Fly-Buck converter PCB layout tips
Synchronous buck converters have been recognized as isolated bias power supplies in the communications and industrial markets. An isolated buck converter, or so-called Fly-Buck ™ converter, uses a coupled inductor instead of a buck converter inductor to create isolated and non-isolated buck outputs. Only one winding, one rectifier diode, and one output capacitor are needed for each isolated output. You can use this topology to generate multiple semi-regulated isolated or non-isolated outputs in a simple, low-cost way.
There are some major current differences between a buck converter and a Fly-Buck converter. We are already familiar with switching current loops in buck converters, as shown in Figure 1. The input loop containing the input bypass capacitor, the VIN pin, the high- and low-side switches, and the ground return pin carries the switching current. The loop should be optimized for quiet operation to achieve the minimum trace length and minimum loop area. The output loop containing the low-side switch, inductor, output capacitor, and ground return path actually carries low ripple DC current. Although it is important to keep all current paths as short as possible to achieve low DC voltage drop, low loss, and low regulation error, the area of this loop is not as important as the input current loop.
Figure 1. Current loop in a buck converter. The VIN loop is a high di / dt loop.
The primary side of a Fly-Buck converter looks similar to a buck converter, as shown in Figure 2. Like the buck converter, the VIN loop here is a high di / dt loop. However, the current in the VOUT1 loop is very different from that of a buck converter. In addition to the primary inductor magnetizing current, the loop also contains the reflected current from the secondary winding. The reflected current contains only the leakage inductance of the coupled inductor in its path, so di / dt is significantly higher than the inductor magnetizing current. So it is also very important to minimize the loop area of the VOUT1 loop. For the same reason, the secondary output loop including the secondary inductor winding, rectifier diode, and secondary output capacitor also needs to be minimized because high di / dt current flows through it.
Figure 2 The Fly-Buck converter has two high di / dt loops on the primary side. All secondary loops are high di / dt.
When laying out the Fly-Buck converter, it is also necessary to remember that the secondary winding also has a switching node. This secondary switching node (SW2) is a high dv / dt node and supports voltage conversion of VIN * N2 / N1. Therefore, it is usually necessary to keep the SW2 trace area small to prevent it from emitting noise.
Figure 3 is a layout example incorporating the guidance content of this article. As with the switching node area, high di / dt loops on the primary and secondary sides can also be minimized.
Figure 3 LM5017-based Fly-Buck layout minimizes the di / dt loop and high di / dt SW1,2 node area.