Designing TEC Temperature Loop PID Control Using SPICE

Temperature control using an analog proportional-integral-derivative (PID) controller is a very simple circuit that is an effective way to ensure that the set point of a thermoelectric cooler (TEC) regulates the temperature or laser. The proportional-integral term works together to accurately servo the current of the TEC to maintain the temperature set point of the controller. At the same time, the derivative term adjusts the rate at which this work is done to optimize the overall system response. If the overall system response H (s) can be described, the most convenient and effective way to design a PID controller G (s) for it is to simulate it using SPICE.

Step 1: Determine the TEC/Temp sensor thermal impedance for the SPICE model.
To use SPICE as an effective tool for PID loop design, it is important to obtain the thermal response of the temperature loop in order to obtain the actual thermistor resistance, capacitance, and transfer function of the PCB  TEC  laser diode  temperature sensor wiring. Remember, since the actual thermal characteristics can vary by up to 50%, it is best to inject a thermal step input into the actual system and measure it to obtain the best thermal model for SPICE simulation.

If the hot wires are described, use the Outer Loop, Inner Loop procedure to determine the overall loop response and stability of the control amplifier in the G(s) module. In all cases, a very large inductor is used to interrupt the outer and inner loops, and the loops are excited by a large capacitor and AC power supply.

Step 2: Break the outer loop between G(s) and H(s)
The outer loop is defined as a path around the G(s) and H(s) blocks. The goal of the simulation using Figure 1 is to break the outer loop and obtain H(s), G(s), and the total loop gain to verify the thermal loop stability. In this case, Figure 2 shows that the phase drops below zero degrees and the loop gain becomes 0 dB, which indicates that the entire loop is unstable. Therefore, changing G(s) should strengthen the PID control and increase the stability of the temperature loop.

Figure 1 Simulation circuit to obtain loop gain and phase

Figure 2 Loop gain and phase curves of Figure 1

The improved G(s) module in Figure 3 includes PID components. The angular frequency of the differential circuit is set by R7 and C3; R3 sets the proportional gain; and C2 and R6 set the angular frequency of the integral circuit.

Figure 3 Simulation circuit for compensating G (s)

Step 3: Break the G(s) “Inner Loop” to Determine Local Amplifier Stability
The final step in building the complete PID assembly is to break the inner loop and check the stability of the local amplifier (OPA2314) to ensure that its stability is independent of the overall loop gain. In this case, the amplifier requires a 50 pF capacitor (see Figure 4) to maintain stable operation of the local loop.

Figure 4 Final circuit of the compensated local G(s) loop

Next time, we'll discuss a bad design in which a 20W amplifier ruins a 100W speaker, so stay tuned.

References

• “Op Amp Stability, Part 2 of 15: SPICE Analysis of Op Amp Networks”, Green, Timothy, En-Genius (formerly Analog Zone) 2006.
• “PSPICE Compatible Equivalent Circuit of Thermoelectric Coolers”, Simon Lenvkin, Sam Ben-Yaakov, PESC '05. IEEE 36th 2005. • “
SPICE Models of Thermoelectric Components Including Thermal Effects”, Chavez, JA, Salazar, J, Ortega, JA, and Garcia, MJ, Proceedings of the 17th IEEE Instrumentation Test and Measurement Technology Conference 2000.