High-precision foil power current-sensing resistors handle 10W
Designed to provide optimal performance when mounted on a chassis or cooled heat sink, the VCS331Z, VCS332Z, VFP4Z, and CSNG Series feature low TCR of ±0.2 ppm/°C from -55°C to +125°C, +25°C ref., PCR (ΔR due to self-heating) of 4 ppm/W typical, and tolerances to ±0.01%. For high-power applications, the devices offer power ratings up to 10W (on heatsink) and a four-terminal Kelvin configuration in order to remove the unwanted influence of lead resistance and lead sensitivity to temperature.
The VCS331Z, VCS332Z, VFP4Z, and CSNG offer load-life stability to ±0.005% typical (50 ppm) at 3 W (heat sink) and +25°C for 2,000 hours, a thermal stabilisation time of <1 sec (within 10 ppm of steady state value), and a wide resistance range from 0.25 Ω to 500 Ω. (Lower and higher values are also available.) Any resistance value within this range is available at any tolerance with no additional cost or lead time effect. The resistors offer a rise time of 1.0 nsec with effectively no ringing, current noise of 0.010 µVrms/V of applied voltage (<-40 dB), and a voltage coefficient of <0.1 ppm/V. The devices withstand ESD to at least 25 kV for increased reliability, and they offer a non-inductive (<0.08 μH), non-capacitive design.
"In applications such as force-balance scales, electronic beam deflection systems, and switching power supplies, accuracy and repeatability are likely to be more important than power handling requirements and depend to a major extent on precision current sense resistors with Kelvin connections," said Yuval Hernik, Senior Director of Application Engineering for VFR. "The resistors must have extreme thermal efficiency, well-distributed internal heat from power dissipation, very low thermal EMF, and fast thermal stabilization. All these characteristics are necessary to prevent errors caused by uneven heat from external sources such as the power amplifiers and other circuit elements."
Many instrumentation circuits depend upon current sense resistors for precision measurements and system controls. The new devices can also be used in lithium-ion battery circuits for alternative energy applications where energy that cannot be harvested must be stored. In state-of-the-art battery management circuits, Bulk Metal Foil resistors can be used in the measurement of the "state-of-charge" (SOC) of the battery cells. The characteristics of Li-ion cells complicate SOC measurement, so precise ICs and precision current sense resistors with improved stability and low thermal EMF help to extend battery lifetime.
The Bulk Metal Foil resistor is based on the planar construction of a cold-rolled Ni-Cr foil bonded to a flat ceramic substrate. The monolithic foil, as opposed to the agglomeration of particles in thin film resistors, has a strength and substance independent of the substrate, which gives the metal foil its own specific thermal linear coefficient of expansion (LCE). The foil also has a known and repeatable independent positive temperature coefficient of resistance (TCR). When the foil is bonded to the substrate, it attempts to respond to temperature changes in accordance with its own LCE but is constrained to follow the lower LCE of the ceramic substrate. The resistance change due to the differential LCEs counterbalances the metal’s positive TCR for a near-zero net TCR of the resistor. The composite formation with a planar foil structure makes it possible to very accurately etch a pattern into the resistor that is non-inductive and non-capacitive, with low noise and low thermal EMF.
Vishay Foil Resistors; www.vishayfoilresistors.com
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