Temperature Sensor Readout Circuit for Microheater

TEMPERATURE SENSOR READOUT
CIRCUIT FOR MICROHEATER
PRESENTED BY
SHYAMILI JOHN
ROLL NO: 56
S7 F
1

CONTENTS
 INTRODUCTION
 MEMS BASED MICROHEATER ARRAY ON SOI WAFER
 MICRO HEATER PATTERN STRUCTURE
 HEATER DESIGN
 TEMPERATURE SENSOR
 SELECTION OF NTC THERMISTOR
 INTERFACE CIRCUIT
 ADVANTAGES
 DISADVANTAGES
 CONCLUSION
 REFERENCES
2

INTRODUCTION
 Micro heaters are used to elevate the temperature of gas
sensor.
 Micro heater array design can be more effective using
thin film and MEMS technology.
 Temperature measurement is recorded through resistive
sensor, NTC thermistor.
 For temperature readout circuit, NTC thermistor and
heater is combined together in Wheatstone bridge.
3

MEMS BASED MICROHEATER
ARRAY ON SOI WAFER
 The thickness of the microheater is 0.1µm and total
length is 420µm.
 The microheater is made of platinum.
 Temperature achieved is 200 0C .
 Power consumption is 20mW .
 Uniform temperature distribution over the microheater.
 Minimum heat loss through the substrate.
4

CONTND…
5
Fig 1. Temperature VS power consumption graph of microheater

MICROHEATER PATTERNS
6
Fig 2. Types of Geometries Structure

MICROHEATER PATTERN
STRUCTURES
 S-Shape – uniform temperature profile and sensing film
of small dimensions.
 Double spiral – to avoid radial temperature gradient of
conventional meander types.
 Honeycomb- redistribution of thermal energy.
 Meander shape- demonstrates undistributed hot spot at
high temperature.
 Plane plate- with central hole has a square hole in its
design.
 Fan shape- low power consumption and uniform
temperature profile.
7

HEATER DESIGN
 Copper is the heating element.
 Resistance is calculated by , R= ρL/A , ρ is resistivity
in Ÿ/m, L for length of copper and A is a cross sectional
area of conductor.
 The resistance of copper at different temperature can be
measured by equation,
1. Rm = R0 [1+α 0 (T − T0)]
where R m : Resistance measured at T
R0 : Resistance given at To
α 0 : Temperature coefficient of metal
T0 : Room temperature
8

MEANDER PATTERN HEATER
9
Fig 3. Temperature profile meander heater pattern

CONTD…
 The single MEMS heater is designed with 3μm width,
209 μm length and 0.15 μm thickness.
 The maximum voltage from ambient temperature of
250C (300K) to 100 0C (375K) is 0.2V.
 The amount of current supply is 3.3mA .
 Power consumption is 0.6 mW .
10

CONTD…
11
Fig 4. Temperature vs voltage graph Fig 5. Current vs Voltage graph

TEMPERATURE SENSOR
Why Thermistor?
 Higher sensitivity.
 Less expensive than Resistance Temperature Detector.
 Better accuracy.
 Provides faster response.
 Reasonable output voltages.
 Very reliable and convenient to use.
12

THERMISTORS
 Thermistors are of two types-Negative temperature
coefficient(NTC) and positive temperature
coefficient(PTC)
 NTC thermistor-used for temperature sensing applications
because of its high stability.
 PTC thermistor-used as heating element for small
temperature controlled ovens, circuit protection
applications etc.
 NTC thermistor results non linear measurement.
13

SELECTION OF NTC THERMISTOR
 Minimum and maximum resistance values at the highest
temperature.
 4ŸΩŸNTC thermistor.
 According to Steinhart-Hart equation , the resistance of
thermistor is determined at different temperatures,
2. 1/T= A+ B (loge R) + C (loge R)3
 Dissipation factor should be maintained at low level.
 Excitation current calculated from,
3. Power dissipated= I2R = δ(T)
14

RESISTANCE VS TEMPERATURE
GRAPH
15
Fig 6. Resistance VS temperature graph of 4 Ω thermistor

INTERFACE CIRCUIT
 Wheatstone bridge is preferable than voltage divider.
 Used to connect the heater and thermistor for
temperature measurement.
 Supply noise is fully eliminated.
 Nonlinearity of R/T curve of thermistor leads to the use
of this circuit in order to linearize the signal.
16

WHEATSTONE BRIDGE CIRCUIT
17
V out = Vb – Vc = [R2/(R 1+R 2)] – [R 4/(R 3+R 4)]
Fig 7. Wheatstone bridge circuit

RESULT
18
Fig 8. Circuit diagram of Wheatstone bridge

CONTD…
 Two voltage dividers.
 First voltage divider consists of two heaters with 8 Ω
resistance each.
 Total summed up voltages are 0.4 V.
 Current flow should minimize to avoid self heating
which is 0.1 A .
 Potentiometer of 4 Ω is used.
19

OUTPUT VOLTAGE AND OUTPUT
POWER
20
 Total current consumption of heater and
thermistor is 0.118A
 Constant value of heater and thermistor is selected
to have low power dissipation.
Fig 9. Reading of output voltage and output power

OUTPUT VOLTAGE VS
TEMPERATURE GRAPH
21
Fig 10. Voltage Output VS Temperature graph

ADVANTAGES
 Bridge circuit offers the simplest configuration
 Low power consumption especially in micro heater gas
sensor array application.
 Number of parts can be reduced.
 Sensor measurement is more accurate as the distance is
close with heater.
 Proper thermal isolation between sensor element and
substrate.
 Ease of microheater array fabrication and small size.
22

DISADVANTAGES
 Since thermistors are semiconductor devices, their opera
tion is highly non linear.
 Limited temperature range due to which they are
rendered unsuitable for use at higher temperatures.
23

CONCLUSION
 Meander structure is suitable for heater .
 Resistance and current values to heat up the heater are
obtained from simulation of heater design.
 NTC thermistor resistance is selected based on the range
of temperature and power dissipation constant.
 Potentiometer is used to adjust the current and balance
the circuit.
 Maximum output voltage at 1000C is 0.4153V with
0.0049W requirement.
24

REFERENCES
 Solehah Md Hashim,Umadevi Chandaran,
“Temperature Sensor Readout Circiut for
Microheater”,International Conference on Electronic
Design,2014
 A. Datta. Gupta, C. Roy Chaudhuri, “Design and
Analysis of MEMS Based Microheater Array on SOI
Wafer for Low Power Gas Sensor Applications”,
International Journal of Scientific & Engineering
Research, vol 3, pp. 1-8, 2012.
25

CONTD…
 Avigyan Datta Gupta, Chiirashree Roy
Chaudhuri.“Design and Analysis of MEMS Based
Microheater Array on SOI Wafer Low Power Gas
Sensor Applications” International Journal of Scientific
& Engineering Research, vol 3. No 6. 2012.
26

Temperature Sensor Readout Circuit for Microheater

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