ECE 477Digital Systems Senior Design Project – Grp 11Fall 2004
TABLE OF CONTENTS
Introduction...... / 2Reliability Analysis...... / 3
Microcontroller...... / 4
Step-down Regulator...... / 4
Compressor Interface...... / 6
Temperature Sensor...... / 7
Conclusions...... / 8
FMECA...... / 9
Functional Blocks Schematic...... / 11
Worksheets...... / 12
List of References...... / 14
Introduction
The "Cold as Ice" fridgeis an intelligent refrigerator that consists of an inventory system to keep track of its contents, a digital thermostat system to control the temperature within the refrigerator, and an Ethernet and LCD interface to allow for user interaction. All of the refrigerator's functionality will be controlled by a microcontroller. Themicro-controller will be reading, interpreting, and storing information given by a Radio Frequency Identification Device (RFID), operating a web server to allow for remote user control, driving the LCD panel to allow for local user control, and monitoring the temperature within the refrigerator and in turn controlling the compressor to keep the temperature within a user-specified range. Local user interaction will consist of the user being able to control the thermostat as well as the refrigerator displaying warning messages such as an expired product. Remote user interaction will consist of the user's ability to control the thermostat, see the contents inside, as well as input/access a recipe database that will be continually updated based on the current contents of the refrigerator.
This design interfaces with many external components, and while the failure of these components will result in a loss of functionality (i.e. keypad input, Ethernet accessibility) the failure of these devices will not be analyzed in this document. Instead, the failure of components that will either render the design inoperable, or that pose a potential safety issue, will be analyzed.
Reliability Analysis
Reliability analysis was accomplished using the information provided by the Military Handbook for Reliability Prediction of ElectronicEquipment [1] and Designing for Reliability, Maintainability, and Safety [2]. The Mean Time to Failure (MTTF) and the number of failures per 106 hours were calculated using these data sources. The parameters used throughout this document for the calculation of the MTTF are reiterated below from [1] and [2]:
λppart failure rate / πRpower rating factorλbbase failure rate / πSvoltage stress factor
C1die complexity / πEenvironmental constant
C2a constant based on the number of pins / πLlearning factor
πTtemperature coefficient / πQquality factor
πAapplication factor
Since the design will be controlling the on/off switching of a compressor there is the potential for hazardous conditions to occur should thetemperature sensor, power relay, or the NPN transistor fail. The voltage regulator and the Rabbit 2000 microprocessor are other components of concern since the failure of these devices will render the design inoperable.The power components and microcontroller are also most likely to fail due to operating above room temperature.Although the temperature sensor will not operate above room temperature the analysis of this component is important since the failure of this device could potentially burn out the compressor and the corresponding interface components. Therefore this document will focus on the analysis of the following components:
- Rabbit 2000 Microcontroller
- LM2676 Step-down Regulator
- T77 Power Relay
- TIP122 NPN Transistor
- TC72 Temperature Sensor
Microcontroller
The Rabbit 2000 [3] microprocessor is onboard the Rabbit Core Module 2200which is used in this design. The microprocessor will be used to control user interface (both locally and remotely through the Ethernet), compressor operation using the temperature sensor, and processing of data from the RFID reader. The failure rate for this component is defined as:
λp= (C1πT + C2πE) × πQ× πL Failures/106 Hours
Table 1. Rabbit 2000 MTTF parameters
Parameter / Value / JustificationC1 / 0.14 / The Rabbit 2000 is an 8-bit microprocessor
(MIL-HDBK-217F, Section 5.1)
πT / 0.98 / Digital CMOS device
Operation should not exceed +85°C (normal operation temperature range is -40°C to +85°C)
TJ used +85°C
(MIL-HDBK-217F, Section 5.8)
C2 / 0.040 / 100 pin SMT device
C2 = 2.8 x 10-4 (Np)1.08
(MIL-HDBK-217F, Section 5.9)
πE / 2.0 / Assumed “Ground Fixed” environment
(MIL-HDBK-217F, Section 5.10)
πQ / 10 / Commercial component
(MIL-HDBK-217F, Section 5.10)
πL / 1.0 / Years in Production ≥ 2.0
(MIL-HDBK-217F, Section 5.10)
λp / 2.172 per 106 Hours
MTTF / 460405 hours ≈ 52 years
Step-down Regulator
Thestep-down (buck) switching regulator [4] will be used to convert an unregulated 8-40VDC to a regulated 5VDC that will be supplied to all of the components on the PCB. Figure 1 shows the circuitry for the voltage regulator and the additional components.
Figure 1. 3A Step-Down Voltage Regulator
The failure rate for this component is defined as:
λp= (C1πT + C2πE) × πQ× πL Failures/106 Hours
Table 2. Voltage Regulator MTTF parameters
Parameter / Value / JustificationC1 / 0.02 / Assumed 101 to 300 transistors for a linear MOS device
(MIL-HDBK-217F, Section 5.1)
πT / 58 / Linear MOS device
Operation should not exceed +125°C (normal operation temperature range is -40°C to +125°C)
TJ used +125°C
(MIL-HDBK-217F, Section 5.8)
C2 / 0.0023 / 7 pin device
C2 = 2.8 x 10-4 (Np)1.08
(MIL-HDBK-217F, Section 5.9)
πE / 2.0 / Assumed “Ground Fixed” environment
(MIL-HDBK-217F, Section 5.10)
πQ / 10 / Commercial component
(MIL-HDBK-217F, Section 5.10)
πL / 1 / Years in Production ≥ 2.0
(MIL-HDBK-217F, Section 5.10)
λp / 11.6458 per 106 Hours
MTTF / 85868 hours ≈ 9.8 years
Compressor Interface
This circuitry is used to control the operation of the refrigerator’s compressor. Figure 2 shows the circuitry involving the TIP122 [5] transistor and T77 power relay [6].
Figure 2. Compressor Interface showing T77 Power Relay and TIP122 Transistor
The failure rate for the TIP122 is defined as:
λp= λb × πT× πA× πR× πS× πQ× πE Failures/106 Hours
Table 3. TIP122 Transistor MTTF parameters
Parameter / Value / Justificationλb / .00074 / Base failure rate for NPN and PNP devices
(MIL-HDBK-217F, Section 6.3)
πT / 8.1 / Junction Temperature of TIP122 is +150°C
(MIL-HDBK-217F, Section 6.3)
πA / 0.70 / Switching application
(MIL-HDBK-217F, Section 6.3)
πR / 4.69 / πR= (Pr).37
Prused 65W
(MIL-HDBK-217F, Section 6.3)
πS / 0.21 / VCE 50V
VCEO 100V
VS = VCE / VCEO= 0.5
(MIL-HDBK-217F, Section 6.3)
πQ / 8.0 / Plastic
(MIL-HDBK-217F, Section 6.3)
πE / 6.0 / Assumed “Ground Fixed” environment
(MIL-HDBK-217F, Section 6.3)
λp / 0.198357 per 106 Hours
MTTF / 5041408 hours ≈ 575 years
The failure rate for the T77 is defined as:
λp= λb ×πQ× πE Failures/106 Hours
Table 4. T77 Power Relay MTTF parameters
Parameter / Value / Justificationλb / 0.50 / Assumed Hybrid
(MIL-HDBK-217F, Section 6.3)
πQ / 4.0 / Lower
(MIL-HDBK-217F, Section 6.3)
πE / 3.0 / Assumed “Ground Fixed” environment
(MIL-HDBK-217F, Section 6.3)
λp / 6.0 per 106 Hours
MTTF / 166667 hours ≈ 19 years
Temperature Sensor
The temperature sensor [7] is used by the microcontroller to monitor the temperature within the refrigerator and based on this data control the compressor of the refrigerator. The analysis of this part failing is critical since if this component fails and constantly outputs incorrect temperature readings the compressors operation would be unpredictable. This could lead to food spoilage or to the compressor, the power relay, or the transistor burning out due to constant operation and or switching.
Figure 3. TC72 temperature sensor interface
The failure rate for this component is defined as:
λp= (C1πT + C2πE) × πQ× πL Failures/106 Hours
Table 5. Temperature Sensor MTTF parameters
Parameter / Value / JustificationC1 / 0.040 / Assumed 1001 to 3000gates for a digital MOS device
(MIL-HDBK-217F, Section 5.1)
πT / 5.5 / Digital MOS device
Operation should not exceed +150°C (normal operation temperature range is -65°C to +150°C)
TJ used +150°C
(MIL-HDBK-217F, Section 5.8)
C2 / 0.0026 / 8 pin device
C2 = 2.8 x 10-4 (Np)1.08
(MIL-HDBK-217F, Section 5.9)
πE / 2.0 / Assumed “Ground Fixed” environment
(MIL-HDBK-217F,Section 5.10)
πQ / 10 / Commercial component
(MIL-HDBK-217F, Section 5.10)
πL / 1.0 / Years in Production ≥ 2.0
(MIL-HDBK-217F, Section 5.10)
λp / 2.252 per 106 Hours
MTTF / 444050 hours ≈ 51 years
Conclusions
Table 6 summarizes the preliminary failure rates.From these calculations it can be seen that the devices most likely to fail are the voltage regulator and the power relay with a MTTF of 8.5868E+04 and 1.6667E+05 respectively. The worst of these of approximately 9.8 years was much lower than some of the remaining components, namely the NPN transistor calculated at 575 years. The reason for this low MTTF value was due largely in part to the regulators high Junction Temperature causing the temper-ature coefficient πTto be large. It should be kept in mind that the calculations were done using a junction temperature of +85°C or greater, when in practical applications the temperature would be closer to +40°C to 50°C. These temperatures could also be lowered with the addition of heat sinks and running components in power saving modes, which can help to decrease the probability of failure. It should also be noted that the temperature sensor is rated for approximately 51 years, and due to its operation in a cooler environ-ment the time to failure should be even higher. This MTTF is satisfactory since it is higher than many of the other components and the failure of this device should never become an issue of safety. Even if failure occurs the microcontroller can reduce the risk of the compressor interface failing by performing reality checks on the temperature input. The microcontroller will also only switch the compressor on/off at most once a minute to decrease the probability of failure in the relay and compressor.
Since these components will go into a design for high end consumer appliances it was determined that the MTTF values calculated were satisfactory since changes in consumer trends will likely see the replacement of this design before product failure.
Table 6. Preliminary Failure Rate Calculations
Component / Description / Ip/106 hours / MTTFJ4/J5 / Rabbit 2000 microprocessor / 2.1720 / 4.6041E+05
U1 / Step-down regulator (LM2676) / 11.6458 / 8.5868E+04
Q1A / NPN transistor (TIP122) / 0.1984 / 5.0414E+06
RL1 / Power relay (T77) / 6.0 / 1.6667E+05
J3 / Temperature Sensor (TC72) / 2.2520 / 4.4405E+05
FMECA
As shown in figure 4 the design schematic has been divided into four functional blocks for the Failure Mode, Effects, and Criticality Analysis. Table 7 further describes these blocks.
Table 7. FMECA functional blocks
Block / Type / Main component(s)A / Microcontroller / Rabbit 2000 microprocessor
B / Sensor / TC72 temperature sensor
C / Power circuit / LM 2676 voltage regulator
D / Power circuit / T77 power relay & TIP122 transistor
Two criticality levels have been defined as follows:
Criticality / Failure effect / Maximum probabilityHigh / A critical failure should never happen, potential for personal injury / λ < 10-9
Low / During non-critical failure the system has lost some or all functionality. Customer dissatisfaction results. / λ < 10-5
It is assumed that any failure with the potential to cause personal injury should have a probability rate less than 10-9; this is considered to “never” happen. The following High Criticality failures have been identified for this design:
- A2 – software malfunction
- B2 – temperature sensor
- C2 – power supply over-voltage
- C3 – power supply out of tolerance
- D1 – compressor interface failure
- D3 – compressor interface, software malfunction
The probability of these high criticality failures can be reduced with the use of hardware monitoring or by software and watchdogs using the microcontroller. Being a purely analytical exercise, the implementation of such techniques is beyond the scope of this document.
List of References
[1]U.S. Department of Defense, Reliability Prediction of Electronic Equipment,
MIL-HDBK-217F
[2]George Novacek, Designing for Reliability, Maintainability, and Safety,
Circuit Cellar December 2000.
[3]Rabbit 2000 Microprocessor.
[4]National Semiconductor SIMPLE SWITCHER® High Efficiency 3A Step-Down Voltage Regulator. LM2676S-5.0.
[5]Fairchild Semiconductor NPN Epitaxial Darlington Transistor. TIP122.
[6]Potter & Brumfield/Tyco Electronics 5VDC PC Mount Relay. T77S1D10-05.
[7]Microchip Digital Temperature Sensor with SPI™ Interface. TC72.
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