Specifying high voltage film resistors

June 27, 2018 | Author: Anonymous | Category: Engineering & Technology, Electrical Engineering
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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

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