In the Buck switch circuit, NMOS is usually used as the switch tube. We know that compared with PMOS, NMOS has advantages in on-resistance and price.

In the synchronous structure, MOS tubes can be used as switch tubes in two ways.

1. No external bootstrap circuit is required: the upper tube is PMOS and the lower tube is NMOS

 2. External bootstrap circuit is required: both upper and lower tubes are NMOS

 Why is it necessary to add a bootstrap circuit when using NMOS tubes?

This is related to the conduction condition of NMOS: it is turned on when the gate voltage is higher than the source voltage, so the upper tube must add a bootstrap circuit to ensure full conduction.

Working principle of bootstrap circuit:

When the upper tube is turned off and the lower tube is turned on, the voltage at the switch node is pulled down to GND, and then the internal power supply charges the capacitor through the diode.

When the upper tube is turned on and the lower tube is turned off, the switch node voltage becomes high, the diode is cut off at this time, and the bootstrap capacitor starts to work after being fully charged.

We know that the voltage across the capacitor cannot change suddenly, the gate voltage of the upper tube is greater than the source voltage Vin, and the upper tube is turned on smoothly.

Note:

In practical applications, when using a bootstrap circuit, if the duty cycle is too large, instability will occur.

When the duty cycle is large, the lower tube conduction time is relatively short.

The bootstrap capacitor is charged only when the lower tube is turned on, which will result in the off time to charge the capacitor is not enough to drive consumption, causing the Boot pin voltage to continue to drop. When it drops to the point where it cannot fully drive the upper tube, the upper tube drive circuit will be turned off and instability will occur.

Solution

1. Use PMOS for the upper tube

2. The upper tube uses NMOS, but a charge pump circuit is needed inside to replace the bootstrap capacitor. However, this requires a voltage higher than Vin to be continuously provided, which will be more complicated in actual application and increase the cost.

 Click on the homepage to get more technical knowledge about MOSFET. VBsemi editor updates it daily~Thank you for reading!