We are going to end our latest trilogy of LTSpice simulations with another demonstration of the powerful computing capabilities of this SPICE software in the field of NTC thermistor temperature control.
First, we will simulate a LT1040 fully electronic room thermostat. After that, we will go back to a basic pulse width modulation (PWM) circuit. We will conclude with an example of LED current control, where the temperature control will be applied to the LT8391 (Linear Technology, release date April 2016) using the LTSpice XVII 64-bit program.
The first circuit will use a LTC10401 electronic thermostat with an NTC temperature sensor. The circuit is shown in Figure 1
If you want to see the live emulation of this circuit simulation, a five-minute video can be viewed here.
In a nutshell, the simulation (performed according to a Monte Carlo analysis) shows how the LTC1040 will regulate the temperature of a system between two separate temperatures (24.5o C for the heating and around 25.5o C, with a hysteresis of 0.25o C around these two temperatures), for an external temperature first above, and then under, the target temperature.
In order to avoid duplicating the information available online, I will provide some complementary simulations not performed in the aforementioned video. Take, for example, the effect of the sensor response time. In stirred air, the response time of the thermistor NTCLE203 is 7 s. But if you build a housing around this component, the thermal response could easily be 15 s.
In Figure 2, the responses of the system in these two situations — different overshoots and duty times — are visible.
With the help of measurements recorded in the “spice log file” (Figure 3), one can compute an increase in overshoot of about 0.32o C when going from 7 s to 15 s. Also, the cooler will be on for 126 s for tau = 7 s and 169 s for tau = 15 s.
The second circuit in this article will bring back a bit of nostalgia, as it presents simple temperature control with PWM generated by a 555 (instead of amplitude modulation). The circuit is reprinted in Figure 4, and the results are in Figure 5. By modulating the duty time of a constant current with R3 tuning, it is possible to precisely control the temperature at a fixed value without oscillation (V(tsystem) can be tuned between 50 V and 130 V or o C).
Upper pane: pulsed current at max when V(ntc) > V(c1) in the middle pane, resulting in V(Tsystem) stabilizing exponentially in time (lower pane)
The third and last circuit in this article will deal with one of the most recent LED drivers from Linear Technology, the LT83912 buck boost LED controller.
Extending the macro-model of this power IC, we obtain the circuit in Figure 6, where a thermistor U1 NTCS0805E3104_MT measures the temperature of the LED (V(THERM)). This thermistor voltage is applied to the CTRL2 control pin of the LT8391 and compared to the voltage of the CTRL1 pin. As the voltage at the CTRL1 pin is defined by another reference thermistor U2 fixed at a certain temperature (here 70o C), the LED current will be constant up to this temperature of 70o C, and will be derated for higher temperatures. The U2 device is there for simulation only.
In Figures 7 and 8, we apply the same method explained on page three for the simulation of the LT3474. Fixing the initial LED temperature at increasing values, we look after a few ms at how this temperature further changes. When V(Therm)-V(Tin) shows a flat line, this means we are already in steady state. Figure 8 shows a constant average current under 70o C, a threshold above which the LED current will derate down to 0 at 125o C
Upper pane: the successive voltage values at the CTRL2 pin compared to the fixed CTRL1 pin voltage when Tin increases from 25o C to 125o C
Middle pane: initial LED temperature variation
Lower pane: current constant under 70o C and derating above 70o C
Upper pane: the RMS current pulse into the LED
Lower pane: the initial temperature increasing
This ends our trilogy of LTSpice simulations of temperature control circuits based on dynamic NTC thermistor models. Feedback is welcome at firstname.lastname@example.org, where all simulations of these articles series can be obtained.
Although many applications in thermal control are now microprocessor-based, for which SPICE is not used, there are still many cases (such as those described here) where the SPICE technology analysis can help. We can thus state loud and clear that the analog age and SPICE analysis still have good days ahead.
1) LTC1040, web 2017
2) LT8391, web 2017