Technical Information Site of Power Supply Design

2017.08.10 DC/DC

Current Paths During Step-Down Converter Operation

DC/DC Converter PCB Layout

During PCB layout of a DC/DC converter, it is necessary to know the current paths during step-down converter operation in order to understand what matters need to be taken into consideration, and why.

A switching regulator is an analog circuit, but in contrast with circuits that operate mainly in linear modes, current and voltage are switched, that is, turned on and off. PCB layout is a task in which optimal paths are designed while considering what kinds and quality of voltages are applied to which nodes and lines, and what kind of currents will be flowing.

In this section, we start of by explaining "current paths during step-down converter operation". Moreover, in this chapter we plan to address each of the following topics.

Figure 1-a. Current paths when the switching element Q1 is turned on

Figure 1-b. Current paths when the switching element Q1 is turned off

Figure 1-c. Important places relating to current differences and layout

Current Paths During Step-Down Converter Operation

The circuit diagram on the right shows a step-down DC/DC converter IC with diode rectification, also called non-synchronous rectification, and an external circuit. The capacitor connected to the BOOT pin is for bootstrapping to drive an internal Nch MOSFET. The resistor and capacitor connected to the COMP pin are components used for phase compensation. These pins may or may not be present, depending on the IC. Other pins and components are basic terminals and necessary external components, and need no explanation.

The red lines in Figure 1-a show the paths and directions of the main currents flowing when the switch Q1 is turned on. CBYPASS is a high-frequency decoupling capacitor, and CIN is a capacitor with a large capacitance.

The instant that the switch Q1 is turned on, a steeply rising current flows, but most of the current is supplied from CBYPASS, and then from CIN. A gently changing current is supplied from the input power supply.

The red lines in Figure 1-b show the current paths when the switch Q1 is turned off. The diode D1 is turned on, and energy accumulated in the inductor L is discharged to the output side.

An inductor is inserted in series with the output of the step-down converter, and so the current in the output capacitor rises and falls, but is smooth.

The red lines in Figure 1-c represent the difference between Figures 1-a and 1-b. Each time the switch Q1 switches from off to on or from on to off, current flowing in the parts indicated by red lines changes suddenly. Changes in this system are steep, and so the current waveform includes numerous harmonics.

These different parts are especially important and must be given the greatest amount of attention in the PCB layout process.

Overall, the same flows result even for a synchronous rectification design, and even if an external switching transistor is supplied. Subsequent explanations will assume these current flows, and so the current paths should be clearly understood.

Key Points:

・When engaging in PCB layout (design), it is important to understand the current paths in the step-down converter.

・The sudden on-off of the currents in the switching operation of a switching regulator can adversely affect circuit operation and the like if measures are not taken in an appropriate PCB layout.

PCB Layout and Selection of Peripheral Parts

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