1 Traditional Heater Control

Traditional heater control uses a temperature sensor to measure the temperature of the heating element as shown in Figure 1-1. This measurement is fed back and used to adjust the drive circuitry to alter the current through the heating element maintaining the temperature at the required set point. This approach has a number of challenges. Firstly, the temperature sensor must be mounted close to or in contact with the heating element, which can be mechanically difficult. Secondly, high temperature measurement usually requires a thermocouple that needs complex interface circuitry.

Temperature responds relatively slowly compared to changes in the electrical signals, so it has been usual to use a simple FET switch PWM to modulate the current through the heating element at a higher electrical frequency and allow the slower thermal response to act as the loop low pass filter. This works perfectly well, but the fast switching edges can result in electrical noise. In addition, in a battery powered system, the large current pulses pulled from the source during the PWM pulses, can reduce battery life between charges and overall battery lifetime.

Figure 1-1 Traditional Heating Element Control Loop With Temperature Sensor