Switch-mode power supplies (SMPS) continue
to replace linear-regulator types in a host of
applications. As the need for more efficient
electronics accelerates and as a result of their
size, weight, and energy-saving advantages, SMPS are being
widely used in applications such as LCD TV monitors, PC/
laptop displays, portable electronics chargers, printers, DVD
recorders, and even automotive electronics and industrial.
Yet because these new SMPS lack the inherent resistance of
linear-regulator designs, there’s a growing need for proper circuit
protection. Engineering schools generally do not emphasize
selection and application of circuit protection devices. As a
result, too often many SMPS lack adequate protection.
Whether they are external or internal, forward or fly-back
conversion units, continuous or discontinuous, each type of
SMPS is subject to regulatory requirements. For instance, a
power supply for a telecom installation will potentially need
to comply with Telcordia or ITU requirements, depending on
the target region and application. In the consumer sector, IEC,
UL, and CSA standards govern at the equipment level, and
a host of other safety testing will be required. There are also
IEC, UL, and CSA standards that apply specifically to circuitprotection
components.
Fuses are ideal candidates for overcurrent protection in
SMPS because of their proven safety, reliability, low resistance,
small size, and cost effectiveness. Just as there are system-level
requirements for the SMPS, there are regulatory requirements
for safety and performance at the component level. For example,
IEC-60127 gives specific dimensional requirements and
also specifies a series of fuse tests.
CIRCUIT PARAMETERS OVERVIEW
Figure 1 shows the location of a fuse in the ac-mains input of
an ac-dc SMPS, such as in a cell-phone charger. The fuse positively
interrupts the current flow when a component such as
the radio-frequency interference (RFI) choke or filter capacitor
fails in the short circuit mode.
A metal oxide varistor (MOV) in the ac-mains input suppresses
transient voltages associated with lightning or load
switching. Additional common components, transient-suppression
diodes on the internal dc bus, further suppress any
transients, providing a higher degree of protection for the dcdc
converter circuitry.
Figure 2 shows an embedded ac-dc SMPS that may be
found in a server. In addition to the fuse at the ac mains input,
fuses find employment on the high-voltage dc bus and in the
housekeeping power supply.
To select the proper circuit protection device, it is necessary
to accurately define the key parameters of the application.
These include circuit voltage, maximum normal operating
current, maximum potential fault current, maximum operating
temperature, pulse currents, and mounting/form factor.
Circuit voltage is the source voltage driving the circuit. For
safety concerns, it is critical to know because the fuse’s voltage
rating must be equal to or greater than the circuit voltage. It is
also extremely important that fuses in dc applications have an
adequate dc-voltage rating.
Maximum normal operating current is the maximum RMS
current under full load in normal operation. Maximum potential
fault current is the expected maximum current with the
source voltage shorted out. Obviously, maximum operating
temperature is the anticipated operating
temperature of the circuitry near
the protection device, under full load,
with all shields and covers in place, at
maximum ambient temperature.
Pulse currents are the transients
induced by normal switching events in
the circuitry as well as those coupled
to the ac mains from lightning and
load switching. Both the magnitude
and duration of the transients and the
anticipated number of transients over
the lifetime of the equipment need
careful consideration.
Finally, choosing the mounting
method and/or form factor of the
fuse comes into play. Options vary to
include surface-mount devices, pin-through-hole, pigtail leads, or simply a fuse
and a holder. Also critical is the amount of
space available for mounting the device.
FUSE SELECTION
The fuse selection process usually
begins by satisfying three basic selection
criteria. First, what standards must be
satisfied? For most SMPS products, the
answer is a wide variety of international
standards, which leads to a fuse that complies
with IEC-60127. Several families of
fuses have European, Asian, and North
American safety agency approvals to
IEC-60127.
Second, what is the maximum operating
source voltage? The most common
answer for ac-mains input protection for a
SMPS destined for the international market
is 250 V ac. Third, what is the desired
mounting/form factor? The most popular
choice for a SMPS is radial or axial lead for
direct circuit-board attachment, as small as
possible, driven largely by economic and
available space concerns.
The next step in the selection process is
to determine the maximum potential fault
current. It is best to do this by analyzing or
measuring the path impedance across the
input to the SMPS with the SMPS disconnected
from the ac mains. This impedance,
when added to the estimated impedance of
the ac mains, enables one to calculate the
maximum potential fault current.
Continue to page 2