Supplement－Current Paths during Synchronous Rectifying Step-Down Converter Operation

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

• The red lines in the above diagram indicate the main currents flowing in the converter when the high-side switch (in actuality a MOSFET) Q1 is turned on.
• C_BYPASS is a high-frequency decoupling capacitor, and C_IN is a large-value capacitor.
• Most of the sudden current that flows at the moment when the high-side switch Q1 is turned on is supplied from C_BYPASS, and then from C_IN.
• Slowly changing current is supplied from the input power supply.

■Current path when the low-side switch is ON

• In synchronous rectification, the high-side and low-side switches are turned on and off in alternation.
• The red line in the diagram indicates the flow of current when the high-side switch Q1 is turned off and the low-side switch Q2 is turned on.
• The output voltage V_O is smoothed by the inductor and the output capacitor C_O.
• An inductor is inserted in series with the output of the step-down converter, so that the current in the output capacitor is smooth.

■Switching current waveform

Below are shown switching current waveforms and inductor current waveforms. I_HG and I_LG are the currents of the high-side and low-side switches. They should be compared with the explanation of circuit operation.

The inductor current I_L is the synthesis of I_HG and I_LG, and the output current I_O is the average of I_L.

■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.

• Each time the high-side switch Q1 changes from off to on and from on to off, the current in the paths indicated by the blue lines changes rapidly.
• Because there are sudden changes in this system, waveforms containing numerous harmonics appear.

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.