What is the difference between direct drive circuits of NMOS and PMOS?

There are many ways to drive the high-side non-isolated gate, such as direct drive, level shift drive, bootstrap technology, etc., but they all have their own differences. What are the differences between the direct drive circuits of NMOS and PMOS?

When the maximum input voltage is lower than the gate-source breakdown voltage of the device, PMOS direct drive can be used.

 It is applied in PMOS 12V input DC/DC converter.

The difference between it and NMOS is the path of the gate drive current , which does not flow into the ground connection. Instead, the positive rail interconnect conducts the high charging and discharging currents of the gate. Using wide traces or planes to provide the positive input can minimize the loop inductance in the gate drive.

NMOS direct drive can be achieved through a PWM controller or grounding.

 The switching of this circuit occurs at the source terminal of the device because the drain is connected to the positive DC input voltage rail. It is the same clamped inductive switch, but note here that the gate drive current cannot return to ground at the source terminal.

Instead, it must flow through the load connected to the source of the device.

In discontinuous inductor current mode, the gate charging current must flow through the output inductor and the load. In continuous inductor current mode, the loop can be closed through the conducting pn junction of the rectifier diode.

During the off time, the gate discharge current flows through the rectifier diode connected between ground and the MOSFET source.

This results in some differences, the gate drive circuit requires more components, the loop area becomes larger, and the parasitic source inductance increases.

The above are the design situations of these two circuits. In practical applications, NMOS is often used as the main power supply because of its lower price, faster speed and lower on-resistance.

The above is all the content of this issue. In the next issue, we will explain the level shift drive circuit of MOSFET in the high-side non-isolated gate drive.