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High Density PCB Layout of DC/DC Converters, Part 2

Timothy Hegarty
As I mentioned in part 1, the printed circuit board (PCB) area dedicated to power management is an immense
constraint for system designers. Reducing converter losses is an essential requirement to enable a compact
realization in space-constrained applications with limited PCB real estate.
The ability to flexibly deploy a converter at a strategic location on the board is also important – take for instancea high-current point-of-load (POL) module, optimally located adjacent to a load for smaller conduction drop andbetter load transient performance.
Consider the power stage layout in Figure 1 of a miniaturized form-factor buck converter. As an embedded POL
module implementation, it uses an all-ceramic capacitor design, an efficient shielded inductor, vertically stacked
MOSFETs, a voltage-mode controller and a six-layer PCB with 2oz copper.

Figure 1. 25A Synchronous Buck Converter PCB Layout and Implementation.
The main tenets of this design are high power density and low bill-of-materials (BOM) cost. It occupies a total
PCB area of 2.2cm2 (0.34in2), yielding an effective current density per unit area of 11.3A/cm2 (75A/in2). Powerdensity per unit volume at 3.3V output is 57W/cm3 (930W/in3).
The normal approach to attaining high power density is to increase switching frequency. By contrast, youcan achieve miniaturization through strategic component selection while retaining a relatively low switchingfrequency of 300kHz to lessen frequency proportional losses such as MOSFET switching loss and inductor coreloss. Table 1 lists the essential components for this design.

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High Density PCB Layout of DC/DC Converters, Part 2
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Table 1. POL Module Components, Package Sizes and Recommended Pad Dimensions.
Power train components

Footprint and profile (mm)Recommended land pattern outerdimensions (mm)CSD86350Q5D NexFETÔ Power Block

5.0 x 6.0 x 1.5 (SON5x6)

5.15 x 6.24

LM27402 3V-20V PWM controller

4.0 x 4.0 x 0.8 (WQFN-16)

4.2 x 4.20.68µH 1.6mΩ 33A filter inductor

11.5 x 10.3 x 4.0

4.1 x 13.622µF input and 47µF output X5R capacitors

2.0 x 1.25 x 1.35 (0805)

2.2 x 1.3

Terminal connections

2.0 x 3.0

2.0 x 3.0 (on host board)

Value Proposition of High-density PCB Designs
Clearly, the PCB is an important (and sometimes most expensive) component in a design. The value propositionof a well-planned and carefully executed PCB layout for a high-density DC/DC converter lies in:••••••More functionality in space-constrained designs (reduced solution volume and footprint).
Reduced switching-loop parasitic inductance, contributing to:– Lower power MOSFET voltage stress (switch-node voltage spike) and ringing.
– Reduced switching loss.
– Lower electromagnetic interference (EMI), magnetic field coupling and output noise signature.
– Extra margin to survive input rail-transient-voltage disturbances, especially in wide-VIN-range applications.
Increased reliability and robustness (lower component temperatures).
Cost savings related to a smaller PCB, fewer filtering components and the elimination of snubbers.
Differentiated designs provide a competitive advantage, capture customer attention, and increase revenue.

It’s fair to say that PCB layout defines the performance ultimately achieved from a switching power converter. Ofcourse, the designer is quite happy to avoid countless hours of debugging time for EMI, noise, signal integrity,and other issues related to a poor layout.
Additional Resources:• Read parts 1, 2 and 3 of “DC/DC Converter PCB Layout” on EDN.
• Watch a video on the salient characteristics of high-density buck converter solutions.
• Review the schematic, layout and test reports of these high-density designs from the PowerLabreference
design library:– High-efficiency small form factor 112W sync buck TI Designs reference design.
– High power density voltage regulator module for CPU core power in enterprise switching TI Designs
reference design.
– High power density 12Vin, 100W synchronous DC/DC step-down buck converter with inductor-on-top TI
Designs reference design.
• Order the high-density LM27403EVM-POL600 30A 600kHz POL evaluation module (EVM).
• Start a design now with WEBENCH® Power Designer.

2High Density PCB Layout of DC/DC Converters, Part 2
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