Design of uncooled infrared focal plane temperature control circuit based on ADN8830

As an important means of discovering, detecting and identifying targets, infrared technology has been widely used in military and civilian dual-use technologies. The development of uncooled infrared focal plane array technology has greatly improved the performance of the system. Uncooled infrared thermal imagers use a thermal detector focal plane array that does not require refrigeration. It uses infrared radiation to change the temperature of sensitive pixels on the focal plane, thereby changing the resistance to detect the temperature characteristics of the target. Therefore, only by ensuring that the reference temperature of each sensitive pixel in the focal plane array is stable and consistent as much as possible can the detection sensitivity of the thermal imager be improved, the difficulty of the system's later non-uniformity correction be reduced, and ultimately the detection sensitivity of the thermal imager be fundamentally improved, and the imaging performance of the thermal imager be improved. At present, semiconductor thermoelectric coolers (TECs) are used in actual uncooled infrared focal plane array detectors to stabilize the reference temperature. This article focuses on introducing a high-performance TEC temperature control circuit based on ADN8830 and its PID compensation network adjustment method.

1 Temperature control circuit design

TEC (Thermo Electric Cooler) is a PN junction composed of two different semiconductor materials (P-type and N-type). When direct current passes through the PN junction, the electrons and holes in the two materials absorb or release heat (Peltier effect) in the process of moving across the PN junction, which will make the PN junction show cooling or heating effect. The cooling or heating of TEC can be achieved by changing the direction of current, and the heating and cooling output can be controlled by adjusting the current size.

The method of using TEC to stabilize the target temperature is shown in Figure 1.

The first part in Figure 1 is the temperature sensor. This sensor is used to measure the temperature of the target object placed at the end of the TEC. The desired target object temperature is represented by a set point voltage, which is compared with the voltage generated by the temperature sensor representing the actual target object temperature through a high-precision operational amplifier to generate an error voltage. This voltage is amplified by a high-gain amplifier, and the phase delay caused by the hot and cold ends of the target object is also compensated. It then drives the H-bridge output, which controls the direction and magnitude of the TEC current at the same time. When the temperature of the target object is lower than the set point temperature, the H-bridge drives the current in the direction of the TEC heating at a certain amplitude; when the temperature of the target object is higher than the set point temperature, the H-bridge will reduce the TEC current or even reverse the TEC current direction to reduce the target object temperature. When the control loop reaches equilibrium, the TEC current direction and amplitude are adjusted, and the target object temperature is equal to the set temperature.

In this design, ADN8830, a TEC controller from ADI, is used for TEC control. ADN8830 is one of the most outstanding single-chip TEC power driver modules with high integration, high output efficiency and high performance. It is used to set and stabilize the temperature of TEC. In typical applications, the maximum temperature drift voltage is less than 250 mV, which can make the target temperature error less than ±0.01°C. Each voltage loaded on the input of ADN8830 corresponds to a target temperature set point. Appropriate current will drive TEC to heat or cool the infrared focal plane through TEC. The temperature of the infrared focal plane is measured by a negative temperature coefficient thermistor and fed back to ADN8830 for adjusting the system loop and driving TEC operation.

The uncooled infrared focal plane temperature control circuit designed here using ADN8830 is shown in Figure 2.

The resistor RTH in Figure 2 is the thermistor built into the uncooled infrared focal plane assembly. The selection of the resistance value of resistor R4 _is related to the temperature characteristics of the thermistor RTH and the ambient temperature. The resistance value of the thermistor RTH _does not decrease linearly with the increase of temperature. The resistance value of resistor R4 should be calculated according to formula (1):

Where: R _T1 and R _T3 represent the resistance of the thermistor at the two upper and lower limits of the working temperature, respectively, and R _T2 represents the resistance of the thermistor at the average temperature. In practical applications, the two limits of the working temperature can be taken as 5°C and 45°C, respectively, and the average temperature is 25°C. By consulting the thermistor temperature curve, we can get R _T1 = 10.735 kΩ, R _T2 = 4.700 kΩ, and R _T3 = 2.250 kΩ, so the value of the resistor R ₄ is calculated to be 3.304 kΩ, and R ₄ = 3.300 kΩ.

The control principle of the ADN8830 temperature control circuit is to compare the voltage on the thermistor with the temperature set for normal operation of the uncooled infrared focal plane, thereby adjusting the direction and magnitude of the current flowing through the refrigerator to control the temperature. The set voltage value of pin 4 (TEMPSET) of ADN8830 should be calculated according to formula (2):

When the set temperature is 25°C, the thermistor R _TH is 4.7 kΩ, the reference voltage V _REF is provided by the chip internally and is 2.47 V, then V _SET is 1.45 V.

2 PID network adjustment and parameter setting

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PID (Proportion Integrator Differentiator) integral differential proportional regulation compensation network is the most critical part of TEC temperature control and a key factor affecting the response speed and temperature stability of the TEC controller. PID control technology is used as the core to reduce static errors and improve control accuracy. PID is equivalent to an amplifier with adjustable amplification factor. It uses proportional and integral operations to improve regulation accuracy and differential operations to accelerate the transition process, which better solves the contradiction between regulation speed and accuracy. The mathematical model of PID can be expressed by formula (3):

Where: Kp is the proportional coefficient; T1 is the integral time constant; TD is the differential time constant.

The ADN8830 TEC controller uses an external compensation network that requires only a few resistors and capacitors, as shown in Figure 3. Different application designers can adjust the compensation network according to their own thermal load characteristics to achieve the best temperature setting time and stability tolerance, but the switching cycle of the compensation network has a greater impact on the stability of the control system. In order to ensure the stability of temperature control, the switching cycle of the compensation network must be less than the thermal time constant of the TEC and temperature sensor. However, the thermal time constant of the TEC and temperature sensor is a difficult factor to describe, and the network parameters cannot be designed by calculation. For the PID network of Figure 3, the parameters can usually be optimized through the following debugging steps:

(1) Short-circuit capacitor C9 _and open -circuit _{capacitor C11} , leaving only resistors R6 _and R5 _to form a simple compensation ratio network;

(2) Increase the ratio of resistors R6 _and R5 _to increase the gain until the voltage across the TEC begins to oscillate, and then reduce the ratio of R6 _to R5 _to 1/2 of the original ratio;

(3) Connect capacitor C9 in _series to the compensation network and reduce the value of the capacitor until the voltage across the TEC begins to oscillate, then increase the value of capacitor C9 by a factor of 1. The initial value of capacitor C9 _{is based} on equation (4) to give a unity gain of 0.1 Hz.

(4) Short-circuit resistor R7 _and add capacitor C11 _to make the voltage across the TEC begin to oscillate. At this time, you can reduce the capacitor C11 _or reconnect resistor R7 _to stabilize the voltage across the TEC.

(5) Change the voltage value of TEMPSET to adjust the voltage stabilization time across the TEC. The change of TEMPSET is about 100 mV. Then reduce the capacitor C11 _, C9 _and resistor R7 _to reduce the stabilization time, but it will cause the output voltage to overcharge.

(6) Add feedback capacitor C10 in parallel with R6 _and C9 _. Feedback _capacitor C10 _can improve the stability of the system without increasing the stabilization time. Generally, a capacitor of 330 pF to 1 nF is suitable.

The temperature control circuit designed in this paper uses the PID network structure of Figure 3. When C ₉ =22μF, C ₁₀ =330 pF, C ₁₁ =1μF, R ₇ =1.388 MΩ, R ₅ =1.092 MΩ, and R ₆ =175 kΩ, the system’s settling time from ambient temperature to target temperature is within 10 s, with an accuracy of 0.01°C and can maintain long-term stability.

3 Performance Testing

The experimental test was carried out at room temperature. The signal shown in Figure 4 is the voltage change of pin 30 (TEMPOUT) of ADN8830. The voltage change is consistent with the temperature change detected by the sensor, so the temperature change characteristics can be obtained from the voltage change characteristics. As shown in Figure 4, it can be seen that after 8.4 s, the voltage stabilizes at the preset voltage of 1.45 V, which means that the establishment time for the temperature to change from the ambient temperature to the target temperature of 25°C is 8.4 s, and the overcharge is small, and it has reached stability. The circuit has the functions of normal operation indication and work failure alarm indication. When the temperature detected by the thermistor reaches the set temperature (the set temperature of this circuit is 25°C), pin 5 (TEMPLOCK) of ADN8830 outputs a high level, indicating that the working temperature of the uncooled infrared focal plane has reached the set temperature, and the light-emitting diode D1 is illuminated; when pin 1 (THERMFAULT) outputs a high level, it indicates that the circuit is abnormal, and the light-emitting diode D2 is lit.

4 Conclusion

The uncooled infrared focal plane temperature control circuit based on ADN8830 designed in this paper has high efficiency, low power consumption and small size. It has been proved through practical application that it can control the temperature at the preset temperature with an accuracy of 0.01℃. The external compensation network composed of several simple resistors and capacitors can control the temperature at the preset temperature within 10 s and keep the entire temperature control system in a stable working state for a long time.