MOS tube oscillation suppression method (3) Selection of Rboot
1.Selection of Rboot
Synchronous Buck converters are generally powered by a bootstrap circuit, as shown in the figure. The oscillation on the rising edge of the switch node is closely related to the turn-on of the upper tube. The drive current path when the upper tube is turned on is as shown in the figure. Therefore, the turn-on speed of the upper tube can be slowed down by increasing Rboot, thereby suppressing the oscillation of the switching node. Similarly, the experimental platform uses the synchronous Buck evaluation version TPS549D22EVM in the figure. When the input voltage is 12V and the output is 1V/20A, the value of Rboot is changed, and the test results are obtained as shown in the chart. Increasing Rboot can significantly suppress the switch node oscillation, but the switching loss will increase significantly, and the value of Rboot should be weighed in practical applications.
Let's estimate the loss on Rboot. Taking TPS549D22 as an example, its internal integrated MOS is 16V/40A. Therefore, using CSD16401Q5 (25V/38A) with similar specifications as a reference, its gate charge Qg is 21nC. Therefore, it can be considered that the internal The Qg of the integrated MOS is also 21nC, and the driving voltage VBP is 5.07V.
When the upper tube is turned on, as shown in the figure, the energy released by Cboot is: