5. FUSE SELECTION
The total let-through energy of the fuse (pre-arcing plus arcing) also varies enormously.
Further, it depends on the fusible link material, construction of the fusible element, applied
voltage, type of fault, and other circuit-linked parameters.
5.3 TYPES OF FUSES
Time-Delay Fuse (Slow-Blow)
A time-delay fuse will have a relatively massive fuse element, usually of low-melting-point
alloy. As a result, these fuses can provide large currents for relatively long periods without
rupture. They are widely used for circuits with large inrush currents, such as motors, sole-
noids, and transformers.
These fuses are low-cost and generally of more conventional construction, using copper
elements, often in clear glass enclosures. They can handle short-term high-current tran-
sients, and because of their low cost, they are widely used. Very often the size is selected
for short-circuit protection only.
Very Fast Acting Fuses (HRC, or High Rupture Capacity,
These fuses are intended for the protection of semiconductor devices. As such, they are
required to give the minimum let-through energy during an overload condition. Fuse ele-
ments will have little mass and will often be surrounded by some form of filler. The purpose
of the filler is to conduct heat away from the fuse element during long-term current stress
to provide good long-term reliability, and to quickly quench the arc when the fuse element
melts under fault conditions. For short-term high-current transients, the thermal conductiv-
ity of the filler is relatively poor. This allows the fuse element to reach melting temperature
rapidly, with the minimum energy input. Such fuses will clear very rapidly under transient
Other important fuse properties, sometimes neglected, are the long-term reliability and
power loss. Low-cost fast-clearance fuses often rely on a single strand of extremely thin
wire. This wire is fragile and is often sensitive to mechanical stress and vibration; in any
event, such fuse elements will deteriorate over the long term, even at currents below the
rated value. A typical operating life of 1000 h is often quoted for this type of fuse at its
The more expensive quartz sand-filled fuses will provide much longer life, since the
heat generated by the thin element is conducted away under normal conditions. Also, the
mechanical degradation of the fuse element under vibration is not so rapid, as the filling
gives mechanical support.
Slow-blow fuses, on the other hand, are generally much more robust and have longer
working lives at their rated currents. However, these fuses, with their high “let-through”
energies, will not give very effective protection to sensitive semiconductor circuits.
This brief description covers only a very few of the ingenious methods that are used in
modern fuse technology to obtain special characteristics. It serves to illustrate the number
of different properties that fuses can exhibit, and perhaps will draw a little more attention
to the importance of correct fuse selection and replacement.
5.4 SELECTING FUSES
Off-Line Switchmode Supplies
The initial fuse selection for off-line switching supplies should be made as follows:
For the line input fuse, study the turn-on characteristics of the supply and the action of
the inrush-limiting circuitry at maximum and minimum input voltages and full current-
limited load. Choose a standard- or slow-blow fuse that provides sufficient current margin
to give reliable operation and satisfy the inrush requirements. Its continuous current rat-
ing should be low enough to provide good protection in the event of a genuine failure.
However, for long fuse life, the current rating should not be too close to the maximum
rms equipment input current measured at minimum input voltage and maximum load
(perhaps 150% of I
maximum). Note: Use measured or calculated rms currents, and
allow for the power factor (approximately 0.6 for capacitor input filters) when calculating
The voltage rating of the fuse must exceed at least the peak supply voltage. This rating
is important, as excessive arcing will take place if the voltage rating is too low. Arcing can
let through considerable amounts of energy, and may result in explosive rupture of the fuse,
with a risk of fire in the equipment.
5.5 SCR CROWBAR FUSES
If SCR-type overvoltage protection is provided, it is often supplemented by a series fuse.
This fuse should have an I2t rating considerably less (perhaps 60% less) than the SCR I2t
rating, to ensure that the fuse will clear before SCR failure. Of course, a fast-blow fuse
should be selected in this case. The user should understand that fuses degrade with age,
and there should be a periodic replacement policy. The failure of a fuse in older equip-
ment is not necessarily an indication that the equipment has developed a fault (other than
a tired fuse).
5.6 TRANSFORMER INPUT FUSES
The selection of fuses for 60-Hz transformer input supplies, such as linear regulator sup-
plies, is not as straightforward as may have been expected.
Very often inrush limiting is not provided in linear power supply applications, and
inrush currents can be large. Further, if grain-oriented C cores or similar cores are used,
there is a possibility of partial core saturation during the first half cycle as a result of mag-
netic memory of the previous operation. These effects must be considered when selecting
fuses. Slow-blow fuses may be necessary.
It can be seen from the preceding discussion that the selection of fuse rating and type for
optimum protection and long life is a task to be carried out with some care. For continued
optimum protection, the user must ensure that fuses are always replaced by others of the
same type and rating.
5. FUSE SELECTION
1. Quote the three major selection criteria for supply or output fuses.
2. Why is the voltage rating of a fuse so important?
3. Under what conditions may the fuse voltage rating exceed the supply voltage?
4. Why is the I2t rating of a fuse an important selection criterion?
5. Why is it important to replace a fuse with another of the same type and rating?
This page intentionally left blank
LINE RECTIFICATION AND
CAPACITOR INPUT FILTERS
As previously mentioned, the “direct-off-line” switchmode supply is so called because it
takes its power input directly from the ac power lines, without using the rather large low-
frequency (50–60 Hz) isolation transformer normally found in linear power supplies.
In the switchmode system, the input-to-output galvanic isolation is provided by a much
smaller high-frequency transformer, driven by a semiconductor inverter circuit so as to
provide some form of DC-to-DC conversion. To provide a DC input to the converter, it
is normal practice to rectify and smooth the 50/60-Hz ac supply, using semiconductor
power rectifiers and large electrolytic capacitors. (Exceptions to this would be special low-
distortion systems, in which input boost regulators are used to improve the power factor.
These special systems will not be considered here.)
For dual input voltage operation (nominally 120/240 V ac), it is common practice to
use a full-bridge rectifier for the high-input-voltage conditions, and various link arrange-
ments to obtain voltage doubler action for the low-input-voltage conditions. Using this
approach, the high-frequency DC-to-DC converter can be designed for a nominal DC input
of approximately 310 V for both input voltages.
An important aspect of the system design is the correct sizing of input inductors, recti-
fier current ratings, input switch ratings, filter component size, and input fuse ratings. To
size these components correctly, a full knowledge of the relevant applied stress is required.
For example, to size the rectifier diodes, input fuses, and filter inductors correctly, the
values of peak and rms input currents will be required, while the correct sizing of reservoir
and/or filter capacitors requires the effective rms capacitor current. However, these stress
values are in turn a function of source resistance, loading, and actual component values.
A rigorous mathematical analysis of the input rectifier and filter is possible, but tedious.83
Further, previous graphical methods26 assume a fixed load resistance with an exponential capac-
itor discharge. In power supply applications, the load applied to the capacitor input filter is the
input loading of the regulated DC-to-DC converter section. This is a constant-power load in the
case of a switching regulator, or a constant-current load in the case of a linear regulator. Hence,
this previous work is not directly applicable except where ripple voltages are relatively small.
Note: A constant-power load takes an increasing current as the input voltage falls, the
reverse of a resistive load.
To meet this sizing need, a number of graphs have been empirically developed from actual
system measurements. These will assist the designer in the initial component selection.
6.2 TYPICAL DUAL-VOLTAGE CAPACITOR
INPUT FILTER CIRCUIT
Figure 1.6.1 shows a typical dual-voltage rectifier capacitor input filter circuit. A link
option LK1 is provided, which allows the rectifier capacitor circuit to be configured as a
voltage doubler for 120-V operation or as a bridge rectifier for 240-V operation. The basic
rectifier capacitor input filter and energy storage circuit (C5, C6, and D1 through D4) has
been supplemented with an input fuse, an inrush-limiting thermistor NTC1, and a high-
frequency noise filter (L1, L2, L3, C1, C2, C3, and C4).
FIG. 1.6.1 Example of a direct-off-line, link-selected dual-voltage, capacitive input
filter and rectifier circuit, with additional high-frequency conducted-mode input filter.
For 240-V operation, the link LK1 will not be fitted, and diodes D1 through D4 act as
a full-bridge rectifier. This will provide approximately 310 V DC to the constant-power
DC-to-DC converter load. Low-frequency smoothing is provided by capacitors C5 and C6,
which act in series across the load.
For 120-V operation, the link LK1 is fitted, connecting diodes D3 and D4 in parallel
with C5 and C6. Since these diodes now remain reverse-biased throughout the cycle, they
are no longer active. However, during a positive half cycle, D1 conducts to charge C5 (top
positive), and during a negative half cycle, D2 conducts to charge C6 (bottom negative).
Since C5 and C6 are in series when the diodes are off, the output voltage is the sum of the
two capacitor voltages, giving the required voltage doubling. (In this configuration, the
voltage doubler can be considered as two half-wave rectifier circuits in series, with alter-
nate half cycle charging for the reservoir capacitors.)
6.3 EFFECTIVE SERIES RESISTANCE R
The effective series resistance R
is made up of all the various series components, including
the prime power source resistance, which appear between the prime power source voltage
and the reservoir capacitors C5 and C6. To simplify the analysis, the various resistances are
lumped into a single effective resistance R
. To further reduce peak currents, additional series
resistance may be added to provide a final optimum effective series resistance. It will be
6. LINE RECTIFICATION AND CAPACITOR INPUT FILTERS
shown that the performance of the rectifier capacitor input filter and energy storage circuit is
very much dependent on this final optimum effective series resistance.
A simplified version of the bridge circuit is shown in Fig. 1.6.2. In this simplified circuit,
the series reservoir capacitors C5 and C6 are replaced by their equivalent capacitance C
and the effective series resistance R
has been positioned on the output side of the bridge
rectifier to further ease the analysis.
In the example shown in Fig. 1.6.2, the effective series resistance R
is made up as
The prime source resistance R
` is the resistance of the power supply line itself. Its value
will depend on the location of the supply, the size of utility transformer, and the distance
from the service entrance. Values between 20 and 600 m7 have been found in typical
industrial and office locations. Although this may appear to be quite low, it can still have a
significant effect in large power systems. In any event, the value of the source resistance is
generally outside the control of the power supply designer, and at least this range must be
accommodated by any practical supply design.
A second and usually larger series resistance component is usually introduced by the
input fuse, filter inductors, rectifier diodes, and inrush-limiting devices. In the 100-W
example shown in Fig. 1.6.1, the inrush-limiting thermistor NTC1 is the major contributor,
with a typical “hot resistance” of 1 7. In higher-power supplies, the inrush-limiting resistor
or thermistor will often be shorted out by a triac or SCR after initial start-up, to reduce the
source resistance and power loss.
6.4 CONSTANT-POWER LOAD
By design, the switchmode power supply will maintain its output voltage constant for a
wide range of input voltages. Since the output voltage is fixed, under steady loading con-
ditions, the output power remains constant as the input voltage changes. Hence, since the
converter efficiency also remains nearly constant, so does the converter input power.
In order to maintain constant input power as the input voltage to the converter falls, the
input current must rise. Thus the voltage discharge characteristic VC
of the storage capacitor
is like a reverse exponential, the voltage starting at its maximum initial value V
diode conduction period.
FIG. 1.6.2 Simplified capacitive input filter circuit, with full-wave
bridge rectifier and lumped total effective source resistance R
Documents you may be interested
Documents you may be interested