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AN LCLC RESONANT TOPOLOGY BASED FILAMENT POWER SUPPLY
FOR 300 KEV DC ACCELERATOR
A. Kasliwal , V.K. Gauttam, R. Banwari, T.G. Pandit, A.C. Thakurta
Power Supplies and Industrial Accelerator Division, Raja Ramanna Centre for Advanced
Technology, Indore 452 013, India
Abstract
A compact, low energy dc accelerator for industrial
applications requiring beam energy in the range of 100 to
300 keV is under development at RRCAT, Indore. The
accelerator uses an indirectly heated LaB6 disc type
filament of 4 mm diameter as an electron emitter which is
floating at terminal voltage of the accelerator. A power
supply is required to heat the filament for its full range of
emission. A high frequency inverter operating at fixed
frequency feeds the power to the filament through high
frequency transformers and capacitive isolation column.
A buck chopper controls the dc bus voltage of the inverter
so as to control the terminal voltage of the filament thus
controlling the beam current.
This paper presents the analysis and design of the
filament supply that implements a 40 kHz high order
LCLC series parallel resonant inverter that utilizes the
reflected capacitance of the HV transformer and
capacitive isolation column as its tank circuit component.
The operating characteristics and analysis of series
resonant (SRC), parallel resonant (PRC) and series
parallel (SPRC) resonant converters have been reported
for fixed frequency operation. It has been shown that
SPRC takes the advantage of both SRC and PRC
curtailing their disadvantages. Hence a series parallel
LCLC combination has been used as it gives the
advantage of low device currents and a better load
regulation.
characteristics and sinusoidal voltage and currents,
resonant inverters have proved to be a better proposition.
RESONANT TOPOLOGY
A series resonant inverter has the disadvantage of high
circulating currents that increases the switching device
ratings. Furthermore, a series resonant inverter cannot
actually be developed due to the large parasitic
capacitance generally existing in a high voltage
transformer. As in a SPRC the output is controllable for
no-load or light-loads, and light load efficiency is better, a
series parallel LCLC combination has been used.
The converter utilizes the leakage inductance and the
interwinding capacitance of the high voltage transformer
it feeds to and the parasitic capacitance of the capacitive
isolation column, for its resonant operation. As indicated
in Fig. 1, the selected LCLC resonant network provides
two resonant peaks at 23 kHz and 59 kHz and load
independent response at operating frequency of 40 kHz.
This LCLC resonant tank functions to position zero
voltage across the switching device prior to turn on,
eliminating any power loss due to the simultaneous
overlap of switch current and voltage at each transition.
The 12 stage capacitive isolation column basically
consists of two columns of high voltage capacitors
connected in series to achieve high voltage isolation of
300 kV. This isolation column is fed from a step up
centre tapped high voltage transformer which imposes a
high distributed capacitance.
INTRODUCTION
Hard switching PWM inverters for high voltage
application need to feed a very high voltage transformer.
The HV transformer requires a relatively large spacing
between the primary and secondary windings, which
leads to a relatively large leakage inductance. In this case,
it is generally difficult to employ a PWM type inverter
due to output voltage lost during the reversal of current in
the large leakage inductance of a high voltage
transformer. On the other hand, an operating frequency
higher than 30 kHz is generally desired to reduce voltage
drop across the capacitive isolation column and to avoid
any audio frequency noise that may be generated. This in
turn improves the load regulation of the system. But it
also enhances the effect of parasitic components. A
sinusoidal voltage is desired as high frequency harmonics
in the square wave voltage impose high dv/dt stress. For
lower switching losses, high power density, better EMI
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Figure 1: Frequency response of the LCLC network.
OPERATION
When measured at the primary of the HV Transformer
with the capacitive isolation column connected at its
secondary,
INVERTER
FILAMENT
-300kV
C2:REFLECTED CAPACITANCE
L1
~
C1
L2
C2
BUCK CONVERTER
300kV ISOLATION COLUMN
RESONANT NETWORK
INVERTER
FILAMENT
Figure 2: Basic power scheme of the supply.
the reflected load at inverter output was found to be
purely capacitive with a value of
74 nF. The
compensating inductance required then was 250 µH to
keep the resonating frequency very close to switching
frequency of 40 kHz. This selection kept the no load
circulating current demand at the switch end to the
minimum possible. A full bridge topology with fixed
frequency operation is used in this inverter. The diagonal
switches are driven together alternately to place an AC
voltage across the resonant tank. Power is only transferred
to the output section during the ON times of the switches.
When the switch pair, which is currently on is switched
off the primary current flows into the switch output
capacitance causing the switch drain voltage to resonate
to the opposite input rail. At the same time a current flows
through the output capacitance of the other pair of the
switch discharging it to zero voltage enabling zero voltage
switching (ZVS) upon its turn on. Rather than turn on the
switch instantly when the zero voltage is attained, the
switch is held off while the primary current circulates
through the body diode. This has been done to achieve the
ZVS condition for the full range of operation of the
inverter. The resonant inductive energy was sufficient to
discharge the capacitor before the device was put on thus
ensuring the ZVS operation. The inverter output is varied
by varying the input DC, which is tapped from a buck
converter. The switching frequency is kept at 40 kHz. The
driver card designed for the switching device blocks the
firing pulses in case the capacitor is not completely
discharged by the load current. The inbuilt anti-parallel
diode of the IGBT module has been used to provide path
for the freewheeling current.
POWER CIRCUIT
The required accelerating voltage for this accelerator is
generated using high frequency cockraft walton based
multi doubler circuits. A 30 kHz, 10 kW driver inverter
feeds the power to the multiplier column through a centretap ferrite core high voltage transformer.
The accelerator uses an indirectly heated LaB6 disc
type cathode of 4 mm diameter as an electron emitter
which is floating at the accelerating voltage. The filament
power supply has been designed and developed with a
maximum rating of 5 V and 20 A. The supply as shown in
Fig. 2 comprises of a buck converter, a 30 kHz fixed
frequency inverter, a centre tapped ferrite core step up
transformer, a 12-stage capacitive isolation column and a
ferrite core step down transformer. The output voltage of
the inverter is stepped up through a 10 kV high voltage
transformer and fed to the isolation column, which
provides required DC isolation of 300 kV between the
inverter and electron emitter. The AC voltage available at
the top of the isolation column is stepped down and fed to
the cathode heater directly. An emission stabilising circuit
controls the output of the buck chopper in a feed back
loop so as to stabilise the emission current.
CONCLUSION
The supply was successfully integrated with the
electron gun of the accelerator and tested for its full
power long term operation with actual filament load.
Beam trials were then carried out at 300 keV beam
energy.
ACKNOWLEDGMENT
Authors are sincerely thankful to Mr. Rajesh Nagdeve,
Mr. Deepchand, Mr. Nathan Singh for their help in
fabrication, assembly, testing and installation of the
power supply.
REFERENCES
[1] A. Kasliwal , R. Banwari, S. Kotaiah, T.G. Pandit and A.
Upadhyay, “Utilizing multiplier stack’s reflected parasitic
capacitance to achieve ZVS operation of resonant inverter
for 750 keV DC accelerator,” APAC 2007, Raja Ramanna
Centre for Advanced Technology(RRCAT), Indore, India,
THPMA036, p. 677; http://www.JACoW.org.
[2] Ned Mohan,Tore M. Undeland,William P Robbins “Power
Electronics: Converters, Applications , and Design”, 3rd
Edition (2003).