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Evolving Trends in Power Converters for Accelerators Speaker: Anirban De Accelerator Technology Group Focus will be evolution in… • Pulse Step Modulation Technique in Converters • DSP based Controller for Power Converters • Optical Fibre and Optoelectronics Applications Accelerators Labs… Present 1931 1870 • LHC • Lawrence’s 13cm Cyclotron • Crookes tube …a Paradise for Power Converters from the perspective of a Power Converter Technologist… Accelerator High Current Magnet Systems High Voltage Electrode Systems High Power Amplifier Systems Illustration with a Cyclotron • Injection System – • Magnet System – • Confinement of accelerated beam RF System – • Introducing the ionized beam to the central region Beam acceleration Extraction System – Taking out the energetic beam to be used for experiments Why do we require Power Converters? • Requirement: Accelerators require DC Power Sources of varied ratings – Not conventional AC power What is so Critical about them? • Two illustrations – K-500 SCC Beam Stability requires 1000A DC Main Magnet current not fluctuating by greater than 10 parts per million = 10mA – The Deflector Power Supply is designed for -100kV output with fluctuation < 0.005% = 5V only The precision aspect was initially addressed by Linear Power Supply (LPS) Illustration: A Constant Voltage DC Regulated Power Supply Regulator Input AC AC to DC Conversion Filter Feedback & Control to Load Schematic to Load Simplified Regulator Input AC AC to DC Conversion Filter Feedback & Control Principle of regulation: dissipating excess power in the regulator Merits of LPS • • • • • Extremely Precise (clean) Output Voltage/Current achieved Excellent transient response Simple Mild high frequency interference due to rectifier switches Inexpensive Disadvantages of LPS • Continous dissipation of excess power in series regulator – Very poor efficiency < 50% • Rectifier introduced multiples (1x, 2x, 6x, 12x, 24x only) of power line frequency (50/60Hz) – Cut-off frequency for a LC filter = 1 2 LC , requires large L &/or C • Input transformer supplying output power + losses, operates at 50/60 Hz – Bulky Introduction of Switching Mode Power Supply (SMPS) Regulation achieved by changing duty ratio of switching Merits of SMPS • Loss is only during switching – Efficiency is higher • Attenuation of harmonics in the range of (tens of kHz) – Filter size considerably diminishes than its LPS counterpart • Output transformer operates at switching frequency – Lighter ferrite core transformer • Portability increases due to decrease in size Disadvantages of SMPS • Increased complexity of control • Increased EMI/RFI • Expensive Disadvantage of Topology • SMPS output stage always consists of a LC • These devices store energy in the form of 0.5LI2 and 0.5CV2 • So a high voltage output at hundreds of kilovolts will force the output capacitor to store energy ~ 100s of joules • In case of an internal arc in the load end, this energy will be deposited to the load, causing irreparable damage to it Accelerator required Higher Power RF Amplifiers of increasing efficiency • Klystron and Inductive Output Tube (IOT) was introduced – This required HV Power Supplies • The voltage boosting was now not a problem – But the protection against internal tube arcing imposes a stiff criteria on response time of the crowbar Solution • Increase frequency to decrease capacitor value Problem • Increased switching loss due to increase in frequency – Recovery time of switches becomes a constraint • Inductor core fabrication at such high frequency becomes costlier • Increased effect of ESL of capacitors Pulse Step Modulation: A Novelty • Though resonant converters, ZVS/ZCS switching were being explored but for higher voltages a novel solution came by adopting PSM • Introduced by Thomson & Multimedia, Switzerland during 1980s as solid-state replacement for audio modulators in radio broadcast transmitters PSM Illustration with 4 Modules • Rectifier + filter of each Switched Module outputs Vdc • Only a single power electronic switch (IGBT) / module • The modules are series connected a power diode • Each module switch at the same frequency, f = 1/T • Each module switching is phase shifted by T/N PSM Illustration….contd Duty Cycle = 62.5% N4 T phase shift 0.25T 4 Duty cycle of each module, t ON D 0.625 T Load output 2.5Vdc D x N x Vdc peak to peak Ripple Vdc Ripple freq 1 4f N x f 0.25T PSM Illustration….contd Duty Cycle = 62.5% N4 T phase shift 0.25T 4 Duty cycle of each module, t ON D 0.625 T Load output 2.5Vdc D x N x Vdc peak to peak Ripple Vdc Ripple freq 1 4f N x f 0.25T PSM Illustration….contd Duty Cycle = 75% N4 T phase shift 0.25T 4 Duty cycle of each module, t ON D 0.75 T Load output 3.0Vdc D x N x Vdc peak to peak Ripple 0 PSM Illustration: Observations Compared to a single SMPS, for the same output voltage • PSM output ripple magnitude is limited to Vdc /N and in some cases even vanishes ideally • f↑ → C↓ → lower stored energy → crowbarless operation → easier protection just by cutting off gate drive • Though output frequency is Nf, each power switch is subjected to frequency f only, so no requirement of faster switches PSM: Regulation • Among the several philosophies that has evolved – Output regulation may be achieved by PWM modulation of all the modules at once – If MVdc is the required output • (M-1) modules are kept ON continuously → Coarse modulation • Mth module is PWM controlled → Fine modulation – Several other techniques evolving from standard SMPS control philosophies PSM : Added advantage • • • Inherent modularity helps in generalized design, faster production and efficient maintenance Provision of extra modules increases redundancy and decreases downtime Application became more robust Implementation of control strategy in evolving Converters: A new challenge LPS Simple SMPS Control Complexity PSM type units SMPS and PSM: an interesting difference with LPS • In LPS, regulated output is an amplified version of the control output – Actual output linearly corresponds to bias at base-emitter junction of the regulating transistor • In SMPS and PSM, regulated output is average determined by frequency and duty ratio of actual control output (ON/OFF type) – while gate of MOSFET / IGBT are switched ON and OFF the actual output depends on the relative period of ON and OFF or on the relative variation of one period with the next How this may help? • As the actuator signal to the power switches are binary type, a digital circuitry can be used to generate the firing pulses • In that case, the logic of the firing can be computed by a microcomputer Additional support required from digital data acquisition circuits Migration from Analog to Digital Govering Differential Equation VOUT ( t ) Multiply and Sum/Accumulate (MAC) 1 VIN ( t )dt RC How to Compute? (Fast & Efficient Processor) PastDigital Instance VOUT (n) c1VIN (n) c 2 VIN (n 1) d1VOUT (n 1) Derived from R, C Data Conversion & Acquisition & sample time ( Fast ADC) Present Instance Data Storage (Memory) ADC Requirement: Data Acquisition High freq components missed in slower sampling Slower Sampling Faster Sampling Data Acquisition rate to be chosen so as not to miss the Highest Frequency Component of the Data @ Nyquist Rate ADC: Resolution Bits ↑ → Resolution ↑ → Conversion Time ↑ →Speed ↓ ADC: Architecture • Optimization between resolution, speed, cost 1921 1954 1969 1977 Now • Electo-opto-mechanical • 5-bit Flash type • Vacuum Tube system • 11-bit 50ksps SAR type • Semiconductor based • 12-bit 10ms SAR ADC • Integrated Circuits • Hybrid ADC • ADCs are integrated inside controller chips • 16-bit, 18-bit, 24-bit sigma-delta, pipeline, time-interleaved, time-stretched, 12.5MSPS Microcontroller with ADC integrated Evolution of Processor Technology Von Neumann architecture Harvard architecture Modified Harvard & Super Harvard architecture Evolution of Digital Controller Technology to match Power Converter Requirement 8-, 16-bit mC 32-bit Digital Signal Controllers Fixed Point Processors Multiprocessor DSPs Floating Point Processors 32 x 32 MAC, 16 x 16 Dual MAC 30MHz Clock 150MHz Clock, single cycle instruction 16 bit Timer 4 PWM channels 32 bit Timer 16 PWM channels 18 PWM channels Processing Power Processing Precision Processing Speed / BW Processing Speed / BW Duty Cycle Precision Switching Power Modules Control complexity addressed by • Switching to Digital control Advantages • Flexibility: Easy to configure & reconfigure by firmware • Static Operation: Less prone to ageing & environmental influences. • Scaling: Programme can scale to the limits of memory & storage space. • Adaptive: Firmware can be made self-tuning with time. • Non-linear control: Easier implementation of non-linear control algorithm. • Cheap. Next challenge • • • • • Output @ kV → MV Regulation @ 3.3V → 1.8V Control Room Supervision @ communication level How to bridge/isolate this huge gap? Transformers can isolate AC but how to isolate DC signals that are more abundant? • Solution in Optoelectronics & Fibre Optic technology Basic Requirement Field Mains Output Feedback Isolation Supervision Power Electronic Switch Isolation Isolation Control Room Regulation and Control Optocoupler A light source (LED) that converts input electrical signal to light Electrical i/p Linearity Problem! Light Electrical o/p Output photosensor detects the incoming light and modulates the electrical current flowing through it Linearity Problem solved to some extent Photo-compensation networks inside IC gave linearity ~ 0.01% Advantages if we go Digital •High Current Transfer Ratio ~100% to 600% (in photo-darlington configuration / integrated driver systems) allowed its use in efficient Digital Data Transfer •Voltage withstand capability ~ 10kV to 50kV and surge capability of 10kV/ms allowed extensive use in HV units •Response time ~ nsec (fastest being PIN photodiode in photoconductive mode) helped in high speed applications Optical fibre Light output Light Input Optical guide following the principle of total internal reflection to contain the light beam Compared to electrical communication, optical communication provides advantages like… Less Signal Attenuation High Data Rate Higher BW EMI/RFI Immunity Simple and Cheaper Higher Availability Complete Galvanic Low Power Lightweight Isolation Application philosophy • Convert the electrical signal to optical signal by LEDs • Transmit the light signal through optical link • Revert back to electrical signal using optical sensors Implementation Field Isolation Supervision Mains Power Electronic Switch Output Feedback Optical fibre links terminated by Optocouplers Linear Optocouplers Control Room Switching Optocouplers and/or Optical fibre links Isolation Isolation Regulation and Control Summary of Evolution Electrical Engineering Optoelectroni cs Power Electronics Power Converter Optical Fibre Communicati on Analog Instrumentati on Digital Control Activities of Accelerator Technology Group in These Fields • Digital Controller based Dynamic Voltage Restorer (both single & three phase) for Sag Mitigation in SMES Project Mains Load 1-f DVR 3-f DVR Activities of Accelerator Technology Group in These Fields • 50 x 800V, 5A PSM based Power Converter (200kW) for IOT of SCRF Linac Cavity Project Activities of Accelerator Technology Group in These Fields Response Trigger • Optical Fiber based ~1ms Crowbar Unit with Fast Acting MOSFET for High Voltage Power Supply References • • • • • • • • • • • • • • • • • • • • • • • http://www.wikipedia.org/ http://www.nobelprize.org Anirban De, “Power Supplies for Different Systems at VECC and their safety” http://www.kepcopower.com/fowler.htm http://www.powerqualityworld.com/2011/07/switched-mode-power-supply-smps.html http://www.engineersgarage.com/articles/smps-switched-mode-power-supply?page=2 P.J. Patel, D. P. Thakkar, L.N. Gupta, V. B. Patel, V. Tripathi, N.P.Singh and U.K. Baruah, “A Regulated Power Supply for Accelerator Driven System” http://pemclab.cn.nctu.edu.tw/peclub/w3cnotes/cn06.%E9%9B%BB%E5%8A%9B%E9%9B%BB%E5%AD%90%E7%B0%A1%E4%BB%8B/html/cn06.ht m J. Alex, M. Bader, J. Troxler, Thomson Broadcast & Multimedia, Turgi, Switzerland, “A NEW KLYSTRON MODULATOR FOR XFEL BASED ON PSM TECHNOLOGY”, Proceedings of PAC07, Albuquerque, New Mexico, USA Paul Scherrer Institut, “Modern and Crowbarless HVPS”, Fourth CW and High Average Power RF Workshop / May 2006 / W. Tron http://www.maxim-ic.com/app-notes/index.mvp/id/733 http://www.technologyuk.net/computing/software_development/programming_languages.shtml http://andrewharvey4.wordpress.com/2009/03/13/comp2121-wk01/ http://www.beis.de/Elektronik/DeltaSigma/DeltaSigma.html http://www.ni.com/white-paper/9078/en http://www.ti.com/product/msp430afe253 Anirban De, “Design of a Generalized and Modular Architecture for Embedded Controller for Power Supplies”, SACET09 http://www.digikey.com/us/en/techzone/lighting/resources/articles/adding-intelligence-and-flexibility.html http://computer.howstuffworks.com/fiber-optic4.htm Mohammad Towhidul Islam, “Fundamentals of Optical Fiber Systems”, North South University http://www.analog.com/library/analogDialogue/archives/39-06/Chapter%201%20Data%20Converter%20History%20F.pdf Acknowledgements Organizers, UFCYC12, Kum. Santwana Kumari, Shri M.L.V. Krishnan, Shri Samit Bandyopadhyay, Shri S.K. Thakur, Shri Subimal Saha Thank You Very Much