Temperature Monitoring

Temperature monitoring is mainly used as a safety feature to switch off the unit should the temperature rise above a predefined threshold. Temperature monitoring can be implemented at the Level 1 Control. The cause of an over-temperature event can be:

  • Insufficient airflow in the system, perhaps caused by a failure of the cooling fan
  • Operation of the system at a high ambient temperature

It's a good practice for the power converter designers to implement temperature monitoring of the power switches due to protection and performance (Current consumption, DC, AC characteristics) reasons.

Temperature Monitoring: Analog Solution

A temperature sensor, such as the MCP9700A, outputs an analog voltage proportional to the measured temperature. This temperature knowledge may be used to change the loop performance in order to compensate for it. In order to prevent damage due to excessive temperature, a comparator may also be used to rapidly detect the threshold and immediately shut down the unit.

The output of the temperature sensor can be connected to an analog input of the dsPIC® DSC. The board temperature is measured in the ADC Interrupt Service Routine (ISR) and checked for over-temperature protection in the fault loop. The figure below shows an example of an MCP9700A temperature sensor interfacing to a PIC microcontroller.

temp-sensor-interface.png

The maximum temperature set point is configured in reference to the maximum operating temperature, which can be found in the data sheet. If the measured temperature exceeds the maximum set point, a fault is generated and the PWM outputs are turned OFF.

Note:

  • Industrial operating temperature ranges from -40 C to +85 C.
  • Depending on the part numbers, some dsPIC® DSCs offer extended temperature ranges (-40 C to +125 C). The product identification system allows users to identify which parts offer such ranges. See figure below.
temperature-range.png

Temperature Monitoring: Digital Solution

On the dsPIC® DSCs side, the low and high temperature thresholds can be changed. In case of over-temperature, the voltage can be driven smoothly to zero. Upon detection of an over-temperature event, the dsPIC DSC can wait for a certain amount of time (or check the continuous presence of a high temperature) before concluding that a real over-temperature event has occurred. Variable http://microchip.wikidot.com/asp0107:comparators-hysteresis | hysteresis]]] can be also added. The system can easily make a predetermined number of attempts to restart from an over-temperature status. If these attempts fail, the system will shut down permanently. Redundant power supplies can balance temperature by adjusting their output currents, optimizing the current sharing.

The dsPIC® DSC can easily accommodate any of them. Because temperature is simply a number inside the dsPIC, it is very easy to set the temperature threshold and also set up a hysteresis cycle in software. The temperature information can be used to change the loop performance. In an over-temp event the information can be used to smoothly shut down the unit (soft start). Moreover, there is a much higher versatility, since, for instance, the designer could define that an over-temperature event only occurs when the temperature remains higher than the threshold for a predetermined period of time. Even in an over-temperature event, the system could make a predetermined number of attempts to start again the unit. If the system is always in over-temperature state, after the predefined number of attempts, the system remains shut down.

Temperature Sensor: Diode

A diode can be used as a temperature sensor. The MCP9700A provides an accurate temperature measurement by monitoring the voltage of a diode located inside the IC. The figure below shows the cross-sectional diagram of the temperature sensing diode based upon a PNP transistor. The details of the voltage to temperature transfer function can be found in the MCP9700A data sheet.

diode-temp-sensor.png

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