Transcript Power Converters in Accelerators

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
•
•
•
•
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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%
N4
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%
N4
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%
N4
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
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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
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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