SDAA195 January   2026 LMG3522R030

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Operating Principles in a Flying Capacitor Switching Cell
    1. 2.1 Switching Pattern of a 3-Level Flying Capacitor Switching Cell
  6. 3Design Considerations of a Flying Capacitor Switching Cell
    1. 3.1 Conduction and Switching Losses
      1. 3.1.1 AC Conduction Losses
      2. 3.1.2 AC Switching Losses
    2. 3.2 Passive Components Design
      1. 3.2.1 Boost Inductor Design
      2. 3.2.2 DC Link High Frequency Ripple
      3. 3.2.3 Flying Capacitor Design
    3. 3.3 Layout Considerations
    4. 3.4 Pre-Charging Network
  7. 4Experimental Results
    1. 4.1 Pre-charging of the Flying Capacitor of TIDA-010957
    2. 4.2 Steady State Operation
  8. 5Summary
  9. 6References

3.2.3 Flying Capacitor Design

In this topology, the flying capacitor is a critical component. This component must be selected with a voltage rating greater than half the DC link voltage. Since when the complete inductor current flows into the flying capacitor a large voltage ripple is present. The peak-to-peak voltage ripple across the flying capacitor can be calculated as in:

Equation 6. ∆ V F C = 0.5 - A B S D - 0.5 × I L C F C × f s w

Where CFC is the capacitance value of the flying capacitor.

Note that from this formula that when increasing the switching frequency, the ripple voltage can be decreased quite significantly, thus making GaN transistors more appealing in this topology.

Furthermore, the RMS current can be calculated as follows.

Equation 7. I F C , R M S = 2 0.5 - A B S D - 0.5 × ( I L 2 + ∆ I L 2 12 )

The flying capacitor ripple voltage and the RMS current in function of the duty-cycle are plotted in Figure 3-7 and Figure 3-8.

Flying Capacitor Peak-to-Peak Voltage in Function of the Duty CycleFigure 3-7 Flying Capacitor Peak-to-Peak Voltage in Function of the Duty Cycle

RMS Current of the Flying Capacitor in Function of the Duty Cycle

Figure 3-8 RMS Current of the Flying Capacitor in Function of the Duty Cycle

Note that from Figure 3-7 and Figure 3-8 that the worst-case scenario is happening when the duty cycle is equal to 50%. The fundamental ripple frequency flowing in the flying capacitor is equal to the switching frequency. To achieve a good cost structure of the designed converter, TI recommends using film capacitor because it provides high capacitance while also being able to handle large ripple currents in a low-cost design.