In practical applications, especially in the design of switching power supplies or control circuits, enhanced N-MOSFETs are almost always used as switches. Why is that?

• Why are MOSFETs chosen as switches instead of bipolar transistors?

• Why is enhanced MOSFET almost always chosen, why not depletion-type?

NMOS Schematic Diagram

Firstly, let's address the first question. VBsemi has mentioned many reasons and circuit analyses in previous discussions on switch applications, but let's briefly explain here. (Those already familiar can skip to the second point.)

1. Why are MOSFETs chosen as switches instead of bipolar transistors?

MOSFETs have several advantages over bipolar transistors:

• MOSFETs have lower on-resistance and higher switching speeds.

• They have different input impedances: Bipolar transistors achieve output current through base current control, which results in a lower input impedance due to the presence of base current; whereas MOSFET's output current depends on the gate-source voltage, and the gate does not draw current, resulting in higher input impedance.

• Different equivalent resistances: Compared to bipolar transistors, MOSFETs have a smaller equivalent resistance in saturation state, typically only a few tens of milliohms or a few milliohms, resulting in higher efficiency.

Bipolar transistors are mainly used in low-noise amplification circuits.

2. Why is an enhanced MOSFET almost always chosen, why not depletion-type?

In switching power supplies, the main function of MOSFETs is as a "switch."

They are used in combination with switches, capacitors, inductors, and diodes to form various topological networks, thereby achieving various switching modes and implementing desired circuits such as boost, buck, or buck-boost.

We generally control MOSFETs using microcontrollers or controllers because the output pins of microcontrollers, FPGAs, or other chips are basically positive voltages. Digital circuits operate on a single power supply, meaning that the supply range is from 0 to VCC; in other words, the entire circuit board has signals greater than 0V.

However, depletion-type MOSFETs are basically controlled from negative voltage to 0V. When there is no need for a negative power supply for current, providing a negative voltage power supply for better MOSFET control would make circuit design very inconvenient. Therefore, in the selection process, P-MOSFETs are rarely used.

So, why choose N-MOSFETs over P-MOSFETs?

In fact, this question has been addressed in previous discussions ("Why Are PMOS Applications Much Less Than NMOS?").

This stems from the advantages of NMOS:

1. It has lower on-resistance, suitable for more efficient switching power supplies.

2. It has higher current-carrying capacity, suitable for high-power switching power supplies.

3. Positive voltage is used in power supply systems, making it more compatible with these circuits.

4. NMOS has higher compatibility and broader application areas in integrated circuits, with higher integration and reliability.

Additionally, it's worth noting that there are situations where PMOS is suitable for certain switching circuits. Specific application conditions may have better matches for PMOS, and the specific problem requires specific analysis.