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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 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.

Figure 1: 25A synchronous buck converter PCB layout and implementation.

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). 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.

 

Power train components

Footprint and profile (mm)

Recommended land pattern

outer dimensions (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 4.2

0.68µH 1.6mΩ 33A filter inductor

11.5 x 10.3 x 4.0

4.1 x 13.6

22µF 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)

Table 1: POL module components, package sizes and recommended pad dimensions.

 

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-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. 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.

 

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