SLVAFK1 January 2025 INA228 , INA232 , INA234 , INA236 , INA237 , INA238 , MSPM0C1103 , MSPM0C1103-Q1 , MSPM0C1104 , MSPM0C1104-Q1 , MSPM0G1105 , MSPM0G1106 , MSPM0G1107 , MSPM0G1505 , MSPM0G1506 , MSPM0G1507 , MSPM0G1519 , MSPM0G3105 , MSPM0G3105-Q1 , MSPM0G3106 , MSPM0G3106-Q1 , MSPM0G3107 , MSPM0G3107-Q1 , MSPM0G3505 , MSPM0G3505-Q1 , MSPM0G3506 , MSPM0G3506-Q1 , MSPM0G3507 , MSPM0G3507-Q1 , MSPM0G3519 , MSPM0L1105 , MSPM0L1106 , MSPM0L1117 , MSPM0L1227 , MSPM0L1228 , MSPM0L1228-Q1 , MSPM0L1303 , MSPM0L1304 , MSPM0L1304-Q1 , MSPM0L1305 , MSPM0L1305-Q1 , MSPM0L1306 , MSPM0L1306-Q1 , MSPM0L1343 , MSPM0L1344 , MSPM0L1345 , MSPM0L1346 , MSPM0L2227 , MSPM0L2228 , MSPM0L2228-Q1 , TPS62866 , TPS62868 , TPS62869 , TPS6286A06 , TPS6286B10
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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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.
The temperature of a resistive heating element is directly proportional to the power applied. Measuring the electrical power can be mechanically much simpler than measuring the temperature. Driving the element with constant power delivers constant temperature and adjusting the power adjusts the temperature. Unfortunately, the resistance of the heating element can vary significantly between batches and also changes over temperature. This means that both voltage and current need to be measured and the voltage applied needs to be adjusted to maintain constant power as the heater element resistance changes as seen in Figure 2-1. The DC/DC converter controlling the applied voltage draws an average current from the supply which can extend the battery life.
The constant power drive design requires hardware to measure the voltage across the heating element and the current through the heating element to calculate the power. This is achieved using the INA234 which is a 28V, 12bit, I2C output current/voltage/power monitor. In this design the device measures the voltage directly across the heating element and the current through a high-side 10mΩ, ± 1%, 1W sense resistor. The devices then calculates the power and reports values for voltage, current and power via I2C.
For this example, we assume a 1Ω heating element that can vary by ± 20% across temperature and batch range. Table 3-1 shows the required voltage and current for different power levels across the resistance range. The input voltage is 3.3V to 5.0V. This means a buck or step-down dc/dc regulator can be used for the whole range required. The applied voltage is controlled using the TPS62868 which is a 2.4V to 5.5V input, synchronous buck converter with 4A output capability. Importantly, this device is I2C controlled which allows the output voltage to be easily adjusted.
Power (W) | Current (A) at 0.8Ω | Voltage (V) at 0.8Ω | Current (A) at 1.0Ω | Voltage (V) at 1.0Ω | Current (A) at 1.2Ω | Voltage (V) at 1.2Ω |
---|---|---|---|---|---|---|
4.0 | 2.24 | 1.79 | 2.00 | 2.00 | 1.83 | 2.19 |
5.0 | 2.50 | 2.00 | 2.24 | 2.24 | 2.04 | 2.45 |
6.0 | 2.74 | 2.19 | 2.45 | 2.45 | 2.24 | 2.68 |
7.0 | 2.96 | 2.37 | 2.65 | 2.65 | 2.42 | 2.90 |
8.0 | 3.16 | 2.53 | 2.83 | 2.83 | 2.58 | 3.10 |
9.0 | 3.35 | 2.68 | 3.00 | 3.00 | 2.74 | 3.29 |
The voltage, current and power is read from the INA234 via I2C using an MSPM0L1306. This low cost microprocessor is also responsible for adjusting the output voltage of the TPS62868 via I2C. The simplified and full circuit schematic can be seen respectively in Figure 3-2 and Figure 3-3.