Copper Alloy Inductors Stabilize Current Sensing

Apr 1, 2008 12:00 PM
By Donna Schaefer, Applications Engineer, BI Technologies, Fullerton, Calif., and
Bryan Yarborough, Applications Engineer, IRC, Corpus Christi, Texas

Current sensing is a critical part of microprocessor VR11x computing power as it provides overcurrent protection, phase-to-phase balancing and load line adherence. The trend is to lower voltage and improve efficiency in the power architecture. This requires tighter voltage regulation during transients with the accuracy of current sensing being critical.

Recommended current-sensing methods vary among vendors, but sensing through an inductor's resistance (R_DC) is a popular method because it is considered lossless. Less than 1% is degraded from the power efficiency, and accuracy is good because the inductor is sensing the actual output current to the load. The stability of the inductor's inductance and R_DC over temperature is critical, and tighter tolerances can improve the accuracy of the measurement.

Presently, negative temperature coefficient of resistance (NTC) thermistors are used to sense the inductor's temperature. The thermistor provides correction for the change in resistance of the inductor's copper conductor material, which has a large temperature coefficient of resistance (TCR), 3900 ppm/°C.

However, a thermistor is an additional component on the bill of materials; it complicates the board layout and is not dynamically responsive. By using a copper-alloy material for the inductor conductor, the TCR can be lowered to 700 ppm/°C, and the tolerance at room temperature can be controlled to ±2%, essentially making for an inductor with a constant R_DC.

Thermal Correction with an NTC Thermistor

A closer examination of how an NTC thermistor is used in current sensing can show why it has a large error margin due to nonuniform heat distribution.

Current is sensed across the inductor in each phase of the voltage regulator by detecting the voltage across the inductor's R_DC . An R-C network is connected in parallel to the inductor where the voltage across the capacitor is proportional to the inductor output current. If the time constant of the R-C network matches the time constant of the inductor, L/R_DC , then accurate sensing is accomplished.^[1]

Compensating for the high TCR of the copper-inductor conductor requires a thermal sensor. Thermal sensing is typically done external to the PWM control chip, because the chip is not positioned next to the components that are dissipating high amounts of heat.

An NTC thermistor is located close to one of the output inductors along with a biasing resistor. The number of output inductors depends on the number of voltage regulator phases. Errors in the compensation method occur because there are differences in board and component temperature and the thermistor can't dynamically respond. This error margin is most critical for output-voltage load line regulation.

According to STMicroelectronics' data sheet for the L6756 multiphase controller for VR11x applications^[2], three factors typically affect the load line regulation tolerance band: controller tolerance, current-sense circuit tolerance and time-constant matching error tolerance. The tolerance band for the current-sense circuit is directly related to the inductor DCR tolerance, the accuracy of the NTC thermistor and the accuracy of the temperature measurement for the copper-inductor conductor. The formula for calculating this tolerance is:

where V_AVP is the output voltage (adaptive voltage positioning), k_DCR is the inductor R_DC tolerance, k_Rg is the trans-conductance resistor, k_NTC0 is the tolerance of the NTC thermistor at room temperature, á is the copper temperature coefficient of resistance, k_NTC is the temperature accuracy, ΔT is the change in temperature, DCR is the inductor R_DCat room temperature and N is the number of phases.

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