Intelligent Thermal Management Using Mixed-Signal FPGAs

However, the cost of these devices increases rapidly if the temperature of multiple test points on the board needs to be measured. This in turn creates an urgent need for efficient, compact and low-cost temperature measurement methods, with applications ranging from high-speed computers, telecommunications network switching equipment to industrial temperature control such as portable electronics, biomedical devices, motor control and automotive electronics.

Because timely and accurate temperature correction is critical in many applications, today's intelligent systems use cooling systems and balance their operation based on the system's internal conditions. Such systems have the additional advantage of using on-board temperature sensing diodes (or diode-connected transistors) to track and measure the temperature of specific devices. This can indicate system operation when an abnormal temperature occurs, indicating that a component is not operating correctly. The intelligent system can then respond by taking corrective action and/or providing an out-of-bounds alert to system management.

In addition to performing other system management tasks, today's mixed-signal FPGAs are also intelligent thermal management systems, allowing designers to easily and accurately measure temperatures at multiple locations at a low cost.

Checking and measuring voltages using mixed-signal FPGAs

When studying the relationship between the absolute temperature of a diode and its forward voltage under constant current, the change in the diode forward voltage drop with temperature is approximately 2mV/C. To improve the measurement accuracy and eliminate the difference between different diodes, the ratio data of two known current values ​​and the measured values ​​is used. Figure 1 shows the effect of temperature on the diode voltage and current.

This measurement is expressed by the following equation:

T =DV * q / (n * k * ln(IH / IL) (1)

Where T = absolute temperature, DV = the voltage difference between the diode at high and low currents, q = 1.602×10-19 coulombs (the charge of an electron), n = 1 (ideal factor, assumed to be 1 here), k = 1.38×10-23 J/K (Boltzmann constant), IH = high current intensity, IL = low current intensity.

This article uses Actel's mixed-signal Fusion PSC (Programmable System Chip) in real-world applications as a case study. The mixed-signal FPGA will provide two known current sources (100mA and 10mA) and measure the voltage difference through the built-in analog-to-digital converter (ADC). Assuming the diode is at room temperature, the voltage difference DV is verified.

Solve equation (1) to obtain the conversion voltage to be sent to the ADC, which is equation (2) below; and then obtain the voltage value measured by the mixed signal FPGA.

DV = T * n * k * ln(IH / IL) / q (2)

DV = 298 * 1 * (1.38x10-23 J/K) * ln(10) / (1.602x10-19C)

DV = 298 * 0.00019835 = 59 mV