A Guide to Designing Gate-Drive Transformers

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

A step-by-step procedure optimizes the mechanical, thermal and electrical parameters of the transformers to suit switch-mode power-supply applications.

As electrical circuits become more complex, the demand for expert electrical engineering becomes more critical. Precise engineering is essential at every stage of designing a circuit, and it is equally important in the design of the components. One component that must be carefully designed is the gate-drive transformer in a switch-mode power supply (SMPS).

A gate-drive transformer is needed in a SMPS to control the timing of the circuit. These devices provide electrical pulses for turning on and off semiconductors, such as high-voltage power MOSFETS or IGBTs. They also are used for voltage isolation and impedance matching. Gate-drive transformers are essentially pulse transformers that are used to drive the gate of an electronic switching device. Assuming optimal values for rise time, droop and overshoot, the application is what discriminates the gate-drive transformer from other transformers.

The basic gate-drive transformer has several design variations, each of which is determined by the specific application. Some common schematics and their corresponding turns ratios are listed in Fig. 1.

Typical gate-drive transformers are designed using ferrite cores to reduce cost. Some of the common core packages are EE, EER, ETD and EFD. These types of cores are “E” shaped and have a corresponding bobbin. The bobbins can be surface mount or thru hole. In some cases, a design will use a toroid.

A typical pulse-transformer design requires the parameters shown in Table 1.

If there is a requirement for a safety standard (UL, VDE, CUL, IEC or TUV), then the design must involve certain creepage and clearances. Documentation will need to be purchased from the safety agencies for the required creepage and clearance requirements.

If the application is for military purposes, then the choice of manufacturing materials may need to be noncompliant with the Restriction of Hazardous Substances (RoHS) directive. Magnetics design engineers need to understand this directive because it limits the selection of materials that can be used in the transformer, potentially impacting performance.

The first step is to determine the core material. This is based on the operating frequency. Table 2 lists several core vendors and the recommended ferrite materials for three different frequency ranges.^[1] The operating frequency of the SMPS will determine the amount of inductance that is needed on the primary of the gate-drive transformer. A general guideline is listed in Table 3.^[2]

Two of the critical electrical parameters to control when designing a gate-drive transformer are the leakage inductance and winding capacitance.^[1] A high leakage inductance and winding capacitance may cause an undesirable output signal such as phase shift, timing error, noise and overshoot. Leakage inductance happens when a winding has poor coupling. High winding capacitance results when a winding has many turns and the turns are not laying uniformly during the winding process.

Leakage inductance can be kept to a minimum at the electrical design stage and also in defining the manufacturing specification. There are many formulas to calculate an approximation of what leakage inductance to expect for a particular design.

One of the formulas^[3] used to estimate leakage inductance in the magnetics design is the following:

where I_L equals the leakage inductance of both windings in Henrys, N equals the number of turns in the winding, MT equals the mean length turn for the entire core in inches, n equals the number of dielectrics (insulation) between windings, c equals the thickness of dielectrics (insulation) in inches, a equals the winding height in inches and b equals the winding traverse in inches.

One thing to avoid at all costs is to have a winding with a half-turn. The half-turn is an uncoupled turn and the leakage inductance will be high. The winding capacitance should be kept within the picofarad range (less than 100 pF is desirable).

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