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

3.4 Pre-Charging Network

A critical subject in flying capacitor converter is the precharging method of the flying capacitor itself.

During the nominal operation of the converter, the flying capacitor voltage level is set up to a nominal voltage of Vdc/2 by a voltage control loop. By having the flying capacitor voltage controlled to half the DC bus voltage, the voltage stress across the FETs can be specified to be half the DC bus voltage.

When connecting for the first time the converter to the grid, the MCU and AUX power supplies are off. During the precharging sequence of the converter, DC link voltage starts to rise but the flying capacitor voltage is still equal to zero. If the grid voltage is high and the DC bus voltage reaches value higher than the breakdown voltage of the transistor, the FETs risk damage. This problem is solved by using Zener diodes and resistors in parallel to the S1 and S4 as shown in Figure 3-11.

Precharging Network of the Three-Level Flying Capacitor Converter Figure 3-11 Precharging Network of the Three-Level Flying Capacitor Converter

Having Zener diodes in parallel to the FETS help to clamp the voltage across them. By doing this, once the Zener voltage is reached the flying capacitor starts to charge. To control the charging the current, a resistor of few ohms can be placed in series, thus limiting the charging current. A trade-off between the charging the current and the charging time can be found by changing the value of the resistor based on the capacitance value. The constant time of the circuit can be calculated in Equation 8.

Equation 8. τ = 2 R P R C F C