Synchronous buck converters have gained acceptance as isolated bias supplies in communications and industrial markets. Isolated buck converters, or Fly-Buck converters as they are commonly called, have a coupled inductor in place of the buck converter inductor which is used to create isolated outputs in addition to the non-isolated buck output. Each isolated output only needs a winding, a rectifier diode, and an output capacitor. Multiple semi-regulated isolated, or non-isolated, outputs can be generated using this topology in a simple and cost effective manner.
There are some key differences in the current flow in a buck and a Fly-Buck converter. The switching current loops in a buck converter are well known and are shown in Figure 1. The input loop consisting of input bypass capacitor, VIN pin, high and low side switch, and the ground return pin carries switching currents. This loop should be optimized with minimum trace lengths and loop area for a quiet operation. The output loop consisting of low side switch, inductor, output capacitor and ground return path carries essentially a DC current with low ripple. While it is important to keep all current flow paths small for low dc drops, low losses, and low regulation error, the area of this loop is not as important as the input current loop.
The Fly-Buck converter primary side appears similar to the buck converter as shown in Figure 2. The VIN loop here is a high di/dt loop just like in a buck converter. The VOUT1 loop however has very different current flow then in a buck converter. In addition to the primary inductor magnetization current, this loop also has the reflected current from the secondary windings. The reflected current has only the leakage inductance of the coupled inductor in its path and therefore has much higher di/dt than the magnetization current of the inductor. It is therefore important to minimize the loop area of VOUT1 loop as well. For the same reason, the secondary output loop consisting of secondary inductor winding, rectifier diode, and secondary output capacitor also needs to be minimized because it has high di/dt current flowing through it.
Another important point to remember when laying out a Fly-Buck converter is to realize that the secondary winding also has a switch node. This secondary switch node (SW2) is high dv/dt node and has a voltage transition of VIN*N2/N1. It is customary therefore to keep the SW2 trace area small to keep it from radiating noise.
A layout example incorporating the guidelines in this article is shown in Figure 3. The high di/dt loops on the primary and secondary sides are minimized as are the switch node areas.
