Simplified design of gate drive transformer based on multi-output isolated DC-DC converter

Photocoupler

Overview:
Often, the biggest obstacle to designing isolated DC-DC converters is the transformer design, which designers tend to discourage and choose other, simpler design tasks. With the commercially available gate drive transformer characteristics, four separate isolated DC outputs are available. In fact, for low-power DC-DC power conversion, the gate drive transformer is an ideal choice because the transformer has been optimized for voltage and time products (ET or volt microsecond products) and low leakage inductance.

A magnetic core with high magnetic permeability and low loss at high switching frequency (FSWX) can support a typical primary voltage of 10V~15V, and has a typical on-time of 500ns~5μs at a switching frequency of 100kHz~500kHz. This voltage and time range is exactly what is needed for DC-DC converter design. At the same time, a core geometry and winding structure have been chosen for low leakage inductance to reduce rise and fall times while having low ringing. Finally, the wire gauge used is sufficient for the DC-DC converter to handle winding currents of the order of 10mA without excessive copper losses.

Pulse Electronics' P0585 gate drive transformer contains five windings, each with the same number of turns (Reference 1). One of the windings uses a three-layer insulated wire (TIW) and the other four windings use a standard winding wire. The TIW winding acts as a primary drive and achieves a nominal primary secondary breakdown voltage with an RMS of 3kV. The rated breakdown voltage between the four secondary windings is not determined, but this type of wire insulation is typically used for off-line power conditions where the voltage between the windings can be as high as 400V.

The isolated power output provides great flexibility. In this way, it is easier to interrupt the ground loop at different ground potentials, power the remote circuit, and simplify the selection of positive and negative output voltage polarities. The figure below shows the four secondary windings of this transformer, which produce four independent equal voltage outputs. But the four secondary windings can have multiple strings/and combinations to produce a large combination of output voltages/currents.


Figure IC, the transformer primary is driven by a 50% duty cycle square wave to avoid core saturation (please read the PDF for details)