How can you implement darlington pair using mosfet?

Answer 1

its not too hard... darlington transistors are simply a pair of bipolar transistors which allow for control of a significantly greater current or voltage (or both) via a low power signal such as a microcontroller. Frequently Transistors that can control a larger current or voltage require a higher gate voltage (Vgs) to fully turn on the transistor than the microcontroller is able to deliver so a smaller transistor is used to boost the control signal. You can build your own darlington out of a pair of bipolars - one that runs at logic level and boosts the signal to control the second one that runs at the higher current & voltage of the end device.

The same can be done with mosfets. There are a fair number of mosfets that can control a decent current and voltage but in many cases you'll need to use either discrete boost (charge pump) mosfets, or a control chip that has built in charge pump circuitry. This is a fairly common requirement in building motor controllers and amplifiers which are or can be controlled by PWM or some other form of logic level signal. The international rectifier IR2104 is a good example of logic level mosfet that can be paired with a more powerful mosfet such as IRF540N for the actual current control. As mentioned you can also use a driver circuit such as those found in the HIP408x series which will obviate the need for a boost mosfet - this configuration is found in a number of commercial motor controllers including those by numerous wheelchair manufacturers.