Application of TMP03/04 digital temperature sensor in temperature protection

1 Performance characteristics of tmp03/04

TMP03/04 is a serial output digital temperature sensor produced by Analog Devices (ADI). The duty cycle of the high and low levels of the output data is proportional to the device temperature. The voltage generated by its built-in temperature sensor is accurately proportional to the thermodynamic temperature. After comparison with the internal voltage reference, it is input into the built-in high-precision σ-δ digital modulator. Compared with the currently commonly used serial data modulation technology (such as voltage-frequency conversion), the ratiometric modulation technology used by the built-in modulator of TMP03/04 has better anti-interference performance. Since it is not affected by clock drift errors, the temperature measurement range of the device is generally between -25℃ and +100℃, and the measurement error is ±1.5℃ (typical value) and no calibration is required.

The main difference between tmp03 and tmp04 is that tmp03 is an open collector output, suitable for circuits that need to be isolated from the microprocessor by an optocoupler, while tmp04 is a complementary MOS field effect tube output, and its output level is compatible with CMOS/TTL circuits, suitable for circuits that interact directly with microcontrollers. tmp03/04 can detect temperature and realize temperature control function through a single-chip microcomputer. It is suitable for temperature remote detection, temperature monitor of microcomputers or electronic equipment, and work control process. It has low voltage power supply and low power consumption. The power supply voltage range is +4.5v-+7v. When powered by +5v, the power supply current does not exceed 1.3ma, and its maximum power is only 6.5mw, which is particularly suitable for low-power circuit design.

2 Working principle of tmp03/04

TMP03/04 has three package types: TO-92, SO-8 and RU-8. The pinout is shown in Figure 1, where V+ is connected to the positive pole of the power supply, GND is the common ground, and DOUT is the serial data output terminal.

Application of TMP03/04 digital temperature sensor in temperature protection

The internal structure diagram of tmp03/04 is shown in Figure 2, which mainly includes 4 parts:

Application of TMP03/04 digital temperature sensor in temperature protection

(1) A reference voltage source and a temperature sensor, where the output voltage of the reference voltage source is connected to a 1-bit DAC (not shown in the figure), and the temperature sensor outputs an uptat voltage proportional to the thermodynamic temperature, which is connected to one input of the summer; (2) A σ-δ modulator, which contains an analog summer (also called an adder), an integrator, a comparator (also called a quantizer), and a 1-bit digital-to-analog converter (1-bit DAC);

(3) Digital filter;

(4) High-speed clock oscillator;

The analog summer, integrator, comparator and 1-bit DAC form a closed-loop system. The comparator also plays a negative feedback role. It can change the duty cycle of the comparator output signal according to the changes in the input temperature signal. The negative feedback circuit makes the integrator output voltage uint the lowest. The above circuit also belongs to a charge-balanced converter. After multiple rapid comparisons, the output digital quantity is proportional to the measured temperature.

The working principle of tmp03/04 is to convert the analog quantity of the measured temperature into a digital quantity, and encode the digitized signal into the form of a time ratio (t1/t2) as the output waveform of tmp03/04.

Application of TMP03/04 digital temperature sensor in temperature protection

t1 refers to the high-level duration, which is a fixed value with a nominal value of 10ms and a maximum value of no more than 12ms. t2 refers to the low-level duration, which varies with temperature and has a maximum value of 44ms, corresponding to a maximum temperature of +125°C. t1 and t2 are continuous in time, so the ratio between them can be obtained using the same timer clock.

The ratio of the measured temperature θ to t1 and t2 can be expressed by formulas (1) and (2):

θ=235-(400t1/t2) (1)

θ=455-(720t1/t2) (2)

The unit of the measured temperature in formula (1) is (°C), and the unit of the measured temperature in formula (2) is Fahrenheit (°F).

3 Interface circuit and program design

The thyristor power module plays a core role in the three-phase rectifier circuit. Due to the long-term flow of large current and frequent "on-off" state, the thyristor power module generates a lot of heat. In addition to installing a heat sink to cool down, the heat sink itself generally needs to add an exhaust fan or blower to assist in heat dissipation. However, the heat sink and the fan itself only play the role of heat dissipation, and cannot play the role of over-temperature protection. Therefore, in addition to outputting the thyristor trigger pulse, the microprocessor also needs to detect the temperature of the heat sink through tmp03/04 (the thyristor power module is installed on the heat sink), make an over-temperature judgment, and respond accordingly .
The microprocessor uses each Texas Instruments (TI) advanced msp430 series flash low-power 16-bit single-chip microcomputer, which has the characteristics of ultra-low power consumption, powerful processing capabilities, and rich on-chip peripheral modules, and is widely used in industrial control.

As mentioned above, the output of the tmp03/04 digital temperature sensor is serial data whose duty cycle varies with the measured temperature. The measured temperature is calculated by formula (1) or formula (2). It can be seen that the key to temperature measurement is to obtain the count values ​​of t1 and t2. These two count values ​​are accurately obtained through the capture function of the microprocessor timer, or more accurately obtained through the ordinary i/o port. The following introduces the interface circuits and program designs of these two methods respectively.

3.1 Obtaining count values ​​through the capture port

The timer_a timer of msp430 has powerful functions and can support multiple capture/compare functions at the same time. Each capture/compare module can be programmed independently to generate interrupts by comparing or capturing external signals. The external signal can be the rising edge, falling edge or all transitions of the signal.

The clock source of timer_a comes from the internal clock or the external clock. The frequency division can be set by its internal register. The selected maximum counting frequency must be appropriate to prevent the counter from overflowing within the t2 time. The maximum counting frequency fcpmax can be calculated using formula (3):

fcpmax=nmax/t2max (3)

Using a 16-bit counter, n2max=65535, t2max=44ms (corresponding to the maximum temperature +125℃), from formula (3) we can get fcpmax=65535/44ms=1.5MHz, the operating frequency of msp430 is 8MHz, the divider selects 8 division, so that the timer works at 1MHz, which can ensure that the count value will not overflow and accurately measure the temperature.

Since tmp03/04 works around the thyristor power module, the environment is relatively harsh. Therefore, in order to prevent interference from the working power ground line into the microprocessor, a photocoupler is added between tmp03/04 and the microprocessor for isolation, and the isolated signal is added to the capture port p1.2 of timer_a. After the microprocessor determines that the temperature is over, it drives the corresponding relay through the output level of p1.3, cuts off the working power of the thyristor power module to protect the circuit. The specific circuit diagram is shown in Figure 4, and the program flow is shown in Figure 5.

Application of TMP03/04 digital temperature sensor in temperature protection

Application of TMP03/04 digital temperature sensor in temperature protection

3.2 Get the count value through the normal I/O port

In fact, in the application of temperature protection circuit of thyristor power module, the temperature measurement is not required to be very precise. It only requires the microprocessor to start the over-temperature alarm when the radiator temperature exceeds a certain temperature value. Moreover, in general industrial control, the resources of I/O ports with capture function are very tight. Therefore, the method of obtaining temperature value by connecting ordinary I/O port with TMP03/04 has considerable practical application value.

The interface circuit of this method is similar to that of Figure 4. It only requires replacing the capture port p1.2 with a normal I/O port. The program flow chart is shown in Figure 6.

Application of TMP03/04 digital temperature sensor in temperature protection

In terms of program design, because t1 is fixed and t2 changes, the microprocessor presets an over-temperature value t2, which can be obtained by formula (4). Once the low-level count value input by tmp03/04 to p5.1 is greater than the preset value, the over-temperature protection is activated.

t2=400t1/max/（235-θ） (4)

Among them, t1max=12ms, θ is the over-temperature value.

4 Conclusion

Practice has proved that in the thyristor power module temperature protection circuit, the interface between the tmp03/04 digital temperature sensor and the microprocessor and the program design are quite simple and convenient, and it has high accuracy and strong anti-interference ability, and can effectively play the role of over-temperature protection.