SNVS480J January 2007 – July 2020 LM5022
Selection of the power MOSFET is governed by tradeoffs between cost, size, and efficiency. Breaking down the losses in the MOSFET is one way to determine relative efficiencies between different devices. For this example, the SO 8-pin package provides a balance of a small footprint with good efficiency (see Q1 in Table 2).
Losses in the MOSFET can be broken down into conduction loss, gate charging loss, and switching loss.
Conduction, or I2R loss (PC) is approximately Equation 3.
The factor 1.3 accounts for the increase in MOSFET on-resistance due to heating. Alternatively, the factor of 1.3 can be ignored and the maximum on-resistance of the MOSFET can be used.
Gate charging loss, PG, results from the current required to charge and discharge the gate capacitance of the power MOSFET and is approximated with Equation 4.
QG is the total gate charge of the MOSFET. Gate charge loss differs from conduction and switching losses because the actual dissipation occurs in the LM5022 and not in the MOSFET itself. If no external bias is applied to the VCC pin, additional loss in the LM5022 IC occurs as the MOSFET driving current flows through the VCC regulator. This loss (PVCC) is estimated with Equation 5.
Switching loss (PSW) 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 loss can be approximated with Equation 6.
For this example, the maximum drain-to-source voltage applied across the MOSFET is VO plus the ringing due to parasitic inductance and capacitance. The maximum drive voltage at the gate of the high-side MOSFET is VCC, or 7 V typical. The MOSFET selected must be able to withstand 40 V plus any ringing from drain to source, and be able to handle at least 7 V plus ringing from gate to source. A minimum voltage rating of 50-VD-S and 10-VG-S MOSFET is used. Comparing the losses in a spreadsheet leads to a 60 VD-S rated MOSFET in SO-8 with an RDSON of 22 mΩ (the maximum value is 31 mΩ), a gate charge of 27 nC, and rise and falls times of 10 ns and 12 ns, respectively.