8.1.1 MOSFET Selection

The important MOSFET electrical parameters are the maximum continuous Drain current I_D, the maximum Source current (that is, body diode) I_S, the maximum drain-to-source voltage V_DS(MAX), the gate-to-source threshold voltage V_GS(TH), the drain-to-source reverse breakdown voltage V_(BR)DSS, and the drain-to-source On resistance R_DS(ON).

The maximum continuous drain current, I_D, rating must exceed the maximum continuous load current. The rating for the maximum current through the body diode, I_S, is typically rated the same as, or slightly higher than the drain current, but body diode current only flows while the MOSFET gate is being charged to V_GS(TH).

Gate Charge Time = Q_g / I_GATE(ON)

1. The maximum drain-to-source voltage, V_DS(MAX), must be high enough to withstand the highest differential voltage seen in the application. This would include any anticipated fault conditions.
2. The drain-to-source reverse breakdown voltage, V_(BR)DSS, may provide some transient protection to the OUT pin in low voltage applications by allowing conduction back to the IN pin during positive transients at the OUT pin.
3. The gate-to-source threshold voltage, V_GS(TH), should be compatible with the LM5050-1 gate drive capabilities. Logic level MOSFETs, with R_DS(ON) rated at V_GS(TH) at 5 V, are recommended, but sub-Logic level MOSFETs having R_DS(ON) rated at V_GS(TH) at 2.5 V, can also be used.
4. The dominate MOSFET loss for the LM5050-1 active OR-ing controller is conduction loss due to source-to-drain current to the output load, and the R_DS(ON) of the MOSFET. This conduction loss could be reduced by using a MOSFET with the lowest possible R_DS(ON). However, contrary to popular belief, arbitrarily selecting a MOSFET based solely on having low R_DS(ON) may not always give desirable results for several reasons:
   1. Reverse transition detection. Higher R_DS(ON) will provide increased voltage information to the LM5050-1 Reverse Comparator at a lower reverse current level. This will give an earlier MOSFET turnoff condition should the input voltage become shorted to ground. This will minimize any disturbance of the redundant bus.
   2. Reverse current leakage. In cases where multiple input supplies are closely matched it may be possible for some small current to flow continuously through the MOSFET drain to source (that is, reverse) without activating the LM5050-1 Reverse Comparator. Higher R_DS(ON) will reduce this reverse current level.
   3. Cost. Generally, as the R_DS(ON) rating goes lower, the cost of the MOSFET goes higher.
5. The dominate MOSFET loss for the LM5050-1 active OR-ing controller is conduction loss due to source-to-drain current to the output load, and the R_DS(ON) of the MOSFET. This conduction loss could be reduced by using a MOSFET with the lowest possible R_DS(ON). However, contrary to popular belief, arbitrarily selecting a MOSFET based solely on having low R_DS(ON) may not always give desirable results for several reasons:
   1. Selecting a MOSFET with an R_DS(ON) that is too large will result in excessive power dissipation. Additionally, the MOSFET gate will be charged to the full value that the LM5050-1 can provide as it attempts to drive the Drain to Source voltage down to the V_SD(REG) of 22 mV typical. This increased Gate charge will require some finite amount of additional discharge time when the MOSFET needs to be turned off.
   2. As a guideline, it is suggest that R_DS(ON) be selected to provide at least 22 mV, and no more than 100 mV, at the nominal load current.
   3. (22 mV / I_D) ≤ R_DS(ON) ≤ (100 mV / I_D)
   4. The thermal resistance of the MOSFET package should also be considered against the anticipated dissipation in the MOSFET to ensure that the junction temperature (T_J) is reasonably well controlled, because the R_DS(ON) of the MOSFET increases as the junction temperature increases.
6. P_DISS = I_D² × (R_DS(ON))
7. Operating with a maximum ambient temperature (T_A(MAX)) of 35°C, a load current of 10 A, and an R_DS(ON) of 10 mΩ, and desiring to keep the junction temperature under 100°C, the maximum junction-to-ambient thermal resistance rating (θ_JA) must be:
   1. R_θJA ≤ (T_J(MAX) - T_A(MAX))/(I_D² × R_DS(ON))
   2. R_θJA ≤ (100°C - 35°C)/(10 A × 10 A × 0.01 Ω)
   3. R_θJA ≤ 65°C/W