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OPA567: TEC application on opa567

Part Number: OPA567
Other Parts Discussed in Thread: INA317

Hi,TI expert

    I have a TEC application question when I use opa567. My schematic is shown in the pic below. 

Vset is the voltage corresponding to the set temperature,Vth is the voltage fed back by the thermistor. I replace ad623 in the pic with ina317. R11 is replaced by a laser in real schematics. If Vset=Vth, then VTEC+=VTEC- theoretically.

But in real experiments, I set Vset=1V, coresponding 25℃, and the feedback voltage Vth is smaller than 1V,about 0.9V——translateing to 28° in laser. The temperature is stable at 28°. VTEC+=VTEC-.

Why can laser temperature be stable when Vset and Vth are not equal. 

Thanks

  • Hi Cunxu,

    Why can laser temperature be stable when Vset and Vth are not equal. 

    I replaced AD623 with INA317 IA as shown below. Vth is the thermistor, assume it is 10kΩ thermistor. The best way to drive it is using the constant current source, which I use 10uA current source. This should avoid the self heating in thermistor. 

    The IA's input is a differential input signal in voltage, where Vdiff delta is 100mV. So the INA317's output should be 2.8V as shown below. Also, at 28C, the thermistor should sense at approx. 28C. 

    INA317 Gains3 Thermistor 12152023.TSC

    I assumed that the laser is equipped with TEC temperature controller. The TEC may be implemented inside of the laser diode or it may be used to cool the outer package of the laser. What is the laser's regulated temperature, say 40C or 50C?

    R11 should be the two leads of TEC device, typically in red and black leads. If positive voltage is connected to red lead, then it will heat; the TEC will be cooled if the reversed voltage is applied. Also, the TEC needs to be oriented correctly, where the thermistor is placed in the body of laser diode or the regulated temperature source. 

    So R11 should not be connected to laser diode. The laser diode should have another two leads, where the laser diode is powered separately. So the attached schematic is only a part of temperature regulator of the TEC devices.   

    Why can laser temperature be stable when Vset and Vth are not equal. --> it is likely that OPA567s is applied a voltage delta across the TEC device or R11, and laser diode is cooled during operation. If you provide me with the TEC regulated temperature setpoint, I will be happy to simulate or check the OPA567's operating conditions. 

    If you have other questions, please let me know. 

    Best,

    Raymond

  • Hi Cunxu,

    I checked out your TEC driver, here is what it is doing. 

    The controller's setpoint is 2.5Vdc. If the Vblack node is less than 2.5Vdc, then current is flowing from Vred to Vblack of the TEC. Otherwise, the reverse current is flowing through TEC. The direction of the TEC current flow corresponding to cooling or heating, depending on how TEC is configured. 

    With TEC's DC resistance configured at 3ohm, the controller only works between 2V to 3V range. 2.5-3V may be cooling and 2-2.5V may be heating. The actual heating/cooling configuration depends on how TEC in laser diode or cooling block is configured. 

    OPA567 TEC Driver 12152023.TSC

    Please confirm the TEC's dc resistance and its heating/cooling curve, and it will be good if you can provide me with temperature setpoint for thermal regulation. 

    If you have other questions, please let me know.

    Best,

    Raymond

  • Hi Zhang,

    The signal "TH" is from thermistor of laser, and it is 10KΩ, coresponding 25℃. So the Vth is 1V when temperature is 25℃. I set Vset=1V, coresponding 25℃.

      

  • Hi Cunxu,

    This is a kind of make sense, except the instrumentation amplifier (IA)'s input may not be configured properly. The thermistor is a resistor, your schematic represented as Vth with a voltage source, unless this is the voltage outputted from the laser diode itself. So it is not clear how the thermistor is powered. 

    With IA's gains = 3 V/V, your IA's output has to be 2.5Vdc, which is the temperature setpoint for the TEC to be regulated at 25C. Please inform me what is the input delta voltage at the Vdiff input or pin 2 and 3 of INA317's input terminals (Vdiff = Vpin3 - Vpin2) should be in ~100mV at 25C). 

    INA317 Gains3 Thermistor 12162023.TSC

    If you have other questions, please let me know. 

    Best,

    Raymond

  • Hi, Zhang

        As shown in pic below. The thermistor and R80 divide the output voltage of U30-REF3040, and thermistor is 10KΩ, coresponding 25℃. So the Vth is 1V when temperature is 25℃. I set Vset=1V, coresponding 25℃.

        When I set Vset=1V, I actually tested Vth was about 0.91V. So the input delta voltage at the Vdiff is about 90mv.

       Confusingly, I set Vset=1V, which means I want to set it to 25°C, but the thermistor feedback is Vth=0.9V, which corresponds to 28°C, and the temperature is stable.

    thanks

  • Hi Lin,

        When I set Vset=1V, I actually tested Vth was about 0.91V. So the input delta voltage at the Vdiff is about 90mv.

    As you indicated,  the thermistor is measured at approx. 28C or 8.4759kohm, if you back calculate the thermistor resistance with the voltage divider of 0.90V. 

    There is approx. 4.096V/40.1k = 0.102mA going through thermistor. Assume that there is no thermal heating effects, then this is about right, where the TEC should have be cooled when the laser diode is operating normally. The effect is seen in the simulation below. The Vtec+ terminal is set for 2.5V and Vtec- is measured at approx. 2.14Vdc, and TEC is likely working and laser diode's temperature is table. 

    INA317 Gains3 Thermistor 12182023.TSC

    Why can laser temperature be stable when Vset and Vth are not equal.

    If Vth and Vset are equal, then the Vdiff is approx. 0mV and there is not heating or cooling at TEC. 

    BTW, please matching the input impedance of INA317 IA. 30.1kΩ||10kΩ = 7.51 kΩ

    If you have other questions, please let me know. 

    Best,

    Raymond

  • Hi, Zhang

         Thank you for your rely. 

         Every time I set a temperature, I need to match the input impedance of INA317?

  • Hi Cunxu, 

    Every time I set a temperature, I need to match the input impedance of INA317?

    I think if you are able to match the input impedance at 25C setpoint, it will be good enough. The instrumentation amplifier is only interested to the input differential signals. 

    if the thermistor is driven in constant current source, the thermistor's R vs. temperature will not be issues for the input INA317, because it is a differential measurement in every temperature changes. Since this is voltage divider input, then there may be some small errors at the input, but it is likely insignificant. TEC controller may have a wide range where errors are near zero (delta voltage between TEC+ and TEC-), and no significant heating or cooling effects are taken place. 

    If this is ultra precision thermistor and temperature control application, it would be better off to use constant current source to drive the thermistor. 

    If you have other questions, please let me know. 

    Best,

    Raymond