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Digitally Controlled Converter with Dynamic
Change of Control Law and Power Throughput
Carsten Nesgaard
Michael A. E. Andersen
Nils Nielsen
Technical University of Denmark
in collaboration with
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Outline
•
Power system specifications
•
The microcontroller
•
Control algorithm and efficiency
•
Analytical redundancy concept
•
Reliability
•
Experimental verification
•
Further work
•
Conclusion
2
Power system specifications
•
Simple buck topology with measurements of input voltage,
input current, output voltage and output current
•
Microcontroller for converter control and thermal monitoring
12V Input
Power switch
5V Output
Filter
1AMAX
Duty-cycle
Temp
Input current
Input voltage
Output current
PIC16F877
microcontroller
Output voltage
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The microcontroller
8-bit RISC PIC16F877 microcontroller from Microchip
Core features:
Uses:
8K 14-bit word flash memory
256 E2PROM data memory
Algorithm and look-up table
10-bit PWM module
8 channel 10-bit A/D converter
Converter control
Single cycle operations
20 MHz clock frequency
Execution speed
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Control algorithm and efficiency
•
Simple buck topology with measurements of : Input voltage
Input current
Output voltage
Output current
•
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
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Control algorithm and efficiency
Software data flow diagram:
System init
Measure input
voltage
Main
Interrupt routine responsible
for correct converter control
Main loop responsible for
temperature measurement, calculation of correct control law
and type of calculation method
(look-up or real-time)
Interrupt routine
ADC interrupt
Converter
control in
'real-time'
Within spec.
If n=100
measure
temperature
Outside spec.
Shut-down
converter
Request sample
Within spec.
Sample
Timer interrupt
Outside spec.
Measure VOUT,
VIN, IOUT, IIN and
calculate power
Control law
Converter failed
Outside spec.
Check
temperature
and deduce
converter state
Within spec.
Converter OK
Change in
control law
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Analytical redundancy concept
Analytical redundancy is the concept of deducing a set of variables
able to accurately describe the actual system behavior
Examples:
•
Converter efficiency is related to system temperature
•
Output voltage is related to the inductor current
Result:
•
Continuous converter operation (at a deteriorated level)
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Analytical redundancy concept
Case temperature vs. output current
160
TSense
140
Temperature
120
No heatsink
100
80
60
40
20
0
0
0,2
0,4
0,6
0,8
1
1,2
Output current
In the event of a fault in PWM mode:
The above graph is used to
determine converter state
h
Minimizing the risk of
shutting down a wellfunctioning converter
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Analytical 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:
Single transistor
Increased reliability
Increased cost
Increased complexity
Transistor array
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Analytical 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
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Reliability
Temperature distribution used for reliability assessment:
TSurface - 10°C
TSurface - 30°C
TSurface
Printed circuit board
1 resistor
4 diodes
2 capacitors
5 resistors
1 IC
1 inductor
1 diode
4 capacitors
1 resistor
1 MOSFET
8 resistors
3 transistors
4 capacitors
Probability of survival as a function of time: R(t)  e- t
Reliability data found in MIL-217 (assumes a constant failure rate)
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Reliability
Failure rates for the two configurations:
Failure rate ( )
10000
Analog configuration
8000
Digital configuration
6000
4000
Failure rate in FIT
2000
20
40
60
80
100
120
Temperature
From a reliability point of view:
At temperatures below 120C an analog controller is preferable
At temperatures above 120C a digital controller is preferable
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Reliability
Survivability R(t) for 10,000 hours:
R(t)
R(t)
1.0
0.990
0.8
0.985
0.6
0.980
0.4
0.975
0.2
0.970
60
20
40
60
60
80
100
Temperature
70
80
90
Temperature
0.965
120
Analog configuration
Digital configuration
The digital configuration is 36 times more likely to fail within
10,000 hours than its analog counterpart.
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Experimental verification
Converter efficiency:
82
80
Efficiency
78
The arrows indicate
direction of change
in control law
76
74
72
70
0,25
0,3
0,35
Output current
0,4
0,45
The hysteresis loop prevents oscillatory converter behavior when
operated close to the optimum point of transition.
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Experimental verification
Gate-Source voltage
Output voltage
PWM:
PS:
15
Experimental verification
Inductor current
Input voltage
PWM:
PS:
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Further work
•
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
1
2
Q
L
6
Vin
T
5
D
C
4
I
7
9
PWM
8
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
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
3
V
Block level buck converter
VOUT
C 0
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Conclusion
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.
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