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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

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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

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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

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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

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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)

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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

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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:

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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

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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

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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)

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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

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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

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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

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PWM:

PS:

Experimental verificationExperimental verification

Gate-Source voltage Output voltage

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Inductor current Input voltage

PWM:

PS:

Experimental verificationExperimental verification

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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

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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.