Specifying high voltage film resistors
High voltage precision resistors, a vital component in applications ranging from radar equipment and ATE to power supplies for
TWTS, present the circuit designer with a unique set of problems in selection and specification.
High voltage resistors play an
Resistors are generally rated in
important role in power conversion terms of power dissipation and voltage
designs where voltages from lkV to rating. Power dissipation is generally
several hundred kV must be measured limited by element temperature
and controlled accurately. There are considerations and is proportional to
many types of high voltage resistor the surface area of the product. Voltage
which can provide non-precision rating is based on internal spacing and
performance in the 2 to 20percent the length of the resistive pattern.
range. These include the common Resistor manufacturers vary widely in
carbon film resistor and some metal their rating specification philosophy.
oxide types. One particular high
performance complex metal oxide
technology is specified to give high
voltage resistor performance of
1percent or tighter. This is a ‘total’
performance taking into account
the effects of operational stresses
and environmental exposure over
extended periods.
High voltage resistors are
manufactured in two general form
factors, a common cylindrical core
(solid being preferred) or a flat
substrate. Resistance film patterns
are deposited and fired onto the
ceramic core or flat ceramic
substrate. Spiral patterns are the most Pre-assembled voltage divider assembly
common. High voltage resistors can
be manufactured using non-inductive
resistive patterns to give improved
performance in high frequency power
Maximum power and voltage
conversion designs.
ratings
cannot
be
used
Termination of the element can simultaneously on all resistor values
be with axial or radial leads. The most in a range. The only point at which this
common termination for the cylindrical does occur is at a value called the
core is solder-attached leads. The critical resistance. This can be
resistive element is adjusted before calculated for any model by dividing
or after termination by modification of the square of the voltage rating by the
the element pattern. This commonly power rating. All values above this are
consists of spiralling, bridging, limited by the maximum voltage rating
notching or abrading the element.
and all resistances below are limited
The adjusted element is then by rated power. The maximum
protected from the effects of exposure continuous rated voltage for resistors
by applying a coating. This, in addition below the critical resistance can be
to environmental protection, determined by taking the square root
establishes the insulation resistance of the product of resistance value and
of the device and its dielectric rated power.
withstanding capability. Both of these
characteristics are extremely
Absolute tolerance
important in the design of precision
high voltage resistors, especially for
The nominal resistance
high resistance values.
accuracy, measured at low voltage and
at room temperature is expressed as
the absolute tolerance of a resistor. In
circuits where stress levels are
minimised the resistor precision is
essentially the absolute tolerance.
Resistor tolerances are available
from as wide as 10percent to better
than 0.1 percent. For precision
applications, the designers must be
aware of the contribution to error
caused by temperature coefficient,
voltage coefficient and long term
stability. If the effects of these error
sources on the accuracy of the resistor
are not taken into account, then the
performance of the resistor may
not be as expected.
The temperature coefficient
of a resistor, in ppm/C, indicates
the stability of the element over a
specified temperature range
referenced to an ambient
temperature-normally +25C. There
are many high voltage resistors
available in the
±200ppm/c
to
±500ppm/C
range.
However, these are generally used
in nonprecision applications such
as bleeder resistors. A 500ppm/
C temperature coefficient over a
typical operating range of ±50C would
result in a resistance shift of ±
2.5percent f rom the + 25C reference
value.
The temperature coefficients
used in precision designs should be
less than 100ppm/C. Even at 100ppm/
C, the resistance can vary as much
as ±0.5percent over a 50C range.
Precision high voltage resistors can
have a temperature coefficient of
80ppm/C and for applications which
require even greater precision, low TC
precision high voltage resistors are
available with temperature coefficients
of 25ppm/C from - 55C to + 125C,
referenced to + 25C.
Voltage coefficient, in ppm/V, is
negative for most materials. Voltage
coefficient is generally measured
between 10percent of rated voltage
and full rated voltage.
Care must be taken so that,
when making the resistance
measurement at high voltage, a
temperature rise due to power
dissipation does not create a
temperature coefficient error which
might mask the true VC. Voltage
coefficients for high voltage resistors
vary from immeasurable to over 10ppm/V for some film resistors. A VC
of - 10ppm/V is equivalent to
0.001percent/V or - 1percent/kV. This
is unsatisfactory for precision
applications where total error is
maintained below 1percent, including
all effects. For precision requirements
the voltage coefficient should not
exceed - 1 ppm/V ( - 0. 1 percent/kV).
VC varies in direct proportion to the
resistance value with a constant
physical size (and resistance pattern)
and varies in inverse portion for a
constant value with increasing physical
size.
The effects of self-heating are
related to power dissipation,
temperature coefficient and the
physical size of the resistor. Power
dissipation will result in a temperature
rise in the element which is a function
of the element material and the size of
the part, with a consequent shift in
resistance.
So, for high precision, over a
broad range of temperature and of
applied voltage, (such as that found in
a programmable high voltage power
supply), conservative derating,
coupled with the selection of resistors
with low temperature coefficients and
low voltage coefficients is mandatory.
When using high voltage
resistors one of the most important
resistor characteristics, yet often the
most neglected, is load life stability. This
is the ability of the resistor to retain its
initial absolute value within
acceptable limits over a long period of
time under operational conditions. In
most high voltage applications, the
resistor is protected from the harmful
effects of moisture and surface
contamination. The primary stresses
affecting long term stability are applied
voltage and thermal stresses.
Load life stability is directly
related to the resistance film system
and the manufacturing processes.
Most high voltage resistors have a load
life stability of 1percent to 5percent per
1000hours (at + 70C). Optimised
technologies can produce load life
stabilities of 0.5percent per 1000hours
and extended life stability of as tight
as 0.02percent per 1000hours has
been achieved. It can be seen that the
long term performance of a high
voltage circuit can be affected greatly
by the load life stability of the resistor.
Quality and reliability must be
manufactured into an electronic
component. These characteristics
cannot be tested into a product which
does not already exhibit them. Stability,
which is primarily a function of the
resistance film technology and of the
manufacturing process, can be
improved by additional post
manufacturing conditioning tests and
should be considered for precision
designs where long term stability is of
importance.
One of the most cost-effective
post manufacturing conditioning tests
consists of exposing the resistor to
rated voltage at elevated temperature
for a minimum of 100hours. The
absolute resistance value shift is
measured before and after the
conditioning and a maximum
resistance shift is applied as an
accept/reject criterion. Resistors which
exceed the maximum shift are
removed from the lot.
Many other tests are performed
as required by military specifications
or source control drawings prepared
by the customer. These include the
Group A requirements of MIL-R55102
which specify thermal shock and
overload with a maximum acceptable
resistance shift.
The Group B tests of commonly
referenced military specifications
require a lot sample of parts which
have passed the Group A
requirements of the specification.The
Group B tests of MIL-R-55182 include
measurement of temperature
coefficient, dielectric strength and
insulation resistance.
Group C tests of military
specifications comprise those that are
most often performed during
qualification. These tests, with the
exception of the life test normally
required in Group C,
are of a potentially destructive
nature and are not normally used for
screening. Group C tests include
terminal strength, shock, vibration, low
temperature operation and mechanical
shear.
There are now available precision
high voltage devices that provide the
necessary combination of tight
tolerance, low thermal coefficient,
voltage coefficient, long term stability
and MIL qualification for use in the
most demanding applications.
High voltage dividers
When using precision resistive
voltage dividers at high voltage levels,
there are a number of interactive criteria
that must be understood to obtain the
desired performance.
The ratio of a classical divider
is defined as the total resistance
divided by the tap resistance.
Generally the total voltage is applied
across the divider elements in series
and the tap voltage is taken across
the tap resistor. Other configurations
are used where the ratio is defined as
the resistance ratio. These are used
where the low voltage resistor of the
divider is connected to a reference
voltage and the tap is held at virtual
ground by the control circuitry.
In precision applications, it is
essential that the ratio configuration is
correctly specified. For example, a
resistance ratio of 100: 1 (100MOhm/
1 MOhm) is equivalent to a divider ratio
of 101:1 This is a difference in definition
which would result in an error of 1
percent in addition to other, less easily
corrected errors.
The tap divider ratio is affected
by,, the tap loading. It is important to
keep in mind that a tap load resistance
of 100 times the value of the tap
resistor will decrease the effective
value of the
tap resistor by 1 percent. A
similar effect occurs when a divider
is encapsulated in a compound with
a volumetric resistivity of 100 times
that of the upper or high voltage
resistor. Here again, the effective
value of the resistor is decreased
by 1percent. Where the ratio must
be adjusted, a variable resistor can
be added in series with the low
voltage tap resistor.
In a high voltage divider, the
absolute tolerance is often of
secondary importance compared
with the ratio tolerance. Ratio
tolerance can be defined as the
accuracy of the voltage division in
a divider. For a 1000:1 divider with a
10kV applied voltage, the ideal
voltage at the tap, referenced to
earth, is 10V. The percentage
deviation of the tap voltage from this
ideal represents the ratio tolerance.
Figures
in
precision
applications range from 1percent to
0.25percent. Ratio tolerances tighter
than 0.5percent require special
consideration of several factors,
including voltage coefficient,
self-heating, component mounting,
encapsulation and thermal coupling.
Ratio tolerance is normally
expressed at 25C to differentiate it
from ratio temperature coefficient
track.
The absolute tolerance for the
overall resistance of a high voltage
divider is, essentially, the absolute
tolerance of the high voltage resistor
in the divider. It indicates the total
resistance load variation caused by
the divider on the high voltage being
measured.
One of the most important
parameters for specifying high
voltage dividers is the ratio
temperature coefficient. This is also
called ratio TC or ratio track and
specifies the maximum difference
in the absolute temperature
coefficients between the high
voltage resistor and the low voltage
resistor. Without special matching,
the worst case ratio TC is given by
the sum of the absolute TC of the
high and low voltage resistors.
The effects of ratio track can
be seen by measuring the tap
voltage of the divider at several
It is not recommended that
resistors manufactured in different
technologies are used in precision
applications. Ratio TC performance
in this case could be as poor as
200-400ppm/C which represents a
ratio change of 1percent to 2percent
over a nominal temperature change
of only 50C from the reference
temperature. Even in cases where
resistors are manufactured using the
same technology but are not
specially matched, ratio TC may be
100ppm/C or thereabouts, equivalent
to a 0.5percent ratio change over a
50C range.
Consistent ratio TC
The preferred method to
guarantee consistent ratio TC
performance of 50ppm/C or better
is to select either:
1) Specification of all resistors
used in the divider to have an
absolute TC less than 25ppm/C over
the desired temperature range, or
2) TC characteristics of the
resistors in the divider over the
desired temperature range and
matching of the resistors to obtain
the desired ratio TC.
High voltage resistors with
absolute TCs less than 25ppm/C
over the temperature range from 55 to + 125C (referenced to 25C)
are now available.
A ratio track of 30ppm/C can
be obtained without special
matching, when the 25ppm/C high
voltage resistor is combined with a
low voltage (tap) resistor having a
5ppm/C absolute TC. Ratio TC
performance better than 30ppm/C
requires TC characterisation and
special matching of elements. When
ratio tracking of
15ppm/C is necessary, special
attention should be paid to the effects
of voltage coefficient and self-heating
and their inter-relationships with TC.
In high precision ratio TC
applications, the non-linear absolute
TC characteristic of most high voltage
resistor technologies will cause a
problem if self heating is present. Self
heating of the high voltage resistor
moves it to a different point of the TC
curve, with the associated change of
TC slope. The ratio TC match will be
compromised to a degree directly
related to the temperature gradient
(caused by self heating) that exists
between the high and low voltage
resistors.
Another important parameter is
ratio voltage coefficient (ratio VC).This
is the difference in VC effect between
two or more resistors. The key word
here is ‘effect’. It was stated above that
the effect of VC is related to the applied
voltage.With a high voltage divider the
tap resistor seldom has more than I0V
across it. With a VC of as much as
10ppm/V, the VC effect is to reduce
the value of the tap resistor by 100ppm
or only 0.01percent. Since the VC of
the tap resistor is normally less than
10ppm/ V the effect on the tap resistor
can be ignored in most applications.
High voltage dividers can be
separated into two basic categories for
the discussion of VC effects; where
the input voltage to the divider remains
reasonably fixed with variations
generally less than 2percent, and
where the input voltage varies over
wide range.
With fixed input dividers, the
effec of the voltage coefficient is to
reduce
the value of the high voltage
resistor by an amount equal to the
product of the voltage coefficient and
the applied voltage.This increases the
tap voltage above the ideal value. It
can be compensated for by the
manufacturer varying the value of the
high and/or low resistors.The amount
of compensation is equivalent to the
VC effect expressed as a percentage.
Where the VC effect is a
significant part of the desired ratio
tolerance, it is essential that the
absolute tolerance specification of the
divider resistors is wide enough to
allow this compensation. Again-the
absolute tolerance is secondary to ratio
tolerance.
With wide input
voltage variations, the VC
has a more significant
effect. Consider the case
of a divider where the
input voltage varies from
2kV to 8kV,a 6kV range.
If the VC of the high
voltage resistor is only 1
ppm/V the ratio will vary
by 0.6percent over the
range of operation.While
this may be satisfactory
for some applications, it is
unacceptable in applications that
stipulate a 1percent ratio tolerance
which may also include the effects of
TC over the operating range.
It is apparent in this second
case that the VC must be reduced to
keep its contribution to total
performance within acceptable limits.
Ways of achieving this include the
selection of a large physical size for
the high voltage resistor or by using
several elements in the high voltage
section.The latter course of action may
be implemented by putting two
resistors in series with a VC of 0.5ppm/
V each. The effective VC will be only
0.25ppm/V. In general, the VC of a
series string of equal value resistors
of a specified model can be
approximated by dividing the individual
resistor VC by the number of devices.
Self-heating is associated with
the high voltage elements since these
dissipate most of the power. One result
of self-heating has already been
discussed-the effect on the absolute
tolerance.
Self-heating changes the 25C
ratio of the divider unless there is a
good thermal coupling between the
low and
high voltage elements.
Isothermal packaging of set elements
with good ratio TC match will minimise
this error.
A secondary effect is noticeable
in cases where thermal coupling is not
suf ficient.
The ratio track is noticeable in
cases where thermal coupling is not
sufficient. The ratio track is affected
where the absolute TC of the divider
elements is not linear. The thermal
gradient resulting from the selfheating
of the high voltage section of the
divider will tend to change the TC
slope of the high voltage device.
Where
the
expected
temperature
gradient is less than 10C, the
TC match can be offset to
compensate for the slope change. If
the gradient is unknown or controlled,
it may be impossible to achieve ratio
tracking of better than 50ppm/C. In a
production environment, it is
impossible to duplicate the final thermal
environment which may exist in each
application, yet this environment may
have a considerable effect on the
divider’s overall performance.
Linear elements
Where linear TC elements are
matched, the self-heating effect is
obvious during warm-up as a ratio
change. When isothermal packaging
is utilised, this warm-up drift is
eliminated. With linear TC elements,
the self-heating effect does not change
the TC slope of the resistor to any great
extent.
Perfect isothermal packaging is
not easy to achieve. Possible solutions
include bonding the tap resistor to the
high voltage resistor with a high
thermally conductive epoxy or
mounting the divider elements on a
common substrate. The latter is used
with currently available voltage dividers
and provides ratio tolerances of
as tight as 0.25percent with ratio
tracking of 10ppm/C.
If the, first method is used, then
care must be taken to avoid dielectric
breakdown which can occur due to the
high voltage field stress. The highest
thermal coupling can be obtained by
bonding the tap resistor to the centre
of the high voltage element. However,
at this point the voltage gradient will
be approximately 50percent of the input
voltage, resulting in a large voltage
stress between the two resistors.
Therefore, the tap resistor is most often
bonded to the low voltage end of the
high voltage element.
Another alternative is to employ
a bleed resistor to handle
a large percentage of the
desired load current on
the supply.The high
voltage divider can then
be operated at a current
level which will minimise
self-heating of the high
voltage resistor.
Divider networks
using the same type of
precision film technology
as the discrete devices
mentioned earlier, are an excellent
alternative for many applications. The
Caddock type THV high voltage divider
networks include two standard ratios
of 100: 1 and 1000: 1 with a 10kV rating.
The total resistance of the standard
product is 100M Ohms. Ratio tolerance
at 10kV is from 2percent to
0.25percent and the standard ratio TCs
over the’ - 55 to + 125 temperature
range are either 10 or 25ppm/C.
Custom networks are available
at other ratios with 100M Ohm total
resistance and below. The ratio
tolerances and TCs are similar to those
of the standard products.
The unique problems associated
with using high voltage precision
resistor film resistors and networks can
be overcome by an understanding of
the criteria for successful application.
In radar equipment, ATE, power
supplies and other situations, high
voltage precision resistors are a vital
analogue component.
For further information please
contact
RHOPOINT COMPONENTS
Holland Road, Hurst Green, Oxted,
Surrey, Surrey, RH8 9AX, England
Tel: +44/01883 717988
Fax: +44/01883 712938