Selecting a line input resistor for battery chargers and power supplies
By Stephen Oxley, Senior Applications Engineer, TT electronics
Introduction
Designers of power supplies and battery chargers are faced with many
requirements, driven by the need to minimize manufacturing cost while
maintaining standards of product safety. A key component in realizing
these requirements is the line input resistor.
However, the selection of a line input resistor is not a simple task, as it involves many
conflicting factors. On one hand, the resistor must be sufficiently robust to survive
repeated inrush surges and occasional power line transients. On the other hand, the
resistor is often relied on for failsafe flameproof fusing in the event of a short-circuit
bridge or capacitor failure. Also, while safety is always paramount, certain applications
come with additional requirements like silent fusing, so that in the event of failure, a
consumer is not alarmed.
Functions of the Line Input Resistor
A line input resistor has several potential functions, the relative importance of which
varies between applications.
Inrush Current Limiting
Firstly, a common requirement is to limit the peak value of the inrush current. This is a
transient high current which flows at switch-on of the PSU and is caused by the initial
filling of the energy storage capacity in reactive components. In most cases this is due to
charging of reservoir capacitors, but in some high power applications it relates to the
magnetisation of large transformer cores. The restriction of inrush current protects
rectifiers, prevents unwanted fusing or circuit breaker tripping and avoids pollution of
the line supply.
If the maximum permissible value of peak current is known, the required resistance
value is simply derived from the line voltage divided by the maximum peak current. The
worst case of switch-on at a voltage peak should be considered, and the peak value of
line voltage is the figure to use. This gives a minimum ohmic value and this may be
raised to give a safety margin, but unless there is a relay shorting the inrush resistor
after switch-on, this must be balanced by the need to limit quiescent power loss.
The other critical factor to define is the surge rating required. If the input resistor is
wirewound, the manufacturer should be able to give a surge energy rating in joules. This
represents the energy capacity of the windings alone, but if the inrush time constant is
around 100ms or more, there is time for heat to be conducted into the resistor core, so
the effective energy capacity is greater. The inrush energy dissipated in the resistor is
independent of ohmic value and simply equals that stored on the charged capacitor of C
farads charged to V volts, as given by the formula:
E (joules) = ½ C.V2
Note that maximum values of C and V should be used as the tolerance on both can be
wide.
It should be mentioned at this point that an alternative strategy is to use an NTC
component which presents a high initial ohmic value when cold and then a lower ohmic
value when hot. This offers an improved combination of high inrush suppression and
low quiescent power loss. However, the design for hot running means it is unsuitable for
high efficiency supplies. Furthermore, a repeat switch-on whilst the NTC is still hot will
give little or no inrush limiting.
Transient Overvoltage Protection
Transient surges on a line input are a common source of product failure and some level
of protection against them is normally required. This type of supply line disturbance can,
for example, arise when a lightning
1.2/50 s Pulse Shape
strike occurs close to power lines and a
Voltage
transient high voltage is induced in the
power system. A common standard for
Vpk
simulating the resulting surge is
0.9Vpk
IEC61000-4-5, which describes a
1.2/50 s pulse as illustrated in Figure 1.
0.5Vpk
The maximum permissible peak voltage
of this pulse across a resistor is limited
0.1Vpk
by pulse energy considerations and
50 s
increases with resistance value.
Time
1.2 s
Figure 1
In a typical arrangement the input resistor is followed by a varistor (see Figure 2) which
limits the transient voltage to a safe level. The resistor restricts the peak current in the
varistor and shares the surge energy with it, thereby lengthening its life. When
calculating the peak voltage across the resistor, allowance should be made for the
varistor clamping voltage and, for low resistance values, the source impedance of the
surge generator. The first of these is defined on the varistor datasheet, and should be
subtracted from the peak voltage appearing at the supply line terminals. The second is
normally 2 ohms, and the residual peak voltage is split between this and the resistor in
proportion to their ohmic values. For ohmic values above about 40 ohms the effect of
this source impedance is negligible.
For example, with a surge peak of 4kV and varistor clamping voltage of 700V, the 10R
resistor in Figure 2 will see
= 2750V
Surge
Generator
2R0
Line Input
Resistor
L
N
~
10R
Varistor
~
Bridge
Rectifier
+
Reservoir
Capacitor
-
Figure 2
Failsafe Fusing
The third function which a line input resistor can perform is failsafe fusing in the event
of a short circuit failure of rectifiers or capacitors. In this event, rapid positive opening
with line voltage standoff is called for. In some cases limited overload conditions apply
and low power fusing must be defined.
For certain end markets it is desirable to use a fusible component with UL recognition to
UL1412. This makes obtaining UL approval easier and ensures that safety-critical aspects
of electrical performance and ongoing consistency of manufacture are independently
verified. TT electronics has a range of options in UL File Number E234469.
Case Study
When a high volume manufacturer of consumer appliances faced the challenge of
defining the exact fusing mode under wide range of overload conditions they
approached TT electronics engineers for a solution. For this customer safety was
paramount, but another key requirement was for low impact, silent fusing. The aim was
to avoid consumer concern which could result from visible or audible signs of the failure.
In short, if a fault developed, the product should fail gracefully with isolation from line
voltage achieved both safely and silently.
In wirewound resistors the core acts as a heatsink for the wire element. This can delay
the fusing resulting in the body and coating reaching high temperatures. Depending on
the overload power applied, this can result in fragmentation of the coating and
ionisation of the air close to the point of fusing. If this ionisation occurs close to the cap
edge and at a voltage peak in the mains cycle, it can initiate a momentary flashover
outside the component body. This releases far more energy than is required to fuse the
wire element. Although the opening of the circuit is safe for most applications, it may
not be silent.
TT electronics engineers addressed this issue in close
collaboration with the customer to refine a new
coating material and process which delivers silent
fusing performance without adversely affecting either
safety or cost. The new multi-layer coating has a
flammability rating of UL94-V0 and shares the same
high temperature and insulating properties as
standard silicone cement. However, it is more
compliant and therefore better able to absorb the
thermal and mechanical stresses of an element fusing
without fragmenting.
This silent fuse coating is one of a range of options
available in the WP-S Series of wirewound resistors
covering ratings from 1W to 5W which can be tailored
to meet the most demanding of line input resistor
applications. It is also used on the ULW Series which is
a UL1412 recognised version.
Top Ten Design Tips
1. Flameproof wirewound resistors are commonly the best choice for line input due
to good surge performance coupled with failsafe fusing.
2. Carbon composition technology can be used for extremely high surge energy
density, but cannot fuse safely.
3. If multiple wirewound resistors are needed to meet a surge rating, use a series
combination rather than parallel. Not only does this share the voltage stress as
well as the thermal stress, but also, lower ohmic value wirewound resistors have
higher energy capacity.
4. Take great care if using film resistors in this application. Metal oxide, thick film or
untrimmed metal film can be appropriate but only for moderate surge
requirements. Trimmed metal film types should be avoided.
5. In evaluating the surge and fusing capability of resistors through testing, be
aware that there can be batch to batch variations. Ideally resistors with specified
surge and fusing performance should be used.
6. Flameproof resistors will not ignite. However, under limited overload leading to
slow fusing times, the body temperature can become high enough to ignite
adjacent materials. If this is a possible fault condition, it is vital that sufficient
clearance from plastics and other components be provided. Also PCB standoff by
leadforming could be used.
7. Faster fusing under limited overload conditions can be obtained from a
wirewound resistor with a glass fibre core instead of a ceramic core.
8. In extreme cases of limited overload fusing requirements a series connected
thermal fuse is needed to restrict the body temperature. This can either be
discrete or integrated to form a temperature limited resistor. If discrete,
assembly must ensure good thermal contact.
9. It is possible to reduce component count in UL approved designs by using a UL
recognised fusible resistor instead of a resistor with a separate UL recognised
fuse.
10. The effect of moderate surges on a wirewound resistor is generally a slight
increase in ohmic value due to oxidation. A decrease in value is due to annealing
reducing resistivity and is indicative of higher surge stress. Changes exceeding 5%
should be avoided.
END