28-05-2014, 12:22 PM
High-Voltage Energy Storage: The Key to Efficient Holdup
ABSTRACT
This topic provides a tutorial on how to design a high-voltage-energy storage (HVES) system to minimize
the storage capacitor bank size. The first part of the topic demonstrates the basics of energy and the
benefits/limitations of HVES; it also provides volumetric analysis graphs illustrating volume reduction
and energy density. The second part provides an overview of the critical aspects of an HVES design. It
compares the possible topologies and control techniques, identifies the pitfalls and design challenges of
the recharge and holdup modes, and discusses the impact of design decisions on power losses. Design
guidelines applicable to a preselected power topology and control strategy are also provided, including a
design example for a 48-V application.
Basic Principles
Many high-reliability systems have a require
ment for modules to ride through short input-
power interruptions. Some systems require a
graceful shutdown mechanism while others need a
local bank of energy to supply power during
occasional and brief high-load-current demand.
There are also other applications that require
short-term backup power when the main power
fails—for example, a security system that needs to
record information for a limited time following a
power interruption.
Bridge Configuration 3: Multimode Buck/Boost
The third power-bridge configuration is by far
the most complex. It uses a flyback topology to
recharge the capacitor, but during holdup it
transitions from a buck to a flyback topology and
to a boost topology as the storage voltage goes
down (see Figs. 15 and 16).
With this configuration during holdup mode,
the converter can be run as a buck when Vstorage is
greater than Vbus (bus voltage) and as a boost
when Vstorage is less than Vbus. It is also run as a
flyback when Vstorage is close to Vbus.
Thermal Aspects
When the thermal performance of an HVES
system is analyzed, the following aspects need to
be considered.
• The recharge mode uses the PFM technique,
which implies sleep intervals. The duration of
initial recharge varies with the storage
capacitance and the nominal recharge voltage.
HVES MOSFETs are operated in switch mode
only; there is no linear inrush limiting.
• The holdup mode duration is limited, so dynamic
thermal behavior needs to be considered. This
concerns the MOSFETs, the diodes, and the
PCB when used as a heatsink. Allowed dynamic
stresses can be significantly higher than static
stresses, depending on component types,
packages, and power-pulse duration. See Fig. 23
for an example.
• The mechanism that automatically discharges
the storage capacitors upon unit removal needs
to be able to dissipate all of the stored energy as
heat without exceeding critical temperatures.
• There are three basic methods to move heat
away from the source: conduction, convection,
and radiation.
CONCLUSION
The design of an HVES system is complex and
requires a broad knowledge of power supplies.
This topic has presented the basics. More details
are provided in Appendix A.