Temperature detection in thermal management design

　　Detect PCB temperature

　　Surface mount sensors are ideal for PCB (printed circuit board) temperature measurement. RTD (resistance temperature detector), thermistor and IC (integrated circuit) sensors are all available in surface mount packages and have a temperature range suitable for PCB temperature detection. IC sensors are usually the best choice for such applications due to their inherent high linearity and low cost. IC sensors can also provide other functions, such as digital interface or temperature monitoring functions. These functions give them a great advantage in system cost, design complexity and performance compared to other technologies.

　　One of the keys to accurately measuring PCB temperature is to place the sensor in the right location. In many cases, the focus is on measuring the temperature of a specific device or group of devices to ensure that the temperature does not exceed the safe operating range, or to compensate for changes in device performance due to temperature. If the location of the sensor is very sensitive, it is best to use a temperature sensor with a small package size, such as TDFN, SC70, or SOT23, which can easily place the sensor in the appropriate location. When the sensor needs to be placed in a noisy area or away from other temperature-related circuits, it is best to choose a temperature sensor with a digital output. If you need to monitor the temperature of multiple locations on the PCB, a local digital temperature sensor with an I2C, SMBus, or 1-Wire interface is the best choice, so that devices with different slave addresses can be connected to the same bus. Many common I2C sensors provide inputs to set different slave addresses. For example, both the MAX7500 and DS75LX have three address inputs.

　　Another approach to monitoring multiple PCB locations is to use a multichannel remote temperature sensor with discrete transistor temperature sensing. Figure 1 shows an example where the MAX6697 monitors its internal temperature and uses discrete transistors to monitor six external temperatures, for a total of seven temperature points, with only a single I2C slave address.

　　Detection of ambient temperature

　　Measuring ambient temperature is difficult because the sensor must be at the same temperature as the air and isolated from all other components (PCB, power supply, CPU) that are at different temperatures. Thermistors, thermocouples, and RTDs can have long leads, which can help to isolate the sensing element from the PCB if the leads are long and thin enough. If the sensing element is sufficiently isolated from the PCB, its temperature will be the ambient temperature. Of these three types of sensors, thermistors are widely used for general-purpose ambient temperature sensing due to their low cost and simple signal conditioning requirements. Figure 2 shows an example of how to measure ambient temperature using a thermistor, thermocouple, or RTD. In the figure, the thermistor is well isolated from the surface of the circuit board, and the long leads help to provide thermal isolation from the circuit board.

　　Measuring ambient temperature with a surface-mount temperature sensor IC is more difficult because the best thermal path for the IC sensor is through the pins, which are at the same temperature as the board. If the PCB is in the temperature measurement environment, a sensor mounted on the PCB can be used to measure the ambient temperature. However, if the components on the PCB dissipate too much power, the temperature will be higher than the ambient temperature, and the temperature measured by the IC is the elevated PCB temperature, not the ambient temperature. Note that even with a traditional package, such as TO92, the IC sensor is located above the PCB, and heat is conducted through the pins, so the measured temperature is actually equal to the PCB temperature. Figure 3 shows a temperature sensor IC in a board-mounted TO92 package. Board-mounted sensors are good for measuring PCB temperature, but are not suitable for measuring ambient temperature.

　　Although they are susceptible to PCB temperature, sensor ICs are still the best solution for measuring air temperature because they are more than just sensors—they often provide more functions, such as digital output, addressability, or temperature monitoring. When using a TO92 packaged IC to sense ambient temperature, use twisted-pair wire to isolate the sensor from the PCB. As with a thermistor, if the leads are long and thin enough, sufficient thermal isolation can be achieved to obtain an accurate ambient temperature reading. Figure 4 shows an example of using a TO92 sensor in this manner, using Maxim's 1-Wire interface digital temperature sensor. Diode temperature sensors can also be used to measure ambient temperature. In this case, a discrete transistor can be mounted at the end of the twisted pair. Any of Maxim's remote diode sensors can be used to measure the transistor temperature.

　　Temperature detection of CPU, graphics processor, FPGA, ASIC, power devices

　　As mentioned in the Remote Digital Temperature Sensors section, some components, especially high-performance ICs such as CPUs, GPUs, and FPGAs, have a bipolar transistor for temperature measurement. This transistor is usually a PNP with the collector grounded. The base and emitter form a "diode" connection. Because the temperature-sensing transistor is located on the IC die, the measurement accuracy is far superior to other technologies, and the thermal conduction time constant is quite small. Maxim's remote diode sensors are optimized for temperature monitoring of such devices. Note that the ideality factor and series resistance of the temperature-sensing diodes vary from IC to IC. The effects of these differences are explained in the Remote Diode Sensor Application Guide.

        Some ICs do not have a temperature-sensing diode installed, but do have an integrated thermistor to help monitor the temperature (see the Thermistor section). These thermistors are difficult to use, have very small temperature coefficients, and have poor accuracy. The change in resistance value at a nominal 25°C can produce errors of ±50°C or even more. Therefore, they must be calibrated at one or two temperature points before use. The low temperature coefficient allows a typical thermistor converter (such as the MAX6698) to have a resolution of about 6°C/LSB. Figure 5 shows a typical plot of thermistor channel code vs. temperature when the MAX6698 is measuring an integrated thermistor. Note that although the test results are useful, the resolution is low.

　　Remote diode sensor applications

　　PCB Layout Guidelines for Remote Sensors

 　　When using a remote temperature sensor, following these guidelines will help you achieve the best results. Connect DXP to the anode and DXN to the cathode. Note that accuracy is related to the amount of noise picked up, and its effects are not easy to predict. Be sure to verify that accuracy meets requirements when delivering the final layout.

　　1. Install the remote temperature sensor as close to the temperature diode as possible. In a noisy environment, such as a computer motherboard, the maximum distance can be 20 cm. If strong noise sources can be avoided, the distance can be appropriately extended. Noise sources include: CRT, clock generator, memory bus and PCI bus.

　　2. Do not allow the DXP-DXN leads to cross high-speed data lines or be arranged in parallel near them. Even after good filtering, these signals can easily introduce a +30°C error.

　　3. Keep the DXP and DXN leads parallel and close to each other. Each pair of parallel wires should be connected directly to a temperature measuring diode. Be sure to keep these leads away from any high-voltage traces, such as +12VDC. Leakage current caused by PCB contamination must be minimized. A 20MΩ leakage resistance between DXP and ground can cause an error of about +1°C. If high-voltage traces cannot be avoided, a guard line connected to GND must be arranged outside the DXP-DXN (see Figure 6).

　　4. Avoid using vias and crossing traces to minimize the thermocouple effect of copper/pads.

　　5. Use the widest possible traces—typically 5 mil to 10 mil. Note that if you use long, narrow traces, you need to understand the effect of lead resistance on temperature.

　　6. When using a noisy power supply, add a resistor (47Ω maximum) in series with VCC.

　　7. Connect a filter capacitor across the DXP-DXN input and place it close to the remote sensor IC. Please refer to the recommended value in the sensor data sheet for the capacitor value.

　　Cable-Connected Thermal DiodesSometimes it is necessary to place a thermal diode at a distance that exceeds the normal span of a circuit board—for example, using the diode to measure the temperature at the other end of a large cabinet.When the distance is not very far and the noise is relatively low, a simple twisted-pair connection can be used, which can work properly for distances up to 3m or 4m.For longer distances (up to 30m or so) or when there is a lot of noise, a shielded cable should be used to connect the remote sensor with the shield connected to ground. Belden 8451 cable is suitable for this application.Note that the equivalent series resistance of the cable will affect the temperature reading, so it is best to use a temperature sensor with resistance cancellation or calculate the effect of the lead resistance and subtract this value from the test temperature.Also be aware of the capacitance of the cable, which will reduce the maximum capacitance allowed at the input of the thermal diode.

　　When the remote temperature sensing diode is a discrete transistor, connect its collector and base together. NPN and PNP transistors are well suited for this application. Table 1 lists examples of discrete transistors that can be used with remote temperature sensors. Small signal transistors must be used with relatively high forward bias; otherwise, the A/D input voltage range will be exceeded. The forward bias at the maximum expected temperature must be greater than 0.25V at 10μA, and the forward bias at the lowest expected temperature point must be greater than 0.95V at 100μA. A high-power transistor must be used, and the base resistance must be less than 100Ω. Tight forward current gain specifications (such as 50 < β < 150) indicate that the manufacturer has good process control and the devices have consistent VBE characteristics.

　　Discrete transistor manufacturers do not usually specify or guarantee ideality factors. Since the ideality factors of high-quality discrete transistors are usually within a relatively narrow range, this should not be a problem. We have observed variations in remote temperature readings of less than ±2°C with a variety of discrete transistors. Nevertheless, it is best to verify the consistency of temperature readings across multiple discrete transistors from a selected manufacturer

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　　Some IC manufacturers, such as microprocessor and FPGA manufacturers, have integrated temperature sensing diodes into their products for many years and have mastered the design technology of these devices. For IC designers who are integrating temperature sensing diodes for the first time, this section provides some helpful references:

　　1. Minimize the internal resistance of the diode. As mentioned above, each ohm of series resistance will cause an error of approximately +0.45°C. If the base of the transistor is connected to the collector in a diode-connected configuration, the base resistance will increase the beta value. In this case, the collector resistance is insignificant unless it causes the diode-connected device to saturate at 100μA.

　　2. Minimizing the transistor's beta helps maintain the collector current ratio (and therefore accuracy) over the entire temperature and current range.

　　3. The forward bias voltage of the diode must be within the input range of the temperature detection ADC. In the entire temperature measurement range, when the forward current is 10μA, the forward bias voltage must be greater than 0.25V; when the forward current is 100μA, the forward bias voltage must be less than 0.95V.

　　4. In most processes, there is no isolated P/N junction. If the transistor connected as a diode meets the following restrictions, it can work normally:

　　A. If it is an NPN tube, the three terminals must be isolated from any power source, and the base is connected to the collector to form a diode;

　　B. If it is a PNP tube, the collector can be grounded, but the emitter and base must be isolated from all power supplies.

　　5. The test circuit must be measured to determine if it is working properly. Measurement accuracy is very important—the voltage must be accurate to 100μV and accurate to ±0.1% at 10μA and 100μA bias current. The temperature sensing transistor works with all Maxim remote temperature sensors.

　　6. Noise coupled to the temperature measurement junction will cause temperature measurement errors, and the temperature measurement device needs to be carefully isolated from the noise source, including digital signals and noisy power supplies.

　　A. Physically isolate the temperature sensing device from the leads carrying high-speed digital signals. Physically isolate the digital signals from the metal wires between the temperature sensing transistor and the bonding pad.

　　B. Do not place the bonding pad of the temperature sensing junction close to the pads of high-speed digital signals, especially high-speed buffer outputs. If possible, place the bonding pad of the temperature sensing junction close to the DC input pad (for example, DC logic level input for pin setting).

　　C. Surround the temperature measuring device with n+ and p+ guard rings.

　　7. The typical structure of a vertical PNP transistor with a base collector is shown in Figure 7. 10 emitters are connected together, and each emitter occupies a 20μm × 2.5μm space.