Remote Diode Conditioning ICs Provide Cost Effective Alternatives to Traditional Temperature Sensing Solutions

John Austin, Microchip Technology

Remote Diode temperature monitoring ICs have been a proven temperature sensing technology that has been utilized in the computing and server industries for more than a decade. For many application scenarios this technology can reduce cost, reduce development time and minimize the thermal design expertise required. This article discusses the benefits, limitations and cost effectiveness of remote diode temperature monitoring ICs versus other traditional temperature sensing technologies such as silicon ambient temperature sensor ICs, thermistors, Resistive Thermal Detectors (RTDs) and thermocouples. Key design criteria, such as temperature accuracy, power consumption, system cost and size, and design complexity will be addressed for each of these solutions as well. The article will then discuss how the remote diode solution can be used to eliminate many of the limitations found in the previously mentioned "traditional" solutions. The article will also discuss how to utilize integrated features, such as resistance error correction, dynamic averaging and under-/over- and critical-temperature monitoring to improve system performance. Finally, this article will highlight available tools to help designers identify the product most suited for their application (MAPS).

The article will then discuss how the remote diode solution can be used to eliminate many of the limitations found in the previously mentioned "traditional" solutions. The article will also discuss how to utilize integrated features, such as resistance error correction, dynamic averaging and under-/over- and critical-temperature monitoring to improve system performance. Finally, this article will highlight available tools to help designers identify the product most suited for their application (MAPS). 

Thermistors

Thermistors are the most common method used to sense temperature. Thermistors are made using semiconductor materials and can have either a positive or negative (PTC or NTC, respectively) temperature coefficient. The thermistor’s resistance will change with a corresponding change in temperature. PTC thermistor resistance will increase with an increase in temperature, whereas NTC thermistor resistance will decrease with an increase in temperature.

There are a few advantages to thermistor-based solutions. First, thermistors are highly sensitive to changes in temperature. In addition, they have a quick thermal response and they are inexpensive. The biggest disadvantage is that thermistors are highly nonlinear over wide temperature ranges.

Figure 1 shows a thermistor circuit with a low-pass filter and a fixed-gain buffer amplifier. The low-pass filter (R2 and C1) network filters system noise from the sensor output and the unity-gain buffer is used to drive resistive or capacitive loads. The voltage across the thermistor (VTH) is proportional to the change in temperature. The graph indicates a linear response from 00C to 700C. However, there is significant non-linearity at temperature extremes. The change in resistance with respect to temperature is much less when compared to the linear region. This requires some signal amplification to improve measurement resolution at hot and cold temperature extremes.
Thermistors are inexpensive and provide accurate temperature monitoring over a limited temperature range. To achieve high accuracy over a wider range requires a more complex design. This increases overall system cost. In most circumstances other solutions, such as Silicon ambient or remote diode conditioning ICs, may be better suited for extended temperature applications. For applications monitoring multiple zones, remote diode conditioning ICs have the potential of offering significant cost advantages.

Resistive Temperature Detectors (RTDs)

Resistive Temperature Detectors (RTDs) are a robust temperature monitoring solution. These sensors provide excellent repeatability and stability characteristics. A designer can achieve high accuracy over several 100s of degrees Celsius range using RTDs. This requires careful scaling, calibration and resistance to temperature conversion. Various standards and specifications are adapted worldwide.

A basic RTD circuit requires a constant current source for biasing and an analog instrumentation circuit, such as an instrumentation amplifier, to measure the voltage drop across the RTD. The amplifier output is typically connected to an Analog-to-Digital Converter (ADC) for digitization. Other circuits convert the change in resistance to frequency. For example, the circuit in Figure 2 shows a relaxation oscillator circuit which uses a RC network and a comparator to generate a frequency proportional to the change in temperature. The frequency can be directly connected to a microcontroller for digitization. When designing an RTD circuit, the effect of self-heat must be carefully considered.
RTDs have excellent repeatability and can provide a precise temperature-monitoring solution over a wide temperature range. Downsides of this technology include cost, design complexity and increased system power consumption.

Thermocouples

Thermocouples have an extremely wide operating temperature range of -270°C to 1750° C. The Instrument Society of America (ISA) defines a number of commercially available thermocouple classifications in terms of performance. Types E, J, K and T are base-metal thermocouples and can be used to measure temperatures from about -200° C to 1000° C. Types S, R and B are noble-metal thermocouples and can be used to measure temperatures from about -50° C to 2000° C.

Thermocouples use two metal alloys, such as Alumel and Chromel, to measure temperature. The two metals are welded at one end and open at the other end. The electrical characteristics of the wires at the welded point are temperature dependent. A voltage is generated at the welded tip, which can be measured at the open end using a volt meter. The voltage’s magnitude increases and decreases proportional to changes in temperature. Thermocouples are highly non-linear and require linearization algorithms.

The welded tip is referred to as the hot junction and the open end is the cold junction. Temperature is measured by the difference between the hot junction and the cold junction’s room or ambient temperature. The temperature at the cold junction is used as a reference for the hot junction. The temperature at the cold junction is measured using a variety of temperature sensing technologies.

The full-scale voltage range of a thermocouple is less than 100 mV. Therefore, high-performance analog signal conditioning is required. The circuit in Figure 3 shows a typical thermocouple circuit. The thermocouple is connected to the instrumentation system with EMI filters for industrial applications. It is tied to a positive and negative supply through large resistors, so that the circuit can detect an open circuit. Auto-zero and chopper amplifiers can be used for signal conditioning, due to the low offset voltage and Common Mode Rejection (CMR) specifications. The cold-junction compensation circuit is implemented with a remote diode conditioning IC in conjunction with a remote diode located on the PCB.

Silicon Ambient Temperature Sensors

Many semiconductor manufacturers offer silicon-based temperature sensors. These devices can be categorized by their output type as well as their logic, voltage and serial outputs. IC sensors integrate many useful features that allow system designers to implement the design that best meets the requirement of their application. Temperature-sensor ICs require minimal design effort, and the integrated features can decrease overall system cost and minimize design effort.

Silicon Remote Diode Conditioning ICs

I mentioned previously remote diode conditioning ICs have been used in the PC and server industries for many years. This is a proven technology that has been underutilized in other mainstream applications where the technology has the potential to provide significant cost savings as well as product enhancements by utilizing many of the integrated features.

Remote Diode conditioning ICs monitor delta Vbe of a vertical PNP, on a processor or Graphics Processing Unit (GPU), or a standard diode connected Negative-Positive-Negative (NPN) transistor. One of the benefits of using an industry standard NPN transistor is that in volume the cost of the transistor can be almost free. In applications where multiple temperature zones must be accurately monitored this can provide significant cost savings.

Figure 4 illustrates the typical diode connections for both NPN and Positive-Negative-Positive (PNP) transistors using Microchip’s MCP9904.
In both cases the MCP9904 is forcing two currents, of different magnitudes, and measuring the Vbe of the transistor. These currents are sourced from DP and returned through DN. Taking the difference yields delta Vbe and the temperature is calculated using Equation 1. A more detailed explanation of this equation can be found on the Microchip website in application note AN10.14 – Using Temperature-Sensing Diodes with Remote Thermal Sensors.

A PNP transistor is connected to pins DP1 and DN1. The currents are forced into the emitter and returned from the base of the PNP transistor, while the emitter is grounded. In this configuration the return current is limited by the beta of the transistor (IB=IC/b) , causing limitations in discharging the filtering capacitance between DP and DN. For this reason it is recommended to use a NPNtransistor unless this option is not available, such as an embedded vertical PNP transistor in a GPU/CPU.

Figure 4 also illustrates anti-parallel diode configuration. This is when two diode connected transistors utilize the same two pins. The concept of how this is achieved is easy to comprehend. In the example above the MCP9904 still uses two current magnitudes to determine the temperature of the transistor; however, the device alternates the direction of the currents. In one current direction one diode is “OFF” and one is “ON”. This allows the device to monitor more temperatures with fewer pins. The advantage is smaller packaging and lower device cost. It is worth noting in that in the NPN configuration the base is connected to the collector and the current return path to DN is from the emitter. In this configuration the return current is not dependent on the beta of the transistor. This allows for larger filter capacitance between DP and DN.

The Remote Diode conditioning ICs also have many useful integrated features such as dynamic averaging, series resistance correction, user programmable alert and thermal limits.

The dynamic averaging feature oversamples the delta Vbe measurement and increases the amount of averaging to minimize the effects of conducted noise. Many applications today have multiple sources that could potentially conduct noise onto the Vbe signal measurements. A few examples are back lighting inverters, clock and data lines and switch mode power supplies. It should be noted that a Vbe change of 250µV corresponds to a temperature change of 1 degree Celsius.

A series resistance of 1 ohm will generate a positive temperature offset of 0.656oC. A few examples of a series resistance source include a substrate resistance, package lead resistance, trace resistance and wire resistance for off board temperature measurement. The remote diode conditioning ICs are able to eliminate up to 100 ohms of series resistance by forcing two additional currents and choosing a ratio that eliminates the resistance term in Equation 2.

More details can be found on the Microchip website in application note AN13.19 – Resistance Error Correction.
The alert and thermal limit features allow the host microcontroller to load a temperature-trigger value into an internal register located in the silicon temperature sensor using the serial interface. When the desired temperature value is exceeded, the sensor flags the host controller that an over or under temperature condition occurred. This feature can be used to turn on a light or control a fan, without the need for the microcontroller to monitor temperature continuously, using the serial interface. This increases flexibility by freeing up the host microcontroller from having to continuously monitor the system. It also simplifies software and hardware development.

Summary

Temperature sensors bring with them a variety of advantages and disadvantages. No one type of sensor is appropriate for all temperature-sensing applications. Microchip, for example, offers very useful selection of tools to assist designers in choosing the most appropriate thermal management solution. MAPS (Microchip Advanced Parts Selector) allows the user to input specific product specification requirements to narrow down to the most suitable Microchip products. MAPS can be located online at http://www.microchip.com/maps/.

Thermistors provide a useful, low-cost temperature-sensing solution in applications that operate over a limited temperature range. RTDs can be highly accurate over several 100s of degree Celsius range, requiring careful scaling and calibration. Thermocouples are most useful in applications that must operate in temperature extremes. However, RTD and thermocouple solutions can be quite costly and design intensive. Silicon IC-based temperature sensors and remote diode conditioning ICs simplify designs, while offering fairly high accuracy over a wide temperature range. They also provide many integrated features that enhance system flexibility and performance. In conclusion, choosing the correct temperature sensing solution can be complicated with the array of options available. Microchip offers a diverse portfolio of products to address each designer’s needs. In this article we specially looked at Remote Diode Conditioning ICs and provided clear reasons why they are a cost effective alternative to traditional temperature sensing solutions.