Electronic

### How to Apply Converter Design Metrics to Fly-Buck Circuit Design

From a practical standpoint, power supply designers need to consider many aspects, which may need to be adjusted based on board test results. These design steps are a good starting point for creating an LM5017 Fly-Buck design. A quick start calculator for the LM5017 Fly-Buck is available here. With it, not only can you design a Fly-Buck with up to 4 isolated outputs, but you can also gain ripple injection network computing capabilities and other external components.

Author: Xiang Fang

This blog is divided into two parts, in Part 1 we explore the important design metrics needed to make the Fly-Buck design stable. In this article we will describe how to apply these design specifications to Fly-Buck circuit design and how this affects the converter operation.

Figure 2. Typical Dual Output LM5017 Fly-Buck Circuit

We assume that you have gathered the required power supply specifications and have decided to use the LM5017 Fly-Buck as the power supply solution (Figure 2). The Fly-Buck design process has a lot in common with a normal buck converter. After determining the primary side inductance and switching frequency, the next step is to design a suitable ripple injection network (Rr, Cr and Cac) to ensure stable operation. The design steps are as follows:

Adjust the output capacitor to ensure stability specifications are met:

Sufficient margin should be provided to obtain information on the left side of the inequality. The true duty cycle is a bit larger if power losses are included, and the derating of the DC bias and the temperature effect on the ceramic capacitor will result in a lower value.

3. Determine if the FB pin ripple is above 25mV:

The ripple amplitude is set by adjusting Rr and Cr. Do not make it too large (<100mV), and return to step 2 to adjust Cout if necessary.

4. Determine if the AC coupling filter cutoff frequency is low enough:

The choice of Cac is not critical, the recommended value is Fsw/10. As long as Fc is significantly lower than Fsw, it does not affect the passage of ripple.

The circuit simulation waveforms in Figures 3 through 5 show the effect of different ripple injection settings. The Fly-Buck converter specifications are as follows: Vin=24V, Vopri=10V, Fsw=275kHz, Lpri=50uH, and the transformer turns ratio is 1:1. In Figure 3, the circuit configuration does not meet the stability criteria, so the switch node voltage showing the double pulse is unstable. In Figure 4, Rr is reduced from 50kΩ to 30kΩ, and the switching performance becomes stable. Figure 5 is another way to improve stability: increase Cout without changing Rr and Cr. This helps keep the ripple voltage low while meeting the specifications.

Figure 3. Rr=50kΩ, Cr=10nF, Cac=1nF, Cout=4.7uF

Figure 4. Rr=30kΩ, Cr=10nF, Cac=1nF, Cout=4.7uF

Figure 5. Rr=50kΩ, Cr=10nF, Cac=1nF, Cout=10uF

From a practical standpoint, power supply designers need to consider many aspects, which may need to be adjusted based on board test results. These design steps are a good starting point for creating an LM5017 Fly-Buck design. A quick start calculator for the LM5017 Fly-Buck is available here. With it, not only can you design a Fly-Buck with up to 4 isolated outputs, but you can also gain ripple injection network computing capabilities and other external components. Please let me know if you have any questions.