Transistor and MOSFET switches

BJT switch — an NPN driven from a microcontroller pin into saturation. The base current is the load current divided by a forced β of about 10 (hard saturation whatever the actual hFE), so R_b = (V_drive − V_be(sat)) ÷ (I_c/β_forced + V_be/R_be), rounded down to E24. The tool checks that the pin can supply the base current, that hFE(min) × I_b exceeds the load current, and gives the dissipation V_ce(sat)·I_c + V_be·I_b.

MOSFET — conduction loss I²·R_ds(on)·duty with R_ds(on) rising about 0.5% per °C, iterated with T_j = T_a + P·θ_JA. With a switching frequency it adds the switching loss ½·V·I·(t_r + t_f)·f, where the transition times come from the Miller charge: t_r ≈ Q_gd·R_g ÷ (V_drive − V_plateau) and t_f ≈ Q_gd·R_g ÷ V_plateau (see Vishay AN608A and TI SLUA618), and the gate-drive power Q_g·V_gs·f. The drive check compares your gate voltage with the V_gs at which R_ds(on) is specified: a MOSFET is only "logic level" for a given drive if its R_ds(on) is guaranteed at that voltage — V_gs(th) is where it barely starts to conduct.

High-side P-channel — to switch the positive supply, an NPN (or a small N-MOSFET) pulls the P-channel gate down and R1 pulls it back to the source. On supplies above about 12 V a second resistor R2 limits |V_gs| = V_s·R1/(R1 + R2) to stay inside V_gs(max).

Inductive loads — the energy ½·L·I² stored in a relay coil, solenoid, or motor must go somewhere at turn-off: a flyback diode across the load releases it slowly (τ = L/R); a diode plus Zener releases it in about L·I/V_z if the transistor can take supply + V_z.