TMS320F280049-Q1: How to determine the threshold for safe junction temperature or ambient temperature?

Hi Will,

The nominal temp. sensor accuracy is 15 deg. C, so you'd want to set your temperature threshold to at most 135C.  It isn't specified in the datasheet, but the worst case sensor accuracy is definitely at maximum cold temp., so you probably don't need much if any additional guardband for the sensor accuracy for an over-temperature trip threshold. 

If you test your completed system in production up to the max temperature, you can instead capture and store a sensor reading at the test temperature.  The sensor readings are very repeatable.  

As far as delay, this will depend on how frequently you are sampling the temperature sensor with the ADC.  Assuming the sensor is sampled in a control loop that is running in the 10's to 100's of kHz, the temperature reaction time should actually be pretty fast relative to how fast the device can physically heat up.

In addition to the internal temperature sensor, some other strategies would be external temperature, voltage, or current sensors.  The temperature sensors can be on/adjacent to the C2000 device or near/on key system components that are prone to overheating (e.g. power FETs).  The voltage and/or current sensors can be used to monitor the board supplies (usually 5V and/or 12V), the controller/logic supplies (3.3V and 1.2V for the C2000 device) and/or the power electronics supplies.  If these sensors have analog outputs they can be fed to the comparator (CMPSS) module to allow extremely fast ePWM shut-down without CPU intervention.  Usually the first line of defense is CMPSS monitoring of the currents and voltages from the power electronics that are being actively controlled by the C2000 device (and they are already going to the ADC inputs, so it is usually just a matter of enabling and configuring the comparator modules).