
Forward (2.05)
Flyback (2.24)

Clicking on one of the family color bars will take you to a page with details on individual sizes within that family.
Family Sizing Factor Chart

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