8.2.1.2.13 MOSFET Selection

Selection of the power MOSFETs is governed by the same tradeoffs as switching frequency. Breaking down the losses in the high-side and low-side MOSFETs is one way to compare the relative efficiencies of different devices. When using discrete SO-8 MOSFETs, generally the output current capability range is 2 A to 10 A. Losses in the power MOSFETs can be broken down into conduction loss, gate charging loss, and switching loss. Conduction loss P_DC is approximately Equation 36 and the example (Equation 37).

Equation 36.

Equation 37.

where

• D is the duty cycle
• The 1.3 factor accounts for the increase in MOSFET ON-resistance due to heating

Alternatively, the factor of 1.3 can be eliminated and the high temperature ON-resistance of the MOSFET can be estimated using the R_DS(ON) vs Temperature curves in the MOSFET datasheet. Gate charging loss, P_GC, results from the current driving the gate capacitance of the power MOSFETs and is approximated with Equation 38.

Equation 38.

where

• Q_g refers to the total gate charge of an individual MOSFET
• n is the number of MOSFETs

Gate charge loss differs from conduction and switching losses in that the actual dissipation occurs in the LM25119 device and not in the MOSFET itself. Further loss in the device is incurred if the gate driving current is supplied by the internal linear regulator.

Switching loss occurs during the brief transition period as the MOSFET turns on and off. During the transition period both current and voltage are present in the channel of the MOSFET. The switching loss can be approximated with Equation 39.

Equation 39.

where

• t_R and t_F are the rise and fall times of the MOSFET

The rise and fall times are usually mentioned in the MOSFET datasheet or can be empirically observed with an oscilloscope. Switching loss is calculated for the high-side MOSFET only. Switching loss in the low-side MOSFET is negligible because the body diode of the low-side MOSFET turns on before the MOSFET itself, minimizing the voltage from drain to source before turnon. For this example, the maximum drain-to-source voltage applied to either MOSFET is 36 V. The selected MOSFETs must be able to withstand 36 V plus any ringing from drain to source, and be able to handle at least the VCC voltage plus any ringing from gate to source. A good choice of MOSFET for the 36-V input design example is the SI7884. It has an R_DS(ON) of 7.5 mΩ and total gate charge of 21 nC. In applications where a high step-down ratio is maintained in normal operation, efficiency may be optimized by choosing a high-side MOSFET with lower Q_g, and low-side MOSFET with lower R_DS(ON).