Gate-Drive and Isolation Transformers

Controlling the current flow between the drain and the source of a MOSFET requires the application of a drive voltage to the gate of the MOSFET. Switching power supplies operate the MOSFET as a current switch by applying a pulsed voltage drive to the gate for turning the drain-source current on and off. Delivering the controlling pulse requires a gate drive transformer to provide isolation between the controlling drive circuit and the MOSFET. Companies like Coilcraft offer off-the-shelf gate drive transformers for the purpose.

Gate drive circuits must provide an isolated or floating bias supply for maintaining the necessary turn-on bias when the MOSFET source rises to the input voltage. While driving the MOSFET gate, not only does the gate drive transformer help in isolating the controlling gate drive circuit from the switch node, it may also scale the output voltage via a suitable turns-ratio between its primary and secondary.

Some applications use optocouplers or digital isolators for driving the MOSFET directly. However, the use of a gate drive transformer is preferable, as it can provide a higher voltage requirement, much lower turn-on and turn-off delay times, and it can scale voltages by the ratio of its turns. These advantages make the simple gate driver transformer the best-performing solution for high-frequency and high-voltage applications that require maintaining accurate and fast signal timing.

Typical low-power applications use a simple single-output, transformer-coupled, high-side gate driver circuit. Additional components like capacitors, resistors, and diodes may be necessary depending on the duty cycle and other circuit conditions. These include preventing the development of a DC voltage across the transformer, as this may cause it to saturate. The additional components also help in the coupling capacitance and magnetizing inductance from resonating with specific duty cycle ratios. For single-ended circuits, the highest duty cycle is preferably 0.5.

Higher power applications may require half-bridge and full-bridge configurations coupled with transformers. Double-ended or DC-coupled bridge configurations may use a theoretical maximum duty cycle of 1.0. Designers use isolation transformers for isolation and voltage scaling in power supply applications. These serve three main purposes.

First, the transformer helps to connect circuits with grounds at different potentials—this prevents ground loop formation. Second, the transformer provides galvanic isolation, thereby preventing any flow of direct current. Lastly, the transformer provides voltage transformation—stepping up or stepping down from one voltage to another.

Isolation transformers may be available as signal transformers, power-supply transformers, communication transformers, data-line transformers, and many others. These are versatile and aptly suited for several industrial and commercial data communications and power supply applications.

Using off-the-shelf gate drive and isolation transformers can simplify the design of the gate drive circuit and significantly reduce the design cycle time. Coilcraft transformers typically use high-permeability ferrite cores for maximizing the inductance and minimizing the magnetizing current.

The designer can determine the required transformer size by the volt-time product of the application. This forms the first selection criterion for a gate drive transformer, as the designer can select the appropriate volt-time or V-µsec rating from the datasheet of the transformer. The rating must be equal to or greater than the highest applicable voltage-time product