This is a supplementary article relating to “Operating Principles of Buck Switching Regulator” in “Switching Regulator Basics.” In “Operating Principles of Buck Switching Regulator”, an explanation of ”Nonsynchronous (diode) rectifying buck converter: Its circuitry and operation” is given. As a supplement, in this article an explanation is given of the current paths during synchronous rectifying step-down converter operation, which may be regarded as standard operation in the high-efficiency DC/DC converters of recent years.
Current Paths during Synchronous Rectifying Step-Down Converter Operation
Taking the abovementioned article as an example, we explain the current paths in a nonsynchronous (diode) rectifying step-down converter. To begin with, current paths during operation are essentially the same in nonsynchronous rectification and in synchronous rectification. This is because in a synchronous rectification circuit, the diode that is the low-side switch in nonsynchronous rectification is replaced with a transistor. Given this premise, we explain the current paths and points to be noted in a synchronous rectifying step-down converter.
■Current path when the high-side switch is ON
■Current path when the low-side switch is ON
■Switching current waveform
Below are shown switching current waveforms and inductor current waveforms. IHG and ILG are the currents of the high-side and low-side switches. They should be compared with the explanation of circuit operation.
The inductor current IL is the synthesis of IHG and ILG, and the output current IO is the average of IL.
■Points to note regarding the switching current and PCB layout
The blue lines in the following diagram indicate the differences in currents flowing when switches are turned on and off in the current paths presented above.
This current has the following features.
Hence in the PCB (printed circuit board) layout, the layout must be designed according to theory, with places where these differences occur identified as places requiring attention. Regarding PCB layout, please refer to “DC/DC Converter PCB Layout” in the Design edition.
・Current paths during operation of a synchronous rectifying step-down converter are basically the same as in a nonsynchronous (diode) rectifying converter.
・Current paths and the lines in which sudden changes in current occur must be understood when undertaking PCB layout.