Closed Loop Constant Power Drive to Simplify Heater Element Control and Extend Battery Life
Abstract
Many applications require accurate temperature control of a heating element. Closed loop control based on temperature requires measurement of the heater temperature using a thermistor or thermocouple. This can sometimes be mechanically challenging and costly. Additionally, in battery powered applications, traditional PWM drive and associated high current pulses, can reduce battery life and lifetime.
Heater temperature is not linear with voltage or current due to changes in resistance with temperature. However, temperature is close to linear with applied power. By implementing a closed loop constant power drive the temperature can be controlled needing only to measure power and not temperature directly.
This reference design uses a closed loop constant power topology to drive a low impedance heater element. This application note includes the choices and challenges within the hardware and software implementation. The document also shows initial results and discusses advantages of this method of temperature control.
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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
