SLYT819 October   2021 BQ25980

 

  1. 1 Introduction
  2. 2 Delivering over 100 W power with USB PD
  3. 3 High-efficiency switched-capacitor DC/DC converters
  4. 4 Output impedance
  5. 5 Switching losses
    1. 5.1 Turnon switching loss
    2. 5.2 Turnoff switching loss
    3. 5.3 Gate driving loss
  6. 6 Dual-phase interleaved switched-capacitor converter
  7. 7 High-power-density switched-capacitor converter
  8. 8 Switched-capacitor converter application: fast charging for portable devices
  9. 9 Conclusion
  10. 10Related Websites
  11. 11Important Notice

3 High-efficiency switched-capacitor DC/DC converters

Switched-capacitor DC/DC converters are switching regulators that use only flying capacitors and switches to transfer charges between the input and output. Compared to inductor-based DC/DC converters, switched-capacitor converters offer higher power density (capacitors store 10 to 100 times more energy per volume than inductors), low electromagnetic interference (EMI) and lower cost.

A 2-to-1 switched-capacitor DC/DC converter uses four switches to alternately charge and discharge flying capacitors to deliver power. Figure 1 shows a simplified switched-capacitor circuit and the charging/discharging phase of the flying capacitor. The output voltage is half of the input voltage, and the output current is twice the input current.

Figure 3-1 2-to-1 switched-capacitor converter: flying capacitor charging phase (a); and discharging phase (b).

It is possible to use an ideal transformer in series with the output impedance (ROUT) to model the switched-capacitor converter efficiency [3], including the charge-sharing loss of the flying capacitor and the conduction loss in the resistive components. Switching loss is not included in the model, however, and requires separate analysis.