IEEE Power Electronics Magazine - June 2021 - 54

Moreover, 48-54-V voltage sources are becoming more
common due to the need to reduce currents on backplanes
and power connectors.
■ Increased complexity of digital PCBs that require
more routing layers with controlled impedance for highspeed
signals, yet need heavy copper to deliver power to
processors and ASICs.
■ More aggressive operating environments than ever
before with higher operating temperatures, reduced airflow,
and greater peak loads with longer duty cycles.
■ Fabrication, processing, and assembly issues such as
contamination, registration offsets, mechanical and thermal
stresses.
■ Relentless drive for cost reduction enables market
entry for low-cost PCB suppliers whose technical capabilities
and quality systems may not yet be on par with
industry standards.
PCB Construction
A typical multilayer PCB consists of layers of copper foil
isolated from each other by layers of dielectric as shown in
Figure 2 [3]. A woven glass fabric reinforced (pre-impregnated)
with epoxy-like resin is used to make cores and prepreg.
A layer of prepreg with copper foils laminated on
both sides and cured makes a core. Multiple cores are laminated
together with layers of prepreg between them. Under
pressure and temperature, a multilayer PCB is formed.
The most ubiquitous PCB dielectric, known as FR-4, is
actually a grade of flame retardant (FR) materials that are
capable of meeting the UL94 V0 flammability rating. FR-4
rated materials represent over 70% of laminates used in
the PCB industry [4]. Other notable materials include polyimide,
which is more durable than FR-4, has higher electric
strength and better thermal conductivity.
FIG 1 Board damage due to a short circuit in the PCB.
System Configuration
A typical rack system shown in Figure 3 consists of blade
servers, cooling fans, and power supplies, all plugged into
the power distribution backplane [5].
The backplane connects all power supplies in parallel
and distributes power to blade servers or networking cards,
fans, and other devices. Therefore, the total power available
on the backplane is the sum of the output power of all the
power supplies connected to the backplane. For example, a
12U HPE Apollo k6000 chassis pictured in Figure 4 uses six
3,400 W power supplies to power up to 24 server trays and a
number of switching cards [6].
FIG 2 Multilayer PCB construction.
Chassis
Side Walls
Tray
Backplane
Blade
Board Protection
Fan
Door
Tray
FIG 3 An example of a modular rack server chassis.
54 IEEE POWER ELECTRONICS MAGAZINE z June 2021
The power input to a blade server or a networking
board needs to be protected from a catastrophic
PCB failure. Fuses, electronic circuit
breakers, and hotswap circuits can be used
for protection.
Power Distribution
Backplane
Power
Supply
Even though each server or switching card consumes
only hundreds of watts, the total amount of power available
on the backplane is 20.4 kW or 1,700 A! Should a short circuit
occur on a given PCB, the current from all six power supplies
will incinerate the board unless the board is adequately
protected. While it is true that power supplies
have short circuit protections, thresholds of
those protections are typically set at 110-120%
of nominal current values. Therefore, a board
designer cannot rely on power supplies for primary
protection of individual boards, but rather
needs to use a protection circuit on each board.
Copper Layer
Dielectric-Prepreg
Copper Layer
Dielectric-Core
Dielectric-Prepreg
Copper Layer
IEEE Power Electronics Magazine - June 2021

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