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Guide to DC/DC converter PCB layout (Part 1)

02 Jul 2015  | Timothy Hegarty

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Fundamental to careful system design of any DC/DC power converter is a well-planned and carefully executed printed circuit board (PCB) layout. An optimised layout leads to better performance, lower cost and faster time-to-market. Additionally, it can constitute a competitive advantage for the end equipment user owing to higher reliability (lower component temperature), easier regulatory compliance (lower conducted and radiated emissions), and improved space utilisation (reduced solution volume and footprint).

A primary objective of this three-part article series is to look closely at PCB layout design, knowing that it represents a critical piece of the power converter puzzle. These articles also offer clear guidance on factors related specifically to PCB design to achieve low noise converter implementations. Using a four-switch synchronous buck-boost DC/DC converter as a case study, PCB layout considerations for fast-switching, high-current applications are highlighted using a step-by-step process. The real purpose here is a practical one. PCB layout, one of the thorniest and most menacing topics for a power supply engineer, can make or break a real design.


Four-switch buck-boost converter review
Let's digress for a moment to introduce the four-switch (non-inverting) synchronous buck-boost topology. This circuit is an excellent example to study DC/DC converter PCB layout. It has numerous applications including industrial computing, LED lighting, RF power amplification, and USB power delivery [1]. The most compelling feature of this particular buck-boost implementation is that buck, boost, and buck-boost transition modes are engaged as needed to achieve high efficiency across wide and overlapping input and output voltage ranges.

One common application scenario is deriving a tightly-regulated 12V rail from an automotive battery source. Even if the battery's DC voltage varies from 9V to 16V, transients arise from start/stop, cold crank or load dump [2]. The voltage during such events can dip as low as 3V or spike to 42V, sometimes even higher. To meet these requirements, the schematic in figure 1 illustrates components for the power stage and controller, including integrated gate drivers, bias supply current sensing, output voltage feedback, loop compensation, programmable under-voltage lockout (UVLO), and dither option for lower noise signature.


Figure 1: Four-switch synchronous buck-boost converter schematic.


The four power MOSFETs in figure 1 are arranged as buck and boost legs in an H-bridge configuration, with switch nodes SW1 and SW2 connected by inductor Lf. Conventional synchronous buck or boost operation occurs when the input voltage lies suitably above or below the output voltage, respectively. Meanwhile, the high-side MOSFET of the opposite, non-switching leg conducts as a pass device. More importantly, as the input voltage approaches the output voltage, the buck or boost leg that is switching reaches an anticipated threshold. This triggers a changeover to buck-boost transition mode, in this case a hybrid of buck and boost modes interleaved as required for an aggregate buck-boost effect [3].


Power converter PCB design flow
PCB design for high-current, fast-switching converters demands more caution than ordinary PCBs, since the voltage drops caused by parasitic impedances become really significant in high-current and high slew rate conditions [4]. Even though the topic of PCB design is fraught with subjectivity, the best practices for a PCB design also facilitate high-density and small-solution size. What is equally relevant is that many of these practices align with safeguards to help curtail electromagnetic interference (EMI), and ease regulatory compliance for emissions and immunity specifications [5].

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