TCR in Bulk Metal® Foil Resistors to ±0.2 ppm/°C

Foil resistor technology has continued to progress over the years, with significant improvements in TCR. Figure 1 shows the typical TCR characteristics of the various foil alloys utilized by Vishay to produce Bulk Metal® Foil resistors.
The original Alloy C Foil exhibits a negative parabolic response to temperature with a positive chord slope on the cold side and a negative chord slope on the hot side.
Following was the Alloy K Foil which produced an opposite parabolic response with temperature with a negative chord slope on the cold side and a positive chord slope on the hot side. In addition, it provides a TCR approximately one half that of Alloy C Foil.
The latest development is the Alloy Z Foil Technology Breakthrough which has a similar parabolic response as the Alloy K Foil but produces TCR characteristics an order of magnitude better than Alloy C and five times better than Alloy K.
Extremely low TCR resistors have been developed that provide virtually zero response to temperature. See the data sheet for the Vishay Thermotropic VHP100, ultra performance Z201, hermetically sealed VH102Z resistors and Foil Surface-Mount Chip resistors: VSMP series, VFCP series and VCS2516Z.
These technological developments have resulted in a major improvement in TCR characteristics compared to what was available from Vishay before, and what is available in any other resistor technology. Use Bulk Metal® Foil resistors for all extremely low TCR requirements.

Datasheets (36)

TCR Tracking

When more than one resistor shares the same substrate, the TCR tracking will be much better than the TCR provided by two discrete resistors. Resistors with different technologies increase or decrease in value when temperatures change.
Resistance ratio tracking is influenced by heat that comes from outside the device (such as a rising ambient temperature or adjacent hotter objects) and from inside the device (as a result of self-heating due to power dissipation). Absolute TCR and Tracking are particularly important figures of merit in the case of two resistances, even with similar TCR characteristics. The amplification ratio will be affected by using a pair of resistors having the same high TCR (and therefore good tracking). The ratio will change when there is a temperature difference between the two resistors. This is why it is important to have also low absolute TCR, not only good tracking.
The best analog design would be using a fundamentally low absolute TCR resistor since it would minimize the effect of ambient temperature and self-heating.
This is impossible to accomplish with high TCR resistors even with good initial TCR Tracking.

Datasheets (36)
Technical Note: Zero TCR Foil Resistor Ten Fold Improvement in Temperature Coefficient


Tight Resistance Tolerances
The accuracy of Bulk Metal® Foil resistors can be made as precise as 0.001% by selectively trimming various adjusting points that have been designed into the photoetched pattern of the resistive element (See Figure 2). They provide predictable step increases in resistance to the desired tolerance level.



Trimming the pattern at one of these adjusting points will force the current to seek another longer path, thus raising the resistance value of the element by a specific percentage.  In the fine adjust areas, trimming affects the final resistance value by smaller and smaller amounts down to 0.001% and finally 0.0005% (5 ppm). This is the trimming resolution (See Figure 3).



Datasheets (36)

Excellent Load Life Stability
Why are designers concerned about stability with applied load? Load-life stability is the characteristic most relied upon to demonstrate a resistor’s long-term reliability. Military testing requirements to 10,000 hours with limits on the amount of shift and the number of failures result in a failure rate demonstration. Precision Bulk Metal® Foil resistors have the tightest allowable limits. Whether military or not, the load-life stability of foil resistors is unparalleled and long-term serviceability is assured. The reason foil resistors are so stable has to do with the materials of construction (Bulk Metal® Foil and high alumina substrate). For example, the S102C resistor is rated at 0.3 W at 125 °C with an allowable ΔR of 150 ppm after 2000 hours under load. (See Figure 5 for the demonstrated behavior.) Conversely, the ΔR is reduced by decreasing the applied power which lowers the element temperature rise in Vishay resistors. Figure 5 shows the drift due to load-life testing at reduced power. Reducing the ambient temperature has a marked effect on load-life results. The combination of lower power and ambient temperature is shown in Figure 7.



Datasheets (36)

Power Coefficient (PCR)

The TCR of a resistor for a given temperature range is established by measuring the resistance at two different ambient temperatures: at room temperature and in a cooling chamber or oven. The ratio of relative resistance change and temperature difference gives the slope of ΔR/R = f (T) curve. This slope is usually expressed in parts per million per degree Centigrade (ppm/°C).
In these conditions, a uniform temperature is achieved in the measured resistance. In practice, however, the temperature rise of the resistor is also partially due to self-heating as a result of the power it is dissipating. Therefore, the TCR alone does not provide the actual resistance change. Hence, another figure of merit, PCR (power coefficient), was introduced. PCR is expressed in parts per million per Watt or in ppm at rated power. In the case of Z-based Bulk Metal® Foil, the PCR is 5 ppm typical at rated power or 4 ppm per watt for power resistors.

Power Current Sensing Resistors

High precision resistors used for current sensing are usually low ohmic value devices suitable for four terminal connections. Two terminals are connected to allow the electrical current to pass through the resistor. Voltage drop (V) is measured on the other two terminals, called “Sense” or “Voltage drop” terminals.
This “Kelvin connections” arrangement reduces, especially for low ohmic resistance values, a measurement error due to the resistance of the lead wires and solder joints as the sensing is performed inside the resistor, in or close to the active resistive foil.
The high precision power current sense foil resistors are best suited for manufacture at low values due to superior stability compared to thick film or thin film resistors.

Vishay achieves very high precision in current sensing by:
   • Using Z-foil for very low TCR (0.2 ppm/ °C typical) over wide temperature ranges
   • Z-Foil also provides a very low PCR (power coefficient of resistance) of 4 ppm/Watt typical for power resistors
   • Applying proprietary methods of manufacturing precision four terminal resistors
   • Applying its expertise in stress analysis

Examples of Vishay high precision power current sensing Z-Foil resistors: Leaded: VHP4Z, VPR247Z, VFP4Z, VCS331Z, VCS332Z, VCS232Z, VPR220Z, VPR221Z SMD: VCS2516Z

Datasheets (13)
Technical Note: New Foil Resistor with Very Low Power Coefficient

For technical questions, please contact: foilsupport1@vishay.com

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