RU2164709C2 - Thermostatic microswitch with posistor heater - Google Patents

Abstract

FIELD: temperature control for precision radio electronic devices. SUBSTANCE: thermostatic microswitch has posistor 2 mounted on controlled-temperature substrate 1 and control transistor 5 whose collector is connected to power supply and emitter, to one of posistor leads. Newly introduced in device are feedback resistor 6 inserted between posistor lead and common bus, operational error amplifier 7, and voltage dividers 8, 9, 10 that functions to determine thermostatic control temperature of posistor 2 and is connected between power supply and common bus. Output of voltage divider 9 is connected to inverting input of operational amplifier 7 whose output is connected to base of control transistor 5 and noninverting input, to feedback resistor 6. Thermostatic switch has reduced number of thermal-inertia sections and incorporates closed-circuit control system that functions to reduce voltage across posistor heater in response to its temperature increase. EFFECT: improved precision of thermostatic control within wide range of ambient temperatures. 1 dwg

Description

 The invention relates to techniques for temperature control in precision electronic devices and can be used to maintain the constancy of the parameters of these devices in a wide range of ambient temperatures (TOC).

 Known microthermostats containing a temperature control circuit and three inertia links: a temperature sensor, a heater and a thermostatically controlled substrate [1]. The presence of a large number of inertia links in such microthermostats is one of the reasons limiting the dynamic accuracy of temperature control, since it is known from the theory of automatic control that for systems with a small number of inertia links, all other things being equal, this accuracy is higher than for systems with a large number inertial links.

Known thermostabilized piezoelectric resonator containing a posistor located on a substrate and connected to a constant voltage power source [2]. It has only two heat-inertia links (a posistor, which is both a temperature sensor and a heater, and a thermostatically controlled substrate). This allows you to get a short time to reach a given thermal regime and high dynamic accuracy of temperature control in the case when the temperature coefficient of resistance (TCS) of the posistor is large. The disadvantage of this device [2] is that due to the insufficiently large TCS of the posistors (≅0.2 ^o C ^-1 ), the achieved accuracy of thermostatting of the substrate in it is small and amounts to ± 6 ^o C when the TOC changes from -50 ^o C to + 65 ^o C [2,3]. This accuracy is much worse than for microthermostats containing a temperature sensor, heater, substrate and control circuit.

The closest technical solution is a thermostat with a posistor heater [3], containing a posistor located on the substrate and a temperature-dependent power source, the voltage of which depends on the TOC. The block diagram of this source consists of a divider, one of the arms of which includes a constant resistor R ₃ , and the other represents a parallel connection of a constant resistor R ₂ with a chain of thermistor R _T connected in series with a negative TCS and a constant resistor R ₁ . The thermistor R _t must have an ambient temperature, which is achieved by placing it outside the thermostat as close as possible to the latter. The voltage U (t _C ) from the arm of the divider containing R _t is supplied to the resistor heater R _n through a power amplifier made in the form of an emitter follower. At the input of the divider is supplied voltage U _I , set in accordance with the technical requirements for the development of the thermostat. Due to the use of a thermally dependent power supply, the temperature control error can be reduced by two to three times [3].

A thermostat with a posistor heater and a temperature-dependent power supply loses its advantages over microthermostats [1], which also contain three heat-inertia elements, due to the additional limitations of thermostatting accuracy described below due to the lack of isolation of the control system. The first additional limitation: due to the fact that the supply voltage and the posistor depend on the TOC (t _C ), and not on the substrate temperature t _P , for the implementation of temperature control, it is necessary to calculate the dependence U (t _C ) at t _P = const. The initial data for the calculation are the temperature-varistor characteristic of the posistor (that is, the dependence of its resistance on temperature and supply voltage), the dimensions and thermophysical parameters of the thermostat design elements. Errors in the values of the values of the initial data, errors in the calculation formulas lead to an increase in the error of thermostating. The second additional increase in this error occurs due to the fact that, at large ranges of variation of t _{С, the} proposed divider scheme does not allow reproducing the calculated dependence U (t _С ) with a sufficiently high accuracy. An additional contribution to increasing the temperature control error is made by the fact that, when calculating U (t _С ), the temperature of the medium is the average temperature of the medium around the thermostat, and the thermistor in the divider of the power source changes its parameters and the output voltage of the source, responding to the local temperature of the medium at its location , which in the general case is not equal to the average temperature of the medium t _С. The authors' recommendations to place the thermistor outside the thermostat as close as possible will lead to the appearance of an additional error, since the average temperature of the thermistor will in this case be approximately equal to half the local temperature of the medium and the temperature of the external surface of the thermostat, and not t _С. It should be noted that the introduction to the temperature control circuit of the thermostat, in addition to the substrate and the posistor, of the third heat-inertia element - the thermistor increases the dynamic error of thermostating. In particular, due to the large difference in time-to-steady heat thermostat mode (according to the authors of approximately two minutes), and the thermistor R _T (for any manufactured thermistors this time approximately one order of magnitude less than two minutes and differently during heating and cooling) occurs thermostatic error associated with the non-synchronization of regulation.

 In the inventive microthermostat with a posistor heater containing a posistor located on a thermostatically controlled substrate, a regulating transistor, the collector of which is connected to a power source, and an emitter with one of the posistor terminals, a feedback resistor is connected between the other terminal of the posistor and the common bus, an operational mismatch amplifier and a voltage divider connected between the power source and the common bus, the output of which is connected to the inverting input of the operational mismatch amplifier, the output is connected to the base divisor regulating transistor and non-inverting input of the amplifier is connected to the feedback resistor.

 Improving the accuracy of thermostating in a wide range of ambient temperatures in the claimed technical solution is due to the reduction in the number of inertia links and the introduction of a closed regulation system that reduces the voltage on the posistor heater with increasing temperature.

 The drawing shows a diagram of the device.

 The inventive device comprises a substrate 1, on one side of which there is a posistor 2 in film or discrete design, a power bus 3, a common bus 4, a control transistor 5, a feedback resistor 6, an operational mismatch amplifier 7, voltage divider resistors 8, 9, and 10 .

 The posistor 2, which is the heater of the thermostatically controlled substrate 1 and therefore has a strong thermal connection with it, is connected to one emitter of the regulating transistor 5, the collector of which is connected to the power bus 3, feedback resistor 6 is connected between the other terminal of the resistor 2 and the common bus 4. The output of the operational mismatch amplifier 7 is connected to the base of the control transistor 5, and the non-inverting input of the amplifier 7 is connected between the resistor 2 and the feedback resistor 6, and the inverting input is connected to the sliding to with a potentiometer 9, two other contacts of which are connected, respectively, with a resistor 8 connected by another output to the power bus 3, and with a resistor 10 connected by another output to the common bus 4. In general, to reduce energy consumption, the design of a microthermostat can be covered with a thermal insulation sheath .

 The device operates as follows.

В этой формуле
U₂ - напряжение на позисторе 2;
U_вых - выходное напряжение операционного усилителя рассогласования 7;
U_БЭ - напряжение на открытом электронно-дырочном переходе между базой и эмиттером регулирующего транзистора 5, составляющее всего несколько десятых Вольта;
U₁ - напряжение на резисторе обратной связи 6.With an increase in the ambient temperature t _С , the temperature of the posistor 2 increases, which, due to the positive TCR, leads to an increase in its resistance R ₂ and a decrease in the power P ₂ released on it, consumed mainly for heating the substrate

In this formula
U ₂ - voltage on the posistor 2;
U _o - the output voltage of the operational amplifier mismatch 7;
U _BE - voltage at the open electron-hole transition between the base and the emitter of the regulating transistor 5, which is only a few tenths of a Volt;
U ₁ - voltage across the feedback resistor 6.

то есть U₁<<U₂ и K_R≈R₆/R₂. Учитывая приведенные соображения, для упрощения описания работы устройства можно считать

Уменьшение мощности нагрева подложки 1 при увеличении температуры окружающей среды t_С приводит к уменьшению перегрева подложки Δt относительно t_С. При этом происходит термостатирование подложки 1, так как ее температура t_П, равная сумме t_С и Δt, почти не меняется при увеличении t_С
t_П = t_С + Δt
Напряжение U₁ на резисторе обратной связи 6 при увеличении t_С уменьшается из-за увеличения сопротивления R₂

При этом уменьшается выходное напряжение U_вых операционного усилителя рассогласования 7, зависящее от величины опорного напряжения U₀ (U₀< U₁), снимаемого с делителя напряжения, составленного из резисторов 8, 9 и 10 и от коэффициента усиления операционного усилителя рассогласования 7

Выполнив преобразование, имеем:

Для того чтобы система была устойчивой, необходимо, чтобы K_U·R₆/R₂ было меньше единицы, так как при (K_U·R₆/R₂) = 1 имеет место эффект самовозбуждения. При уменьшении температуры окружающей среды t_С сопротивление позистора R₂ уменьшается, что приводит к увеличению выделяемой на нем мощности P₂ и увеличению перегрева подложки Δt относительно температуры t_С. Температура подложки t_П, как и в случае увеличения t_С, почти не меняется. Подставив найденное выражение для U_вых в упрощенную формулу для P₂, получим

При этом для задания требуемой величины температуры термостатирования подложки t_П можно использовать потенциометр 9, регулирующий величину U₀, определяющую мощность P₂ нагрева позистора.To prevent self-excitation of the operational mismatch amplifier 7 at a gain of K _U >> 1, the condition

i.e., U ₁ << U ₂ and K _R ≈R ₆ / R ₂ . Given the above considerations, to simplify the description of the operation of the device can be considered

The decrease in the heating power of the substrate 1 with increasing ambient temperature t _With leads to a decrease in the overheating of the substrate Δt relative to t _With . In this case, the thermostatting of the substrate 1 occurs, since its temperature t _P , equal to the sum of t _C and Δt, almost does not change with increasing t _C
t _P = t _C + Δt
The voltage U ₁ on the feedback resistor 6 with increasing t _With decreases due to an increase in resistance R ₂

In this case, the output voltage U _{o of the} operational error amplifier 7 decreases, depending on the value of the reference voltage U ₀ (U ₀ <U ₁ ) taken from the voltage divider composed of resistors 8, 9, and 10 and on the gain of the operational error amplifier 7

Having completed the transformation, we have:

In order for the system to be stable, it is necessary that K _U · R ₆ / R ₂ be less than unity, since for (K _U · R ₆ / R ₂ ) = 1, the self-excitation effect takes place. By reducing the ambient temperature t _{C 2} R thermistor resistance decreases, which leads to an increase in capacity allocated to him P ₂ and the substrate increase superheat Δt relative temperature t _C. The temperature of the substrate t _P , as in the case of increasing t _With , almost does not change. Substituting the found expression for U _out into a simplified formula for P ₂ , we obtain

At the same time, to set the required temperature thermostating temperature of the substrate t _P, you can use potentiometer 9, which controls the value of U ₀ that determines the power P _{2 of the} heating of the posistor.

Для нашего примера получим

Результатом расчета для различных соотношений параметров схемы при устойчивой ее работе в отсутствие самовозбуждения, то есть при (K_U·R₆/R₂)< 1, приведены в таблице.Let us set the temperature coefficient of the resistance of the posistor α = 0.2 ^o C ^-1 and determine using P ₂ how many times the power of the posistor heater will change with increasing temperature Δt = 5 ^o C, when the resistance of the posistor changes from R ₂ (t) = R ₂ to R ₂ (t + Δt) = R ₂ (1 + α · Δt) = 2R ₂

For our example, we get

The calculation result for various ratios of the parameters of the circuit with its stable operation in the absence of self-excitation, that is, with (K _U · R ₆ / R ₂ ) <1, is shown in the table.

An analysis of the numerical values given in the table shows that the smaller the margin of stability of regulation, that is, the closer the value (K _U · R ₆ / R ₂ ) to unity, the greater the gain in accuracy of regulation, proportional, ceteris paribus, to the ratio P (t) to P (t + Δt).

Для таких термостатов

При α = 0,2^oC^-1 и Δt = 5^oC получаем P(t)/ P(t+ Δt) = 2, то есть при одних и тех же условиях точность регулирования температуры, а следовательно и точность термостатирования, в заявляемом устройстве может быть достигнута в 30.25 раза выше, чем в термостате [2], использующем для питания позисторов постоянное напряжение. В приведенных расчетах не учтено, что в заявляемом устройстве уменьшение напряжения питания позистора при увеличении его температуры приводит к дополнительному увеличению сопротивления позистора, то есть не учтен варисторный эффект в позисторе. При учете этого эффекта в расчете заявляемого устройства выигрыш в точности регулирования температуры по сравнению с термостатом, разработанном в [2] окажется еще больше. В прототипе [3] расчетное повышение точности термостатирования по сравнению с термостатом, разработанном в [2], оценивается лишь в 2-3 раза.For a posistor thermostat, powered from a constant voltage [2], the power P allocated by a posistor heater with resistance R ₂ is found from the expression

For such thermostats

When α = 0.2 ^o C ^-1 and Δt = 5 ^o C we get P (t) / P (t + Δt) = 2, that is, under the same conditions, the accuracy of temperature control, and therefore the accuracy of thermostating, in the claimed a device can be achieved 30.25 times higher than in a thermostat [2], which uses a constant voltage to power the posistors. In the above calculations, it is not taken into account that in the inventive device, a decrease in the supply voltage of the posistor with an increase in its temperature leads to an additional increase in the resistance of the posistor, that is, the varistor effect in the posistor is not taken into account. When this effect is taken into account in the calculation of the claimed device, the gain in the accuracy of temperature control in comparison with the thermostat developed in [2] will be even greater. In the prototype [3], the calculated increase in the accuracy of thermostating compared to the thermostat developed in [2] is estimated only 2-3 times.

Literature
1. A.S. USSR N 1672421, M. cl. G 05 D 23/19 Babayan R.R., Okropiridze D.P., Hovhannisyan O.G. Device for thermostating of semiconductor wafers of integrated circuits.

 2. A.S. USSR N 476665, M. cl. H 03 H 3/02 Sokolov B.A., Vorokhovsky Y.L. , Petrosyan I.G., Smirnov E.M., Shatalov O.M. Thermostabilized piezoelectric resonator // BI, 1975, N 25.

 3. Vorokhovsky Y. L., Gruzinenko V. B., Petrosyan I. G. Quartz resonator-thermostat with a self-regulating posistor heater // Electronic Technology. Series 5. Radio components and radio components, 1977, Issue 3 (22).

Claims (1)

 A microthermostat with a posistor heater, containing a posistor located on a thermostatically controlled substrate, regulating a transistor, the collector of which is connected to a power source, and an emitter with one of the terminals of the posistor, characterized in that a feedback resistor connected between the other terminal of the posistor and the common bus, an operational mismatch amplifier and a voltage divider that determines the temperature of the substrate thermostating temperature, connected between the power source and the common bus, the output of which is connected inen with an inverting input of the operational mismatch amplifier, the output of the amplifier is connected to the base of the control transistor, and the non-inverting input of the amplifier is connected to a feedback resistor.

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