High temperature system design considerations

Designers of high-temperature operating circuits must consider IC parameters and changes in passive components over a wide temperature range, with particular attention to their characteristics at extreme temperatures to ensure that the circuit can operate within target limits. For example, offset and input offset drift, gain error, temperature coefficient, voltage rating, power dissipation, board leakage, and inherent leakage of other discrete devices such as ESD devices and overvoltage protection devices. For example, when the high source impedance is connected in series with the input of an amplifier, the unwanted leakage current (not the bias current of the amplifier itself) can cause an offset, which in turn causes a bias current measurement error (Figure 12).

Figure 12. How bias current and leakage current produce offset error

In all cases, high temperature operation can increase board leakage caused by contamination such as flux, dust and condensation. A reasonable layout helps minimize this effect by providing enough space between sensitive nodes, such as separating the amplifier input from the noisy supply rail.

The standard pinout method for op amps and instrumentation amplifiers is to place one of the inputs near the negative supply terminal. This approach greatly reduces the resistance to solder residue after PCB assembly, and these flux residues increase leakage. To reduce leakage and increase high frequency CMRR, the AD8229 uses the same high performance pinout as other Analog Devices instrumentation amplifiers (Figure 13).

Figure 13. Device Pinout Improvements Help Minimize Parasitic Leakage

Diodes, transient voltage suppressors (TVS), and other semiconductor devices leak exponentially with increasing temperature, and in many cases are orders of magnitude higher than the amplifier's input bias current. In these cases, the designer must ensure that leakage at extreme temperatures does not degrade the circuit specification beyond the required limits.

Today, there are a variety of passive components available for use in high temperature work environments. Resistors and capacitors are common in a variety of circuit designs. Table 1 lists some of the devices available on the market.

Table 1. Examples of High-Temperature Resistors and Capacitors

Note that if the surface mount device is placed against the PCB, leakage between the leads can easily occur because the solder residue remains at the bottom of the board after the assembly is completed. These flux residues absorb moisture, increasing the conductivity at high temperatures. At this time, parasitic resistance (characteristics are difficult to predict) occurs in surface mount devices, which may cause other circuit errors. To solve this problem, consider using larger chips, gull-wing pins, or through-hole devices in particularly sensitive circuit areas. Finally, add an effective board cleaning step (usually with ultrasound or saponifier) ​​before the end of the assembly process, and useless residues can be removed almost completely.

Designers must keep in mind the thermal management requirements when designing systems that operate in harsh environments. Even when high-temperature dedicated devices are used, the self-heating effect associated with their power consumption should be considered. For example, the AD8229's guaranteed operating temperature is as high as 210 ° C, which is equivalent to a small output current load. The extra power consumed by driving high load or permanent fault conditions (such as output shorts) increases the junction temperature beyond the maximum rating of the device, greatly reducing the operating life of the amplifier. Be sure to follow the recommended heat dissipation guidelines and pay attention to adjacent heat sources such as power conditioners.

Even at high temperature resistance, the rated power is reduced above 70 °C. Pay special attention to the resistance temperature rating at the target operating temperature, especially if the power consumption is quite large. For example, suppose a resistor rated at 200 °C operates at an ambient temperature of 190 ° C. If it has a self-heating of 20 ° C due to power consumption, it still exceeds the rated value.

Although many passive components can withstand high temperatures, their construction may not be suitable for long-term environments with both shock and high temperatures. In addition, high temperature resistors and capacitor manufacturers also specify their operating life at a given temperature. Keeping the operating life specifications of all devices in place is critical to building a highly reliable system. Finally, don't forget that many devices rated at high temperatures may need to be derated to maintain long-term operation.

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