Digitally Controlled Converter with Dynamic Change of Control

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 1

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 3

The microcontroller 8 -bit RISC PIC 16 F 877 microcontroller from Microchip Core features: Uses: 8 K 14 -bit word flash memory 256 E 2 PROM 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 4

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 5

Control algorithm and efficiency Software data flow diagram: 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) 6

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) 7

Analytical redundancy concept No heatsink 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 8

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: 9

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 10

Reliability 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) 11

Reliability Failure rates for the two configurations: Analog configuration Digital configuration Failure rate in FIT 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 12

Reliability 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. 13

Experimental verification 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. 14

Experimental verification Gate-Source voltage Output voltage PWM: PS: 15

Experimental verification Inductor current Input voltage PWM: PS: 16

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 Block level buck converter 1 2 3 4 5 6 7 8 9 1 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 5 0 Q 0 0 0 6 0 0 0 0 T 7 0 0 P 0 0 8 0 0 P 0 0 9 0 0 P 0 0 17

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. 18