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2021.02.24 DC/DC

Placement of Output Capacitors and Freewheel Diodes

PCB Layout of a Step-Up DC/DC Converter

Following the discussion of input capacitors of the previous article, we explain the placement of output capacitors and freewheel diodes, components which are nearly as important.

Placement of Output Capacitors and Freewheel Diodes

Following the input capacitor, the output capacitor and the freewheel diode are positioned.

This explanation of PCB layout assumes a step-up circuit the output current of which can be up to 1 A. For an output current of this magnitude, a comparatively small capacitance suffices for the output capacitor COUT, and so a single ceramic capacitor can also be used as a high-frequency decoupling capacitor COBYPASS. This is because the frequency characteristic of ceramic capacitors is better for smaller capacitances. However, frequency characteristics of ceramic capacitors differ depending on the capacitor type and the manufacturer, and so the frequency characteristic of the component that will actually be used must be checked.

The freewheel diode D2 is positioned close to the IC and COUT on the same surface. The node connecting the diode and the switching MOSFET Q2 is a switching node, and therefore is a source of high-frequency noise. If the wiring of this node is long, high-frequency spike noise induced by the wiring inductance is superposed on VOUT. Moreover, the loop from the switching MOSFET Q2 to the freewheel diode D2 and the high-frequency decoupling/output capacitor COUT should be made as small as possible, to hold radiation of high-frequency noise to a bare minimum. These components and wiring must always be placed on the same surface. If components are placed and wired on the back surface using vias, noise is worsened due to the effects of the via conductance; vias should never be used. A satisfactory layout example that takes these considerations into account is shown on the right. The relevant layout section is indicated by the darker color.

Example of Satisfactory Layout of Output Capacitor and Freewheel Diode

As indicated above, because wiring inductance increases if the switching node is made longer, high-frequency spike noise is increased, and in most cases this exerts adverse effects. One way to alleviate this high-frequency spike noise is to add an RC snubber circuit.

The snubber circuit must be placed close to the switching MOSFET Q2 and the GND pin of the IC, and so it is recommended that lands for the resistor RS and capacitor CS of the snubber circuit be prepared in advance.

As one point to keep in mind, because losses always occur in a snubber circuit during switching operation, it should be born in mind that they detract from efficiency. In some cases a balance will have to be struck between reducing the spike noise of the switching node and circuit efficiency.

Example of Satisfactory Layout of Snubber Circuit

Key Points

・If the output current is small, only a comparatively small value is needed for the output capacitor. Hence a single ceramic capacitor can be used as both the output capacitor and as a high-frequency decoupling capacitor.

・The freewheel diode is positioned near the IC and the output capacitor on the same surface.

・If the wiring of the node connecting the diode and the switching MOSFET is long, high-frequency spike noise induced by the wiring inductance is superposed on the output.

・A snubber circuit can be used to deal with spike noise, but it should be recognized that losses occur in the snubber circuit.

PCB Layout and Selection of Peripheral Parts

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