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

3.1 Correlating MOSFET Data Sheet Parameters With the Design Calculator Parameters

This section aims to correlate device parameters typically given on a MOSFET data sheet to the parameters included in the design calculator. If a parameter is not on the data sheet, reach out to the device manufacturer.

These parameters here are the inputs required to use the design calculator. The parameters are the same for the buck-phase and the boost-phase switching MOSFETs.

MOSFET Parameter Inputs for the BQ2575x Design CalculatorFigure 3-2 MOSFET Parameter Inputs for the BQ2575x Design Calculator

The first parameter in the design calculator is the on-state resistance (RDS(on)). This is the resistance across the drain and source terminals when the MOSFET is turned-on. The calculator asks for RDS(on) at 4.5V and 10V. RDS(on) is typically given in the data sheet either in the electrical characteristics section or in a graph that plots the Vgs versus RDS(on).

The total gate charge (QG) refers to the charge (in Coulombs) necessary to charge the gate capacitance to turn on the MOSFET so that the actual gate voltage of the MOSFET matches the driving voltage. This is different from the switching charge. QG is typically given in the MOSFET data sheet. The gate-drain charge (QGD) and gate-source charge (QGS) are also usually provided. The output charge, Qoss, is defined in Equation 38.

Equation 38. Qoss=∫0VDSCoss(v)dv

The internal gate resistance RG can be modeled by a resistor in series with the MOSFET gate, as depicted in Figure 2-1.

The transconductance (gfs) refers to the small signal gain of the MOSFET. The threshold voltage (VTH) is the voltage at which the MOSFET enters the active region and conducts current across the drain and the source terminals. Both of these parameters are typically included in the MOSFET data sheet.

The diode forward voltage (VSD) is the voltage drop across the drain and source terminals when the body diode of the MOSFET is forward biased.

The reverse recovery charge (QRR) is the charge that is stored in the body diode that must be depleted before the body diode can block the flow of current in the reverse biased direction.

The thermal resistance is a measure of the thermal performance of the MOSFET. The higher the thermal resistance, the higher the temperature change of the MOSFET for a given power dissipation.