Advanced temperature control: integrating QSPICE simulation and thermistors in digital circuit simulation

By Alain Stas

Vishay Intertechnology

Temperature control circuits, despite the rise of digital technology, continue to rely on analogue sensors such as NTC thermistors and resistance temperature detectors (RTDs), which are widely used in automotive and industrial applications because of their reliability and low cost.


While digital simulation software such as the Synopsys Saber RD or Simulink Simscape is available at a cost, it often fails to provide precise models for passive components such as thermistors. For this reason, SPICE analysis, which is popular and readily accessible, remains a mainstay of the engineer’s circuit analysis toolkit.


Now, however, the introduction of the QSPICE™ simulator, free software created by Mike Engelhardt, developer of the LTspice® tool, has excited interest in the field of thermal management. The QSPICE tool supports digital simulations with VERILOG and C++, bridging the gap between traditional SPICE and modern digital circuit descriptions.


The following three use cases demonstrate how the QSPICE simulator enables the integration of digital controls with analogue temperature sensing in circuit simulations.


Thermal control circuit of an oven with RS latch logic

The first use case is an on/off temperature control circuit (see Figure 1) using a bi-stable RS latch. It features a safety threshold which prevents the circuit from heating indefinitely in the event of an open circuit. The VERILOG module is the grey box marked RS_latchFS (FS stands for fail-safe). An advantage of using the VERILOG module to build the microcontroller is that the fail-safe can only be implemented by programming the ADC’s conversion of the temperature signal.

Vishay — Thermal System Simulation

Fig. 1: Thermal control circuit of an oven with RS latch logic, including failsafe

Vishay — Thermal System Simulation

Fig. 2: Top pane: fail-safe voltage; second pane: output RS; third pane: different voltage; bottom pane: oven and ambient temperatures


Figure 2 shows that the oven’s temperature starts at -40°C. This low temperature triggers the RS latch fail-safe, as indicated by the 1 V signal in the top graph from the 0 h to 2 h mark. Despite the fail-safe being engaged and blocking the heat, the oven gradually warms towards 25°C. After 2 h, when the temperature exceeds 10°C – the lower fail-safe threshold at 2.5 V – the heating circuit starts up.


The oven then heats up to nearly 80°C before the heating cycle turns off, allowing the oven to cool to about 60°C before the heating cycle recommences. This process self-adjusts with changes in the room temperature. Although the simulation runs for only 1 s, it represents 28 h of real operation.


In addition, QSPICE simulation enables customization of the model’s parameters, such as the NTC thermistor’s resistance at 25°C (dR25), and its temperature coefficient (dB), which are accurate to ±5% and ±1% respectively. This feature allows for the examination of various operational extremes.


Thermal control of an oven with optotriac and PWM controller

The second example features a complete VERILOG module, as shown in Figure 3. This module demonstrates a PWM system controlling an optotriac (the Vishay VOT8125), which in turn activates another triac. This second triac is responsible for switching the ac mains voltage to a load, generating the current necessary to heat the oven.


The oven’s temperature is monitored by an NTCS0603E3103JLT NTC thermistor, which has a 10 kΩ rating with ±5% tolerance. Response time is set by the R11C2 coupler. The thermistor’s temperature-induced voltage is read by the controller’s internal ADC, and a counter creates a sawtooth waveform for the PWM signal. This entire process is encoded within the VERILOG module.


The optotriac is activated when the controller output is low. Initially, when heating begins, the PWM duty cycle is high to rapidly increase the temperature (as shown in Figure 4a). It decreases as the temperature approaches the desired steady state (as shown in Figure 4b). The final oven temperature rises from room temperature to a predefined steady state, determined by the 6 kΩ resistor R7. Figure 5 illustrates four scenarios which account for the thermistor’s manufacturing tolerance.

Vishay — Thermal System Simulation

Fig. 3: Thermal control of an oven with optotriac and PWM controller

Vishay — Thermal System Simulation

Fig. 4a: Mid duty cycle

Vishay — Thermal System Simulation

Fig. 4b: Short duty cycle     

Vishay — Thermal System Simulation

Fig. 5: Oven temperature controlled by the circuit in Figure 3



Temperature control circuit with Peltier thermoelectric element and PWM microcontroller

The final use case (see Figure 6) involves a sophisticated microcontroller which adjusts the PWM current to a Peltier device, which is used to cool a 50 W heat source emitting five pulses every 100 s, as shown in the third panel of Figure 7.


On the Peltier device’s cold side, an NTC thermistor monitors its temperature, while a simulated thermistor sets the target temperature to 25°C. Figure 7 shows how effectively the temperature is controlled: each time the heat source pulses, the PWM actively adjusts to maintain the correct temperature. This PWM duty cycle is also dynamically altered based on the slowly varying ambient temperature.


The EFF parameter governs the intensity of the PWM modulation, with two settings illustrated in Figure 7’s top panel: blue for intense PWM and magenta for milder PWM. A stronger PWM setting is indicated by a longer duration where I(R1) remains at its maximum value, as shown in the second panel.

Vishay — Thermal System Simulation

Fig. 6: Temperature control circuit with Peltier thermoelectric element and PWM microcontroller


Fig. 7: Top pane: offset real controlled temperature vs target temp; second pane: heating current; third pane: heat source OTC power peaks; bottom pane: temperatures of cold side C4/hot side H3 and ambient temperature



By preserving the dynamic characteristics of the NTC thermistors in SPICE models and combining them with cutting-edge components such as optotriacs, silicon carbide transistors, and Peltier modules, digital temperature control simulations can be greatly optimized. These simulations serve as a precursor to more rapid real-world experimentation.


All three simulation examples are available to download free from the platform.

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