SLUAAX9 June   2025 BQ25756

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2MOSFET Power Losses in Buck and Boost Chargers
    1. 2.1 Buck Mode Losses
    2. 2.2 Boost Mode Losses
    3. 2.3 Closing Thoughts for FET Losses
  6. 3Evaluating MOSFETs Using the Design Calculator
    1. 3.1 Correlating MOSFET Data Sheet Parameters With the Design Calculator Parameters
    2. 3.2 Design Calculator MOSFET Comparison Example
  7. 4BOM Evaluation
  8. 5Summary
  9. 6References

2.3 Closing Thoughts for FET Losses

As can be inferred by the equations, switching losses are dominant when the charge current is low. As the current increases, the conduction losses increase as well and the conduction losses become the dominant loss term in the equation. Selecting an efficient FET can depend on the switching frequency of the charger and the average charge current.

Also, the recommendation is to limit the switch node capacitance such that the following is true:

Equation 37. CswnF<160Vin

If Vin is 60V, the recommendation is to keep the total switch node capacitance (Csw) under 2.67nF. This can mitigate the switching loss and is a requirement for proper device functionality.

For ease of reference, BQ2575X's gate driver resistance of Ron and Roff and the dead time (tdead_time) are listed in Table 2-1. The dead time can be adjust to be 45ns, 75ns, 105ns, or 135ns.

Table 2-1 BQ2575X IC's Gate Driver Characteristics
ParameterTypical Value
Ron3.4Ω
Roff1.0Ω
tdead_time (Both sides)45ns