3.2 Rectifier and Switch
Choosing the rectifying diode and switching transistor is done in such a way that they exhibit as close to ideal switch behavior as possible. The following is a few points to take into consideration when choosing the switching transistor:
Switching Transistor
- Low figure of merit
- Low ON-resistance
- High switching speed
- VDS rating to handle voltage spikes
- Ability to switch using the logic level of the controller
If the input voltage of the buck converter is higher than the logic level of the microcontroller, additional circuitry is needed to achieve reliable switching. The figures below show two such options for switching circuitry that always satisfies the requirement of operating the switching transistor using the logic level of the switching controller. An N-channel MOSFET will typically have a lower on-resistance than a P-channel MOSFET, meaning that Figure 3-4 will have a higher efficiency than Figure 3-3, but at the cost of some additional complexity in the supporting circuit.
The diode choice is made to minimize the forward voltage, as the power loss in the diode is proportional to the forward voltage and driving current, meaning that the power loss can become substantial for regulators delivering higher currents. Typically, a good diode choice will be a Schottky diode with low forward voltage and sufficient power rating.
For applications that require smaller losses in the rectifying circuit, a synchronous solution can be implemented by replacing the diode with a MOSFET. As the switching transistor and the rectifying MOSFET can never be turned on at the same time, additional logic has to be added to introduce dead time control. However, only the most basic solution with a simple diode is outlined in this application note.