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Digitally Controlled Converter with Dynamic Digitally Controlled Converter with Dynamic Change of Control Law and Power ThroughputChange of Control Law and Power Throughput
Carsten Nesgaard Michael A. E. Andersen Nils Nielsen
Technical University of Denmark
in collaboration with
2
OutlineOutline
• Power system specifications
• The microcontroller
• Control algorithm and efficiency
• Analytical redundancy concept
• Reliability
• Experimental verification
• Further work
• Conclusion
3
Power system specificationsPower system specifications
• Simple buck topology with measurements of input voltage, input current, output voltage and output current
• Microcontroller for converter control and thermal monitoring
Power switch Filter
PIC16F877microcontroller
12V Input 5V Output
Temp
Duty-cycle
Input current
Input voltage
Output current
Output voltage
1A MAX
4
The microcontrollerThe microcontroller
8-bit RISC PIC16F877 microcontroller from Microchip
Core features: Uses:
8K 14-bit word flash memory 256 E2PROM data memory
10-bit PWM module8 channel 10-bit A/D converter
Single cycle operations20 MHz clock frequency
Algorithm and look-up table
Converter control
Execution speed
5
Control algorithm and efficiencyControl algorithm and efficiency
• Simple buck topology with measurements of :
• Thermal monitoring
• PWM control law for power throughput above 1.85 W
• PS control law for power throughput below 1.85 W
• Look-up table control when operated within specifications
Input voltage Input current Output voltage Output current
6
Control algorithm and efficiencyControl algorithm and efficiency
Software data flow diagram:
System init
Measure inputvoltage
ADC interrupt
If n=100measure
temperature
Timer interrupt
Convertercontrol in'real-time'
Checktemperatureand deduce
converter state
Shut-downconverter
Measure V OUT ,V IN , I OUT , I IN andcalculate power
Change incontrol law
Main
Interrupt routine
Request sample
Control law
Sample
Outside spec.
Within spec.
Within spec.
Within spec.
Outside spec.
Outside spec.
Converter OK
Converter failed
Interrupt routine responsible for correct converter control
Main loop responsible for temperature measurement, cal-culation of correct control law and type of calculation method (look-up or real-time)
7
Analytical redundancy conceptAnalytical redundancy concept
Examples:
• Converter efficiency is related to system temperature
• Output voltage is related to the inductor current
Result:
• Continuous converter operation (at a deteriorated level)
Analytical redundancy is the concept of deducing a set of variables able to accurately describe the actual system behavior
8
Case temperature vs. output current
0
20
40
60
80
100
120
140
160
0 0,2 0,4 0,6 0,8 1 1,2
Output currentT
em
pe
ratu
re
T Sense
No heatsink
The above graph is used to determine converter state h
Analytical redundancy conceptAnalytical redundancy concept
Minimizing the risk of shutting down a well-functioning converter
In the event of a fault in PWM mode:
9
Analytical redundancy conceptAnalytical redundancy concept
The system is only partially fault tolerant due to:
• Resilience towards faults described by the mathematical system• Single converter system – one path from input to output
Further improves in system reliability require hardware redundancy
Example:Increased reliability
Increased costIncreased complexitySingle transistor Transistor array
10
Analytical redundancy conceptAnalytical redundancy concept
Further advantages of analytical redundancy:
• Fault indicator in hardware redundant systems
Continuously comparing theoretical system constraints with actual system behavior
Enables the system to respond intelligently to unusual system behavior
Increasing the overall system fault resilience
11
T Surface
T Surface - 10°C
T Surface - 30°C
1 resistor1 MOSFET
5 resistors1 IC1 inductor
1 diode
4 capacitors
1 resistor4 diodes2 capacitors
8 resistors3 transistors4 capacitors
Printed circuit board
ReliabilityReliability
Temperature distribution used for reliability assessment:
Probability of survival as a function of time:
Reliability data found in MIL-217 (assumes a constant failure rate)
t-e R(t)
12
Failure rates for the two configurations:
Analog configuration
Digital configuration
Failure rate in FIT
From a reliability point of view:
At temperatures below 120C an analog controller is preferable At temperatures above 120C a digital controller is preferable
Failure rate ( )
10000
8000
6000
4000
2000
Temperature20 40 60 80 100 120
ReliabilityReliability
13
Survivability R(t) for 10,000 hours:
Analog configuration Digital configuration
The digital configuration is 36 times more likely to fail within 10,000 hours than its analog counterpart.
ReliabilityReliability
0.2
Temperature20 40 6080 100 12060
0.4
0.6
0.8
1.0
R(t)
0.975
Temperature60 80 9070
0.965
0.980
0.985
0.990
R(t)
0.970
14
Converter efficiency:
The arrows indicate direction of change in control law
The hysteresis loop prevents oscillatory converter behavior when operated close to the optimum point of transition.
70
72
74
76
78
80
82
0,25 0,3 0,35 0,4 0,45Output current
Eff
icie
nc
yExperimental verificationExperimental verification
15
PWM:
PS:
Experimental verificationExperimental verification
Gate-Source voltage Output voltage
16
Inductor current Input voltage
PWM:
PS:
Experimental verificationExperimental verification
17
Further workFurther work
Q L
C
VPWM
I
T D
1 2 3
4
5
6
9 7
8
V in VOUT
1 2 3 4 5 6 7 8 9
1 0 Q 0 0 0 Q 0 0 0
2 0 0 L 0 0 Q I 0 0
3 0 0 0 C 0 0 0 V 0
4 0 D C 0 0 0 0 0 0
5 0 Q 0 0 0 Q 0 0 0
6 0 0 0 0 0 0 0 0 T
7 0 0 0 0 P 0 0 0 0
8 0 0 0 0 P 0 0 0 0
9 0 0 0 0 P 0 0 0 0
• Graph theoretical approach is used for thorough system analysis
• Columns identify the lines interconnecting the individual blocks
• Line arrows indicate direction of power or data flow
Block level buck converter
18
ConclusionConclusion
A buck converter controlled by a low-cost PIC microcontroller has been presented. The system use analytical redundancy, change in control law and thermal monitoring for increased reliability.
Also, an introduction to the proposed techniques has been given supported by calculations concerning the pros and cons of the individual techniques.
Finally, a set of measurements has verified that the algorithm is indeed capable of performing the required tasks within the timing limitations of the microcontroller.