A Guide to Designing Gate-Drive Transformers

Jan 1, 2007 12:00 PM
By Patrick Scoggins, Senior Applications Engineer, Datatronics Distribution, Romoland, Calif.

Determining Wire Gauge

The current will be calculated using 50 μH for the minimum primary inductance, as per Table 3.

The input current is calculated using the following formula:

The wire size on the primary is calculated using this equation:

A_WIRE=(circular mils per ampere)(I_IN)(DT),
where A_WIRE equals the wire area in circular mils, I_IN equals the input current and DT equals the duty cycle.

Substituting 500 circular mils per ampere, 255 mA for current and a duty cycle of 50% yields an area of 63.75 circular mils, provided with margin by a wire size of #32. For the primary, #32H wire will be used (MW80C [155°C]). MW80C is chosen because it meets the 130°C upper temperature limit and because it is solder strippable, easy to solder strip in manufacturing.

The secondary wire size will be based on a current level of 3 mA in each winding. The calculated area in circular mils for each winding is 1.5. Therefore, #48H wire will be used on each secondary.

Completing the Design

Once the wire sizes have been calculated, the next task is to ensure all the materials fit into the package. An appropriate adhesive must be chosen to glue the ferrite cores together. The primary open circuit inductance is verified with the following formula:

With the tolerance in the AL value of the core («25%), the minimum inductance would be 79 µH. This inductance is relatively small, but it meets the design requirements. The minimum inductance that had to be met was 50 µH. The turns could be increased to raise the inductance, but the capacitance will be compromised. Therefore, the 20 turns specified will remain.

Finally, a complete outline package is ready to be presented to the customer (Fig. 3). A final bill of materials for the completed design is shown in Table 5.

References

1. Wagner, J. “Solid State Tesla Coils — General Notes,” http://users.tkk.fi/~jwagner/tesla/
   SSTC/general-sstc-notes-gatedrv.htm,
   pp. 2-3.

2. Andreycak, Bill, Unitrode Switching Regulated Power Supply Design Seminar Manual, “1MHz 150W Resonant Converter Design Review,” Unitrode Corp., 1988.

3. Lee, Rueben, Electronic Transformers and Circuits, 2^nd Edition, John Wiley & Sons Inc., 1947, p. 76.

4. Ferroxcube Soft Ferrites and Accessories Data Book, 2004.

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