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DESIGN & DEVELOPMENT OF POWER SUPPLIES FOR HIGH POWER
IOT BASED RF AMPLIFIER
Yashwant Kumar*, S. Kumari, M.K. Ghosh, A. Bera, A. Sadhukhan, S. S. Pal, V.K. Khare, T.P.
Tiwari, S.K. Thakur, S. Saha
Variable Energy Cyclotron Centre, 1/AF Bidhannagar, Kolkata, India
Abstract
Design, development, circuit topology, function of
system components and key system specifications of
different power supplies for biasing electrodes of Thales
Inductive Output Tube (IOT) based high power RF
amplifier are presented in this paper. A high voltage
power
supply (-30 kV, 3.2A dc) with fast
(~microsecond) crowbar protection circuit is designed,
developed and commissioned at VECC for testing the
complete setup. Other power supplies for biasing grid
electrode (300V, 0.5A dc) and Ion Pump (3kV, 0.1mA
dc) of IOT are also designed, developed and tested with
actual load. A HV Deck (60kV Isolation) is specially
designed in house to place these power supplies which are
floating at 30kV. All these power supplies are powered by
an Isolation Transformer (5kVA, 60kV isolation)
designed and developed in VECC.
INTRODUCTION
The
development
of
multi-cell
medium-
Superconducting RF linac cavity has been taken up at
VECC as a R&D project in line with various accelerator
programmes (Indian SNS, ADSS and advanced RIB) of
DAE.As a part of this project, the design and
development of high power RF amplifier has been done
based on IOT.
The state-of-the-art technology of IOT based high
power RF amplifier developed and tested at VECC is the
first of its kind in India. The amplifier can be operated at
704 MHz/650 MHz at maximum output RF power of 60
kW at 50 Ohm load with proper tuning of its primary and
secondary cavities. The necessary high voltage power
supply (-30 kV, 3.2A dc) with fast (~µs) crowbar
protection circuit for biasing Cathode of the IOT is
designed, Installed and commissioned. The IOT fed by
the -30kV/3.2A power supply, is required to be protected
against the internal arcing. The inductive output tubes are
prone to internal arc that can lead to a permanent damage
of it, if stored energy is not removed or diverted through a
switch to ground after occurrences of fault. Thus to
bypass the energy, a crowbar system (with shunt diverter
topology) is used.
Other power supplies for biasing Grid, Ion Pump and
Filament electrodes are connected with respect to the
cathode of the IOT and floating at high voltage (30kV). A
HV Deck is specially designed in house to place these
power supplies which are floating at 30kV. Inside the HV
Deck, these power supplies are powered through an
Isolation transformer. All these power supply are
*[email protected]
controlled by a PLC, which communicates through an
Optical Fibre link to the PC.
DESIGN AND TOPOLOGY
Anode Power Supply
This is a -30kV, 3.2A primary regulated (by SCR) DC
power supply. It incorporate two numbers of power
transformers, each rated at 100 kVA, 415V/12kV, 3phase, 50Hz, vector group Dy11- d0 and force air cooled
operating in 12 pulse mode. The 3-phase power is
connected to each of two transformers via air core choke
and back to back SCR in each line through fast
interrupting circuit breakers. The SCR bank at the input
of the Transformer is used in closed loop as a primary
regulating device. Rectified outputs (15 kV each) of both
transformer’s secondary are connected in series and then
a LC filter (400uH, 1.9uF/60kV capacitor) is used to keep
the ripple in desired limit. The special feature of this
power supply is Crowbar Protection system against
internal arcing in the IOT by using high voltage Ignitron.
Other protections i.e. over current, over voltage, phase
failure, over temperature etc. have been incorporated and
tested with the IOT. The schematic diagram and
photographs of anode power supply shown in Fig. 1and
Fig. 2 respectively.
Figure1: Schematic of anode power supply
Figure 2: Photographs of anode power supply
Development of Crow bar Protection System for
HV anode Power Supply
The crowbar protection system, which is an integral
part of the anode power supply was designed and
developed in VECC. It involves ignitron NL7703EHV as
a switching device rated at 50kV and current transformer
having sensitivity of 0.01V/A with response time 20ns
installed in ground line of power supply for sensing rate
of change of current during fault. When IOT draws the
more current or arc occurs, CT senses the rate of change
of current and fed to the electronic control card which
gives a tripping signal. This signal triggers a SCR through
optical fiber to discharge a charged capacitor (~5J energy)
through a pulse transformer for the generation of fast
rising pulse of 2-3 kV peak. This is used to trigger the
ignitron which short circuit the power supply to protect
the IOT. The same tripping signal is used to trip main
circuit breaker and withdraw the gate pulse of SCR to
protect the power supply. Isolation between control
circuit and power circuit has been done by using optical
fiber. Fig. 3and Fig. 4 shows the schematic diagram and
photographs of crowbar protection system.
switched at high frequency (20 kHz) by push-pull inverter
circuit to give high frequency AC. This AC voltage is fed
to primary terminals of ferrite core transformer to step-up
from 0-50V to 0-1.5kV AC. This is further multiplied to
achieve 3kV DC by using Cockcroft-Walton voltage
multiplier. The voltage regulation is achieved by
comparing the sample of the output voltage with stable
reference voltage. The error signal is fed to the built in
error amplifier of PWM IC (SG3525) which change the
duty cycle of pulse according to error signal. These pulses
are fed to gates of MOSFETs which used as a push-pull
inverter. Fig. 5 and Fig. 6 shows schematic diagram and
photo graphs of ion pump power supply respectively.
Figure 5: Schematic of Ion pump power supply
Figure 3: Schematic diagram of Crowbar Protection
System
Figure 4: Photographs of crowbar protection system
Figure 6: Photographs of Ion Pump Power Supply
Ion pump Power Supply
Grid Power Supply
This is voltage regulated switch mode power supply for
ion pump of IOT and works on the principle of pulse
width modulation (PWM) technique. The power supply is
operated at high frequency (20 kHz) hence the size and
weight of magnetic component, filter capacitors are
reduced with improved efficiency. A single phase line
voltage (230AC, 50Hz) is fed to step down transformer to
achieve 40 V AC. This AC voltage rectified and filtered
to get around 50 V DC. The rectified dc output is then
This power supplies provide negative bias voltage
(adjustable from 0 to -300V) to the control grid of IOT. In
this power supply 230V AC is stepped up to 350V AC by
a transformer. The transformer secondary voltage is
rectified and filtered. Regulation is achieved by using
IGBT in common emitter configuration.
This power supply is kept on high voltage deck and
tested with actual load and running smoothly without
problem. Fig. 7: shows schematic of Grid power supply.
Figure 7: Schematic of Grid power supply
High Voltage Deck with Isolation Transformer
Isolation transformer design & developed for auxiliary
power supplies placed in high voltage deck for biasing
IOT. This is a single phase, 4.5 kVA, 230V/230V, dry
type, air cooled resin cast transformer design at flux
density of 1.25 Tesla. Both the coils wound on fibre glass
bobbin at current density of 1.3A/mm2. Low current
density is taken to keep temperature rise of the
transformer within the limit. A 5 mil Nomex paper is
provided for inter layer insulation and total coil wrapped
by two layer of Kapton tape to give extra reinforcement
of the coil.
To achieve 60kV isolation between secondary to
primary and core, secondary coil encapsulated by dr. beck
brand dobefil -60 & hardener 758 epoxy resins of 20mm
thick. This dr. beck brand epoxy resin has good thermal
conductivity, high dielectric strength and better for
continuous running of the transformer.
We conducted routine test viz. open circuit test, short
circuit tests and percentage impedance test of the
transformer. Herewith we have specially checked
dielectric strength of the transformer up to 80kV as per IS
2026 with precision grade testing equipments. Fig. 8
shows photographs of high voltage deck with isolation
transformer.
CONCLUSION
The high power IOT-based amplifier along with power
supplies and interlocks etc. has been installed and already
tested up to 40 kW with 50 ohm water-cooled dummy
load at 704 MHz/650MHz.
ACKNOWLEDGEMENT
The authors acknowledge the support of Shri Sumit
Som and Shri Aditya Mandal of RF Division, VECC,
Kolkata.
REFERENCES
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Kumar, M.L.V Krishnan, T.P. Tiwari, V.K. Khare,
S. Sinha, S.S. Pal, A. Sadhukhan, S.K. Thakur,
Manoranjan Das, Subimal Saha,” “Power Supplies
for the RF-System of K=130 Cyclotron at VECC,
Kolkata”, Proc. of Indian Particle Accelerator
Conference (InPAC) - 2011, IUAC, Delhi, Feb 1518, 2011.
[2] S. K. Thakur, T. P. Tiwari, J. S. Prasad, A. De, Y
Kumar, S. Som, S. Saha, R. K. Bhandari.
“Development of Power Supplies for 3-Ф, 240 KW
RF System with Crowbar protection for
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2010.
[3] Yashwant
Kumar,
J.S.Prasad,
S.K.Thakur,
T.P.Tiwari, S.Saha. “Design and Development of HV
power supply with Crowbar Protection for screen
electrode of RF amplifier”. Presented in AE-BRNS
Indian Particle Accelerator conference- 2009,INDIA.
Figure 8: Photographs of high voltage deck with isolation
transformer.