I have been posting various descriptions of my ECAT spice model behavior and I continue to get feedback that suggests that I have done a poor job of teaching.
I want to discuss one issue during this post which will allow those sitting on the fence to understand how positive feedback impacts the ECAT. Visualize that you are measuring an ECAT that for this discussion is at a fixed core temperature. By some miracle you have been able to get the device to temporarily stop changing temperature and reach a state where this parameter is constant without moving up or down. This condition can be thought of as a steady state operating point and the heat inputs are exactly balanced by heat escaping. We are supplying some amount of drive to the resistor heating elements which is included in the heat input balance. Now, if we increase the power to the heating resistors slightly, the temperature of the ECAT will begin to rise in response. The delta increase in core temperature causes a corresponding increase in core power generation. The positive feedback with a loop gain of greater than 1 causes the response temperature increase to be slightly greater than the drive that initiated it. This results in a continuous rise in the core temperature which does not cease until some form of limit is encountered. A similar type of behavior is observed with electronic comparators and many other positive feedback devices having a loop gain of greater than 1. This would eventually lead to thermal run away which is encountered when the ECAT looses control and should be avoided. Suppose that we had reduced the drive at that balanced point described above. In that case, the delta temperature would have been negative. Operating the core at a lower temperature results in the generation of less heat. If the reduced core heating leads to a drop in temperature that is greater than the drop that initiated it a process begins which causes the device to cool down in temperature. This cooling rate gets ever greater with time and the ECAT heads toward a temperature set by the drive power heating. It is safe to assume that this resting point will be at a temperature where the positive feedback loop gain is less than 1 in practical cases. With this type of process an uncontrolled system will never be stable unless it becomes located at a stop of some sort where it likely will fail due to heat or becomes too cool to be of much use. This is how my model suggests that the ECAT should behave and why control must be by some well regulated and constantly adjusted duty cycle modulation of the drive waveform. I am limiting the scope of this posting so that this important process can be well understood prior to getting into other model described issues. I hope that I have done a better job of explaining how my model of the ECAT functions, at least for this portion of its operation. Dave
