Sensors and Actuators, A2l-A23 (1990) 655-659

A New Class of Integrated Thermal Oscillators with Duty-cycle Output forApplication in Thermal Sensors

Y. PAN, F. R. RIEDIJK and J. H. HUIJSING

Electrícal Engineeríng Department, Delft Untu:ersity of Technology, P.O. Box 5031,260reA Delft (The Netherlands)

Abstract

A new class of integrated thermal oscillators has

been developed by measuring the heating power inthe time domain. A duty-cycle output proportionalto the heat-transfer coefficient has been obtained tobe compatible with microprocessors. Several differ-ent structures, which belong to the same family,have been constructed. The converter family can beemployed to measure flow velocity, pressure, IRradiation and true RMS with fast response andgood linearity. Experimented results for measuringgas-flow velocity are presented. The structures ofthe oscillator are simple and only a standard bipolartechnique is required.

1. Introduction

The measurement of many physical parameterssuch as flow, pressure or IR radiation is often basedon the use of thermal sensors, which measure theheat transfer between a heated chip and its ambient.A schematic representation of this class of sensorsis depicted in Fig. 1. It contains three domains: (i)physical, (ii) thermal and (iii) electrical. In the lastdecade, three principles have been developed toconvert the physical signal into a thermal signal,these include: (i) the measurement of a temperaturedifference [, 2]; (ii) the measurement of a heatingpower [3, 4] and (iii) the measurement of rise andfall times [5].

The measurement of on-chip heating powerdissipation is a well-known method. A thermalfeedback loop, which is usually constructed by anon-chip heating source, a temperature sensing ele-ment and an amplifier, should be used to keep thechip at a constant temperature in order to achievea linear relation between the heating power and theheat-transfer coefficient. It is expected from previ-ous reports that a linear relation and a fast responsecan be achieved with a very high loop gain. Wefound, however, that this result can only beachieved with an ideal thermal feedback loopwith one dominant pole caused by the thermal

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Fig. t. The three domains of thermal sensors.

capacitance of the chip. In practice, the loop willeasily be brought into oscillation by several non-ideal effects, such as:

(i) delay of the amplifier and of the heattransfer between the heating element and the tem-perature sensing element;

(ii) parastic positive feedback loops;(iii) 50 Hz background interference;(vi) thermal noise in the input of the amplifier.To overcome these disadvantages, a novel ther-

mal sensor for measuring flow velocity has beenrecently reported by the present authors [6]. tnthis configuration, the loop has deliberately beenforced into oscillation by a high loop gain andpositive feedback at small temperature oscillationamplitude. The average heating power is mea-sured in the time domain. A linear relation be-tween duty cycle, heating power and heat-transfercoefficient can therefore be achieved.

The present paper describes the extension of theabove structure to a converter family with im-proved performance and flexibility for applicationin smart sensors based on the measurement ofthermal power. The principle of power measure-ment in the time domain is discussed in Section 2,the members of the conyerter family are distin-guished in Section 3, and some results for measur-ing gas-flow velocity are presented in Section 4.

2. Principle of Measurement

A basic monolithic integrated thermal sensorcan.be realized by an on-chip heating source and

@ Elsevier Sequoia/Printed in The Netherlands

656

a temperature sensor. The on-chip heating poweris dissipated mainly through two mechanisms in achip with a well designed substrate contact:

(i) through an increase in the chip tempera-ture;

(ii) through compensation of the heat convec-tion between the heated chip and ambient.

The power balance equation can be written as:

P6: C,amff * n.01, - r^1 (l)

where Po: the heating power; Ctt : the thermalcapacitance of the chip; A,: the chip surfacearea; h : the heat transfer coefficient; m : themass of the chip; 7 : the temperature of the chip;and [: the temperature of the ambient.

If the chip temperature oscillates around aconstant level at a small amplitude, we obtain

T(t) : To+ LT(r) (2\

where Io is a constant and ÀI(t) is a small signalperiodic oscillation function of time. Equation (l)can be rewritten as

Pn(r) : c,^*(ry)* n.ot ,+ Lr(t)- "r,r,

Both sides of eqn. (3) are averaged in the timedomain, then we obtain

p . : c..^(d^r(o\\ dr )",,+ A"h(To * (Àr(l)).," - 7.; (4)

where the subscript'ave' means the average valuein one oscillation period. It is obvious that for aperiodic oscillation function with a sufficientlysmall amplitude around the zero point,/d^ríÍ)\(.- *-,)",. = (ar(r))"". nv o (5)

Therefore, we achieve

P,u.N A.h(To- T,) (6)

Equation (6) demonstrates clearly that the aver-age heating power is linearly related to the heat-transfer coefficient with constant average chiptemperature. In addition, also a very short responsetime can be achieved because of the very high loopgain. Such disadvantages as nonlinearity and longresponse time caused in previous stable-state heat-ing power measurements no longer exist.

Although the chip temperature could oscillatenaturally at a high loop gain because of the timedelay of the heat transfer between the on-chipheating source and the temperature sensing ele-ment, a well-defined pulse-modulated block wave isusually preferred in practical applications whichcan be transmitted to a microprocessor directly.

This can lead to the fabrication of a so-called 'smartsensor'. Several configurations have been deve-loped to provide a flexible and well-defined outputsignal. They are discussed in the next section.

3. Converter Family

Several structures have been developed to forcethe thermal feedback loop into oscillation with ablock wave output. They are:

(i) free-running thermal oscillator with positive feedback as published before by the authors

[6];(ii) thermal oscillator with positive feedback,

synchronized by an external clock;(iii) sigma-delta thermal modulator with an

additional clocked flip-flop.These three structures are discussed below.

3. l. Free-running Thermal Oscillatorwith Posith;e Feedback

The basic sensor circuit is depicted in Fig. 2(removing the external clock V" and resistor (r).It contains a diode D, for measuring the chiptemperature and an SP resistor Ro for heating thechip. The diode D2 on another chip is utilised tomeasure the ambient temperature and resistors R,and R, are used to provide the bias current for D,and Dr. The emitter/base offset is set up by thereference resistor R., which makes the sensor chipoperate at an overheat of the ambient. A powertransistor Qn with a collector bias resistor \ isemployed as a switch. A comparator with a posi-tive feedback resistor R6 forces the thermal feed-back loop to function as a block-wave oscillator.

In the initial state, the output of the compara-tor is set high and the heating resistor will beginto heat the chip. As a result, the temperature ofthe chip will increase and V, will decrease until theoutput of the comparator switches low. Then thechip temperature will decrease due to heat ex-change with the ambient until the output of the

Fig. 2. Basic oscillation circuits.

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Tc

____ r\ -___J:>-____l-_ __ V2

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Fig.3.wavefo.-r-,n*--*,"tÏnr"...

oscillation of the comparator is high again. Thecycle is then repeated. The waveforms of thesensor are depicted in Fig. 3. Because of thepositive feedback and the very high gain of thecomparator, the output is a square wave and theoscillation of the chip temperature is approxi-mately a triangular wave. The jump of V, iscaused by the hysteresis of the feedback resistorRr. The temperature oscillation amplitude can bechosen to be sufficiently small. Therefore, theaverage temperature 7.," is almost constant.

The average heating power can therefore bewritten as

tPu," : P*u* |: A.h(T^,. - T^) (7)

tp

where Í,/Í, is the duty cycle. Equation (7) showsthat the duty-cycle output is proportional to theheating power and therefore to the heat-transfercoefficient.

3.2. Thermal Oscillator with Positioe FeedbackSynchronized by an External Clock

In the previous basic configuration, the oscilla-tion frequency of the thermal oscillator depends

T_

sensor caíp J-

%mTime

Fig. 4. Waveforms of the oscillator with an external clock.

mainly on the thermal capacitance of the chip andthe transition region of the comparator. The oscil-lation frequency of the oscillator can be synchro-nized by an external clock in a certain range whenthis signal is added to the sensor signal, as de-picted in Fig. 2. The waveforms of the thermaloscillator with positive feedback synchronized byan external clock are depicted in Fig. 4.

3.3. Sigma - Delta Thermal ModulatorSensors with direct digital signal output are

becoming more and more important in presentmicroprocessor-based measurement systems. Avery popular analog-to-digital converter is thesigma-delta modulator, which can also be ex-tended to the thermal domain. This can easily bedone by inserting a clocked master/slave flip-flopbetween the comparator and the heating resistor(see Fig. 5). This combination performs a one bitD/A conversion into the thermal domain by pro-ducing a heat pulse of one clock period. The

Reference chip Sensor chip

Clock

Fig. 5. Sigma-delta thermal modulators.

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Fig. 6. Waveforms of the sigma-delta modulators.

thermal capacitance of the chip now serves as theintegrating loop filter. The average rate ofthe heatpulses is proportional to the heating power, whichis again related to the flow by eqn. (6). Thewaveforms are depicted by Fig. 6. By using asimple digital low-pass filter like a counter, a fullydigital signal can be obtained. This signal can betransmitted by a bus which is connected to amicroprocessor.

4. Experimental Results

The thermal converter family has been utilisedto measure gas-flow velocity. A photograph of theintegrated silicon flow sensor is depicted in Fig. 7.The chip measures 4 x 3 mm and was processedby a standard bipolar [C process. It contains atransistor for measuring the on-chip temperature,a DP resistor for heating the chip, and a DP-diffusion Seebeck sensor (which is not used in ourconfiguration). The sensor temperature is mea-sured using the temperature dependence of the'V*' of the transistor.

Direct contact between the sensor chip and thefluid may be undesirable since the wire bondingwill disturb the flow over the sensor. A simple

+ Baslc co.figration

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Flow velocity(m/s)

Fig. 8. Measured duty cycle and on-state probability as afunction of flow velocity.

configuration was obtained by mounting the sen-sor chip onto a ceramic AlrO, substrate carryinga gold interconnection pattern. This assembly ismounted upside down in a printed board which isplaced in the mouth of a small wind tunnel. Thesensor was operated at an overheat l0 K (emitter/base offset V*:25 mV). The power supply was5V.

The measured duty cycle as a function of thegas-flow velocity is depicted in Fig. 8. For thefree-running thermal oscillators with positive feed-back (with or without the external trigger signal),the oscillation of the chip temperature is a trian-gular wave with an amplitude of about I K. Theaverage sensitivity is 3.16/mW/K. With a blockwave as a trigger signal, the output frequency ofthe oscillator can be selected from 20 Hz to800 KHz in our experiments. Moreover, the wave-forms are more stable.

The sigma-delta thermal modulators are cur-rently being evaluated. The results will be pub-lished in the future. In principle, the converterfamily described above should be able to measureIR radiation, vacuum and true RMS [7].

5. Conclusions

A new class of integrated thermal oscillatorswith duty-cycle output for application in thermalsensors has been developed. A well-defined duty-cycle output is achieved by measuring the heatingpower in the time domain with a constant averagechip temperature. A linear relation between thelleating power and heat-transfer coefficient is ob-tained and the response time is greatly reduced.Fig. 7. Photograph of the sensor chip

Several different structures can be utilised to con-struct thermal oscillators with different character-istics. Some experiment results for measuringflow velocity are presented. The principle canalso be used to measure the vacuum, IR radia-tion and a true RMS.

The principle of the measurement has beenwell established, while the accuracy should beimproved further. The power dissipation by aheating resistor is not a very accurate quantity ina standard bipolar technique. Moreover, the fluc-tuation of the power supply will further alsoreduce the accuracy of on-chip power.

Future work will concentrate on construct-ing an accurate on-chip heating source andon integrating all external elements on onesingle chip leading to the so-called 'smart sen-sors'.

659

References

I J. H. Huijsing, J. P. Schuddemat and W. Verhoef, Mono-lithic integrated direction-sensitive flow sensor, IEEi Trans.Electron Deoices, ED-29 (1992') t33-136.

2 O..Tabata, Fast-response silicon flow sensor with on-chipfluid temperature sensitive element, IEEE Trans. ElectonDeuices ED-3i (1986) 361-365.

3 9. E. Plstger and M. Southfield, Solid-state fluid sensor,U.S. Patent i992940 (Nov. 23, 1976).

4 Tong Qin-Yi and Huang Jin-Biao, A novel CMOS flowsensor with constant chip temperature (CCT) operation,Sensors and Actuaíots, 12 (1988) 9-2r.

5 G. Stemme, A CMOS integrated silicon gas flow sensor withpulse-modulation output, Proc. 4th tni. Conf. Sotid-StateS_ensors and Actuators (lransducer'87), Tokyo, Japan, June2*5, 1987, pp.364-367.

6 Y. ?an and J. H. Huijsing, New integrated gas-flow sensorwith duty-cycle outpnt, Electron. kti., 24 (Í9gg) 542_543.

7 A. W. Van Herwaarden and P. M. Sarro, Thermal sensors

!ll"A-gl Seebeck effects, Sensors and Actuators, l0 (t986)321-346.