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 instance a high-current point-of-load (POL) module, optimally located adjacent to a load for smaller conduction drop and better 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.

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). Power density 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, you can achieve miniaturization through strategic component selection while retaining a relatively low switching frequency of 300kHz to lessen frequency proportional losses such as MOSFET switching loss and inductor core loss. Table 1 lists the essential components for this design.

Value Proposition of High-density PCB Designs

Clearly, the PCB is an important (and sometimes most expensive) component in a design. The value proposition of 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-V_IN-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. Of course, 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.