A Guide to Designing Gate-Drive Transformers

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

In manufacturing a gate-drive transformer, the leakage inductance can be limited by using the proper winding method during manufacture. Fig. 2 shows a winding specification that displays the method of winding from the center. The winding specification produces the configuration reflected in Fig. 1c. Note that the line between the endpoints Start 1 and Finish 6 are on the primary side.

This winding-from-center method will increase the inductive coupling of the turns. The step-by-step process of how to wind this part is as follows:

• With two wires in hand, red and green (start points Start 2 and Start 3), bifilar wind 10 turns.

• Now add the natural wire and wind trifilar, with three wires in hand (this stage is noted with the start point Start 1) for an additional 20 turns. Stop winding the natural wire after a total of 30 turns. This stage is reflected by the endpoint Finish 6.

• Bifilar wind 10 more turns with the red and green wires together, bringing these wires to a total of 40 turns. This stage is marked by endpoints Finish 4 and Finish 5.

Now that the basic information has been presented for designing and winding a gate-drive transformer, the design can be completed using Fig. 1c as a reference design example. The inductance range will be kept between 50 µH and 500 µH, per Table 3. The electrical requirements for this transformer are shown in Table 4.

As previously mentioned, the first step in designing a gate-drive transformer is to decide what size core to use and what core material. The most compatible material for this particular application is 3F3, or equivalent, from Ferroxcube, as listed in Table 2.^[4]

Because the power level is listed as less than 5 W, an E5.3/2.7/2-3F3 is chosen. The AL value (rated in milli-Henrys per thousand turns) of the core is listed as 265 « 25% (199 to 331). The core area is 0.0265 cm².

The turns are calculated using the following formula:

where B equals the flux density in Gauss, A_CORE equals the core area in square centimeters and ET equals the volt-microsecond constant in volt-microseconds.

ET measures the energy-handling ability of a transformer or inductor, and depends on core area, core material, number of turns and the duty cycle of the applied pulse.

A Gauss level of 2000 will be used in this design. At this Gauss level, there is no risk of saturating the core, because B_SAT of the 3F3 material from Ferroxcube is 4000 Gauss. Also, because this is an E core, there is a small gap at the mating surfaces that aids in preventing the core from entering saturation.

Establishing Turns Numbers

With all the parameters known, using the following equation, the primary turns can be calculated:

where B equals 2000 Gauss, A_CORE equals 0.0265 sq cm (core area) and ET equals 10.5 V µs. With the turns on the primary being 20, the secondary will be 40 turns to meet the 1-to-2-to-2 turns ratio.

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© 2007 Penton Media, Inc.