Fly-Buck converter PCB layout skills



The synchronous buck converter has been recognized as an isolated bias power supply in the communications and industrial markets. The isolated buck converter, or the so-called Fly-Buck™ converter, uses a coupled Inductor instead of the buck converter inductor to create an isolated output and a non-isolated buck output. Each isolated output requires only one winding, one Rectifier diode and one output capacitor. This topology can be used to generate multiple semi-regulated isolated or non-isolated outputs in a low-cost and simple manner.

The synchronous buck converter has been recognized as an isolated bias power supply in the communications and industrial markets. The isolated buck converter, or the so-called Fly-Buck™ converter, uses a coupled inductor instead of the buck converter inductor to create an isolated output and a non-isolated buck output. Each isolated output requires only one winding, one rectifier diode and one output capacitor. This topology can be used to generate multiple semi-regulated isolated or non-isolated outputs in a low-cost and simple manner.

There are some major current differences between the buck converter and the Fly-Buck converter. We are already familiar with the switched current loop in a buck converter, as shown in Figure 1. The input loop including 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 mute operation to achieve the minimum trace length and minimum loop area. The output loop including the low-side switch, inductor, output capacitor, and ground return path actually carries a 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 voltage regulation error, the area of ​​this loop is not as important as the input current loop.

Fly-Buck converter PCB layout skills
Figure 1. The current loop in a buck converter. The VIN loop is a high di/dt loop.

The primary side of the Fly-Buck converter looks similar to a buck converter, as shown in Figure 2. The VIN loop here, like the buck converter, is also a high di/dt loop. However, the current in the VOUT1 loop is very different from the buck converter. In addition to the magnetizing current of the primary inductor, the loop also contains the reflected current from the secondary winding. The reflected current only contains the leakage inductance of the coupled inductor in its path, so the di/dt is significantly higher than the magnetizing current of the inductor. Therefore, it is also very important to minimize the loop area of ​​the VOUT1 loop. In the same way, the secondary output loop including the secondary inductor winding, rectifier diode, and secondary output capacitor also needs to be minimized because of the high di/dt current flowing through it.

Fly-Buck converter PCB layout skills
Figure 2. The Fly-Buck converter has two high di/dt loops on the primary side. All secondary loops are high di/dt.

Another thing to remember when laying out Fly-Buck converters: the secondary winding also has a switching node. The secondary switch node (SW2) is a high dv/dt node, which supports voltage conversion of VIN*N2/N1. Therefore, it is usually necessary to make the SW2 trace area small to prevent it from emitting noise.

Figure 3 is an example of a layout that incorporates the guidance content of this article. As with the switch node area, the high di/dt loop on the primary and secondary sides can also be minimized.

The synchronous buck converter has been recognized as an isolated bias power supply in the communications and industrial markets. The isolated buck converter, or the so-called Fly-Buck™ converter, uses a coupled inductor instead of the buck converter inductor to create an isolated output and a non-isolated buck output. Each isolated output requires only one winding, one rectifier diode and one output capacitor. This topology can be used to generate multiple semi-regulated isolated or non-isolated outputs in a low-cost and simple manner.

There are some major current differences between the buck converter and the Fly-Buck converter. We are already familiar with the switched current loop in a buck converter, as shown in Figure 1. The input loop including 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 mute operation to achieve the minimum trace length and minimum loop area. The output loop including the low-side switch, inductor, output capacitor, and ground return path actually carries a 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 voltage regulation error, the area of ​​this loop is not as important as the input current loop.

Fly-Buck converter PCB layout skills
Figure 1. The current loop in a buck converter. The VIN loop is a high di/dt loop.

The primary side of the Fly-Buck converter looks similar to a buck converter, as shown in Figure 2. The VIN loop here, like the buck converter, is also a high di/dt loop. However, the current in the VOUT1 loop is very different from the buck converter. In addition to the magnetizing current of the primary inductor, the loop also contains the reflected current from the secondary winding. The reflected current only contains the leakage inductance of the coupled inductor in its path, so the di/dt is significantly higher than the magnetizing current of the inductor. Therefore, it is also very important to minimize the loop area of ​​the VOUT1 loop. In the same way, the secondary output loop including the secondary inductor winding, rectifier diode, and secondary output capacitor also needs to be minimized because of the high di/dt current flowing through it.

Fly-Buck converter PCB layout skills
Figure 2. The Fly-Buck converter has two high di/dt loops on the primary side. All secondary loops are high di/dt.

Another thing to remember when laying out Fly-Buck converters: the secondary winding also has a switching node. The secondary switch node (SW2) is a high dv/dt node, which supports VIN*N2/N1 voltage conversion. Therefore, it is usually necessary to make the SW2 trace area small to prevent it from emitting noise.

Figure 3 is an example of a layout that incorporates the guidance content of this article. As with the switch node area, the high di/dt loop on the primary and secondary sides can also be minimized.

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