How is a transformer working at 10's of MHz designed?

Question

In this answer a switching power supply using a transformer operating at 10's of MHz range to reduce size is mentioned.

How would such transformer be designed? Would it still have a plain old design with a core and windings, just smaller, or would it have some alternate design?

Answer 1

The design of a transformer at 10s of MHz is not the main issue in making a switching power supply operate at high frequency like that. There are plenty of ferrites that can be used as the core material that won't be particularly lossy at that frequency. The biggest issue will be to minimize capacitive coupling between the primary and secondary and between parts of each winding. This might call for a toroid with each winding not quite spanning half the toroid so that there is some gap between then and the ends of each winding are apart.

The real problem will be switching losses. When the whole switching cycle is only 100 ns or less, then a 10-20 ns switch transition time is significant. If the switch is a FET, then then charging and discharging the gate 10 M times a second represents significant current. Fast parts generally cost more and require more power to drive. While the inductor can be made nicely small, the loss of efficiency at this frequency will limit size due to heat dissipation problems.

It sounds worth looking into a resonant design. That might actually put parasitic capacitance accross each transformer winding to use with some cleverness.

Answer 2

At dozens of megahertz you may design a planar transformer, using PCB traces as your windings.

Answer 3

RF transformers come in many sizes and shapes. For < 1 Watt look at how cable TV transformers are made to operate from 5MHz to 500MHz for example. The direction of winding, number of turns, ferrite properties and configuration of isolated or not each play a role in the performance. Impedance matching gives even more special properties of directional isolation or Return Loss which is useful for two-way signals and getting a small sample such as -20dB or just splitting the signal into two paths and transforming the voltage 2:1. with 75 ohm sources and 150 ohm R internally.

These photos show a similar transformer, home made with a 100 ohm differential for 50 ohm connections as a "directional Coupler or splitter or 2:1 power transformer with 3.5dB loss. RF transformer schematic (Splitter)

For more power different cores and wire size is considered and geometry depends on turns ratio. Bi-filar winding and tri-filar winding is used to reduce capacitance. Dozens of other methods are acceptable.. Seek Mini-Circuits Labs for advice. photos