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How to design a stable Fly-Buck™ converter with COT (Part 2)
In part one of this two-part blog, we discussed the important design criteria required to make a Fly-Buck design stable. In this section, we will show how to apply that design criteria to a Fly-Buck design, and how it will affect the operation of the converter.
Figure 2. A typical two-output LM5017 Fly-Buck circuit
Let’s assume that you gathered the required power specifications and have decided to use the LM5017 Fly-Buck as the power solution (Figure 2). The Fly-Buck design process has a lot in common with a regular buck converter. After determining the primary side inductance and switching frequency, the next step is to design a proper ripple injection network (Rr, Cr and Cac) to ensure stable operation. The design steps are as follows:
Start from some initial values: Rr=10kΩ, Cr=10nF, Cac=1nF
Adjust the output capacitance to make sure the stability criterion is met:
Sufficient margin should be given to make the left side of the inequality. The real duty cycle will be wider if power loss is included, and the DC bias derating and temperature effect on the ceramic caps will make the capacitance even lower.
Determine if the FB pin ripple is above 25mV:
Adjust the Rr and Cr to set the ripple magnitude. Don’t make it too large (<100mV), and go back to step 2 to adjust the Cout if necessary.
Determine if the AC coupling filter cutoff frequency is low enough:
The Cac selection is not critical, and the Fsw/10 is a suggested value. As long as Fc is considerably lower than Fsw, it won’t affect the ripple pass-through.
The circuit simulation waveforms in Figures 3 to 5 show the impact of the different ripple injection setting. The Fly-Buck converter spec is as follows: Vin=24V, Vopri=10V, Fsw=275kHz, Lpri=50uH, and 1:1 transformer turns ratio. In Figure 3, the circuit configuration doesn’t meet the stability criterion, and the switch node voltage showing double-pulse is unstable as a result. In Figure 4, Rr is decreased to 30kΩ from 50kΩ, and the switching becomes stable. Figure 5 shows another way to improve the stability: Increase Cout without changing Rr and Cr. It can help keep the ripple voltage down while meeting the criterion.
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, the power supply designer needs to take many aspects into consideration, and adjustments may be required based on the board test result. These design steps are a good start to creating a LM5017 Fly-Buck design. A quick start calculator for the LM5017 Fly-Buck can be found here. With it, you can design a Fly-Buck with up to four isolated outputs; the ripple injection network calculation and the other external components are also included. Any questions? Please let me know.
How to design a stable Fly-Buck™ converter with COT (Part 1)
Application Note: AN-2292 Designing an Isolated Buck (Fly-Buck) Converter
Tian, S, “Small-signal Analysis and Design of Constant-on- time V2 Control for Ceramic Caps”, Master Thesis, Virginia Tech, 2012
Wide Vin DC/DC Power Solutions from Texas Instruments
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