If we turn ON a MOSFET with Resistive load, current and voltage transition will start simultaneously. But if the load is inductive, say Relay or any DC DC converter, current rises first to max load current and then only voltage starts to fall. Why?

 The behavior you describe occurs because of the properties of inductors.

When a MOSFET is turned on with a resistive load, the current and voltage transition start simultaneously because a resistor does not have any stored energy. As soon as the MOSFET is turned on, current starts flowing through the resistor, and the voltage across the resistor starts dropping.

However, when a MOSFET is turned on with an inductive load, such as a relay or DC-DC converter, the current through the inductor cannot change instantaneously. When the MOSFET is turned on, current starts to flow into the inductor, and the magnetic field around the inductor begins to build up. As the magnetic field builds up, the inductor resists any change in current. Therefore, the current through the inductor increases slowly, taking some time to reach its maximum value.

The voltage across the inductor is proportional to the rate of change of current through it, according to Faraday's Law. Thus, while the current is increasing, the voltage across the inductor remains high, preventing the voltage across the load from dropping quickly. Once the current has reached its maximum value, the voltage across the inductor starts to drop, and the voltage across the load starts to drop as well.

This behavior is described by Lenz's Law, which states that the induced electromotive force in a circuit always opposes the change that caused it. In this case, the induced voltage across the inductor opposes the change in current through it, slowing down the current rise and causing the voltage across the load to fall more slowly.

Therefore, when a MOSFET is turned on with an inductive load, the current through the inductor rises first to the maximum value, and then the voltage across the load starts to fall.