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Timothy Hegarty

Sep 12, 2015

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High Density PCB Layout of DC/DC Converters, Part 1

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High-density PCB layout of DC/DC converters –
part 1

Today, in an era of intense competition, the
challenge for product designers is to stay ahead of the pack and not merely walk
in lockstep with it. This ups the ante for system designers to innovate with
differentiated products.

One important way to innovate is with high-density designs. In the push for
smaller-footprint solutions, power system designers are now focusing on the
question of power density – the output power per unit of area or volume of a
power converter circuit.

The most visible example of DC/DC converter
printed circuit board (PCB) layout for high density relates to power stage
component placement and routing. Careful layout can coincide with better switching performance, lower component
temperatures and reduced electromagnetic interference (EMI) signatures. Consider
the power stage layout and schematic in Figure 1.

Figure 1:
Four-switch buck-boost converter power stage layout and schematic

As I see it, these are the challenges when
designing high-density DC/DC converters:

Component
technology. Advancements in component technology are key to reducing
overall power dissipation, particularly at higher switching frequencies vital
for filter passive component size reduction. For example, power
MOSFETs have seen consistent advances in silicon and packaging, most notably
with the introduction of gallium nitride (GaN)
power devices with very low parasitics. Meanwhile, magnetic component
performance has advanced independently, albeit at a rate arguably lagging that
of power semiconductors. Prudent layout of the control IC – with integrated,
adaptive gate drivers close to the MOSFETs – in
many cases obviates the need for switch-node voltage slew-rate adjustment
using power-dissipating snubber or gate resistor components.

Thermal design.
While a high-density layout is generally positive for conversion efficiency,
it may create a thermal performance bottleneck. The same power dissipation in
a smaller footprint becomes untenable. Increased component temperatures
amplify concerns of higher failure rates and reliability. Lower-profile power
MOSFETs placed on the top side of the PCB – not airflow-shadowed by taller
components like the inductor and electrolytic capacitors – help
thermal performance with convective airflow. For the converter in Figure 1,
the inductor and electrolytics are purposely located on the bottom side of the
multilayer PCB, because they would impede heat transfer if placed on the
top.

EMI performance.
EMI regulatory compliance is a key milestone in a product’s design cycle. A
high-density design typically has little space available for EMI filtering.
However, tight layout improves radiated emissions as well as
immunity to incoming disturbances. Two essential steps are to minimize loop
areas containing high di/dt currents (see the white current paths in Figure 1)
and reduce surface areas with high dv/dt voltages (see the SW1 and SW2 copper
polygons in Figure 1).

High-density PCB
design flow. Clearly, it’s important for power system designers to develop
and hone their PCB design skills. Even though the layout duties are often
delegated to layout specialists, engineers still bear the ultimate
responsibility to review the design and sign off on
it.

With these challenges in mind, I recently wrote
a three-part series for EDN titled “DC/DC Converter PCB Layout” that
delves into PCB layout in detail. It includes a list of PCB layout guidelines
structured as a checklist to aid designers during layout. The essential steps in
the PCB design flow for DC/DC converters are:

Choose the PCB structure
and stack-up specification.

Identify
the high di/dt current loops and high dv/dt voltage nodes from the
schematic.

Perform
power stage component layout and placement.

Place
the control IC and complete the control section layout.

Perform
critical trace routing, including MOSFET gate drive, current sense, and
output-voltage feedback.

Design
the power and GND planes.

In part 2 of this blog series, I’ll unveil a high-density
buck-converter layout – a 25A point-of-load design in a 20mm-by-11mm
footprint.

Additional resources:

Read
parts 1, 2 and 3 of “DC/DC Converter PCB Layout” on
EDN.

Watch
a video featuring the high-density evaluation
module (EVM) for the LM5175 four-switch synchronous
buck-boost controller.

Study the design equations
for a buck-boost converter.

Examine
the schematic, layout and test data of these high-density buck-boost
designs from the PowerLab™ reference
design library:

Four-Switch Buck-Boost Converter with
Adjustable Output Voltage for Military Radio Applications TI Designs
reference design.

LED PWM Dimming Reference Design Using High
Efficiency Buck-Boost Converter TI Designs reference
design.

Start a design now with WEBENCH® Power
Designer.

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