When the input is positive, the power FET conducts, and the load is powered on. The most important characteristic of this block is the low Rds on. Rds is the resistance between the drain and source of a power FET. This resistance can fall to values as low as fraction of an ohm (0.01 ? is found in some devices). This means that practically no power is dissipated in the transistor, which can control high currents easily.
However, the reader must bear in mind that low Rds on is valid only when high control voltages are used. When operating with low-voltage sources, the power FET has the same characteristics of the bipolar transistor when conducing the current, and some power is converted into heat. Another important characteristic of this circuit is its very high input impedance.
