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2kJ/s 1kV, 25Hz PRR Capacitor Charging Power Supply with twin phase shifted
primary windings to achieve high charge transfer rate and stability
Y.Kelkar#, Y.P. singh, A.C.Thakurta
Power Supply and Industrial Accelerator Division, Centre for Advanced Technology, P.O. CAT,
Indore -452 013
Abstract
The capacitor charging power supply (CCPS) was
developed to charge bank of 150uF energy storage
capacitor (15uf , 10nos in parallel) upto 1kV in 35ms
exhibiting a peak charging power of 2kJ/s at a repetition
rate of 25pps. A CCPS observes a large change in load
variations at the output. Initially the capacitor will act as a
short circuit so the topology must be such that it should
withstand short circuit condition repetitively. The High
Voltage capacitor charging power supply consist of two
identical full bridge resonant converters feeding to two
primary windings of a transformer with rectified
secondary
connected to capacitor load. Topology
selection is based on the fact that the series resonant
converter with switching frequency fs, below 50% of the
resonant frequency fr (fs ≤ 0.5 fr) act as a current source.
INTRODUCTION
The CCPS described in the paper will be used to charge
energy storage capacitors of Septum pulse generator
circuit .The complete process is divided into three modes:
(1) High Power charging mode (2) Low Power Refresh
mode (3) Output Pulse delivery and recovery mode.
During High power Charging mode the two resonant
converters operating at fs < 0.5 fr corresponds to
discontinuous mode of operation will feed the two
primary windings of HV step up transformer in same
phase. In this mode of operation all devices are turned ON
and OFF at zero current so switching losses are reduced
to minimum. It also eases critical bearing on switch
commutation time and diode reverse recovery time. The
reflected load capacitor to primary will be n2 CLoad. The
series combination of reflected load capacitor and
resonating capacitor will decide the resonating frequency.
As the reflected load capacitor value is kept high in
comparison with value of resonating capacitor the
resonating frequency is mainly governed by resonating
capacitor value. The equivalent circuit is shown in figure1
Lr
Cr
2
n Cload
VDC
Figure 1
As the output voltage reaches linearly near the targeted
voltage, the Phase Shift PWM IC UCC3895 will start the
phase shifting between the two resonant converters
depending on the gain of the error amplifier. As a result
the net mmf tends to cancel in primary circuit. The phase
shift in the primary circuit will reduce rate of charging
and subsequently will lead to improved stability of output
voltage .Practically complete cancellation of mmf doesn’t
takes place, therefore another loop consisting of
comparator switches off the converter if the output
voltage exceeds desired voltage. During the low power
refresh mode the two primary windings are driven 180 ̊
out of phase and the output voltage will be maintained at
the desired voltage level. During the output delivery mode
the CCPS will be disabled until the pulse output is
delivered and voltage recovery process is over. A voltage
recovery system at the output enables to reduce power
loading on CCPS. The main drawback of Series resonant
converter is high circulating currents as compared with
PWM. As CCPs deploys IGBT’s so that the conduction
losses are function of average rather than rms current so
high peaks are not so detrimental. The paper describes the
prototype capacitor charging power supply that has been
designed, assembled and tested upto full power.
OPERATION
The inverter utilizes two full bridge series resonant
converter topology with fixed on time and fixed
frequency operation (fs < 0.5 fr) as shown in circuit
diagram Figure 2. The DC bus is obtained by rectifying
and filtering three phase AC mains followed by LC filter.
The bridge consist of MITSUBISHI make CM75 DY24H, VCES equal to 1200 V and current rating of 75 A.
The IGBT bridge was driven by Powerex make IGBT
driver M57962CL-01. The tank circuit values were Lr =
25uH and Cr = 0.4uf. The transformer was built on 4 nos
of U93 cores with 10 turns in primary and 100 turns in
secondary wound with Litz wire. The energy storage
capacitor bank consists of 15uf , 10nos energy storage
capacitors in parallel. The output switch used was two
series connected ABB make 5STF08 F2060, 2kV SCR’s.
As their will be voltage reversal during the output pulse
delivery, two IGBT’s (IXGH 16N 170) connected in
series were placed in series with the output rectifier and
will be made disabled during this period. The switches
are driven by Phase Shift PWM IC UCC3895. The output
is rectified with series connected ultrafast diodes each
rated for 1800V.
Vdc
150uF
Control A1
Cr
Control A3
Sense
N/W
Np=10
Control A4
Ns=100
Control A2
.
Control B1
Vdc
Lr
Voltage Ref
Cr
Control B3
Np=10
Control B4
Control B2
.
Gate drive fir IGBT's
Control Card with UCC 3895
Figure 2
The advantage of series resonant circuit is that the
leakage inductance of circuit including transformer are
absorbed into the tank circuit inductance. The reflected
load voltage appears in series with DC bus voltage as a
voltage source. Initially when the capacitor is in
discharged state the tank current in nearly sine wave with
both the lobes of equal magnitude as shown in Figure 3.
As the capacitor charges the current in the forward
direction increases and current in reverse direction
decreases due to increased reflected load voltage.
Figure 3
The stability of the output voltage is governed by error
amplifier followed by voltage comparator (LM311),
which compares voltage feed back from HV energy
storage capacitor and the reference signal. In single SRC
even the tank circuit switches were disabled, the energy
stored in the tank circuit components will overcharge the
output capacitor during the refresh cycle. In twin resonant
scheme initially when bulk charging is required the power
is fed by both the converters in phase which has the
advantage that power handled by each converter is
halved. The error amplifier will shift the phase between
____________________________________________
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Lr
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the two current sources. As the transformer is wound with
two primaries and one secondary on the same core, phase
shifting phenomenon will change the coupling flux
between primary and secondary windings. As the output
voltage reaches near the target voltage the error amplifier
starts phase shifting and more mmf cancellation starts at
the input of transformer, so net output voltage at the
output of transformer will reduce which in turn reduces
the output charging rate as shown in Figure 4. The
reduction in the output charging rate will reduce the
overcharging rate. Practically perfect cancellation of mmf
doesn’t takes place in the primary, so another voltage
comparator loop was added which will disable the
charging by withdrawing drive to bridge switches. During
Output Pulse delivery mode energy of the capacitor is
delivered to load by triggering the switch. All the events
are synchronised with trigger. Application of trigger will
inhibit the charging completely and simultaneously
inhibit the output switch drive. The output circuit
switches are required to prevent short circuit condition
created due to voltage reversal phenomenon due to
inductive load. The next charging cycle starts after 5ms.
Figure 4
The resonating components consist of L = 60 μF (which
includes the main resonating inductor and leakage
inductance of transformer), C= 0.4uF.The LCEff series
combination of these will yields a resonating frequency of
32 kHz. Therefore the switching frequency used was less
than fr /2 (16 kHz). The 1:10 step up transformer is made
up of single 4 nos U93 core core with 10 turns in primary
and 100 turns in secondary. The series resonating inductor
is made up of E42 core with 10 turns and 2mm air gap.
There is no intentional circuit resistance. The expected
resistance of circuit is due to resistance of diodes,
IGBT’s, ESR of capacitors, resistance of transformer and
inductor.
EXPERIMENTAL RESULTS
The CCPS is linearly charging 150uF Energy storage
capacitor shown in Figure 5. The average output current
Iav required to charge the capacitor C to requisite voltage
𝛥V in a given time 𝛥T can be given by
𝐶𝑋∆𝑉
𝐼𝑎𝑣 =
∆𝑇
12:25:45.0
12:30:25.0
12:35:05.0
12:39:45.0
12:44:25.0
12:49:05.0
12:53:45.0
12:58:25.0
13:03:05.0
13:07:45.0
13:12:25.0
13:17:05.0
13:21:45.0
The calculated value of Iav comes out to be is 4.28 A,
when C=150uF, 𝛥V =1kV, 𝛥T = 35ms.
2.9474
2.9472
2.947
2.9468
2.9466
2.9464
2.9462
2.946
2.9458
Figure 7
Acknowledgement
We are grateful to Shri U.Karandikar, Shri R.Barothiya,
Shri Renukanath who helped us in the development of
charger. Authors wish to thank Shri Yunus Khan ,Shri
S.D. Yadav and M.D.Jha for technical assistance for
wiring and fabricating various parts of power supply.
Figure 5
Figure 3and Figure 4 verifies the series resonance
behaviour of tank current during initial part and at end of
charging cycle. Figure 6 shows complete charging voltage
waveform across the load and charging current waveform.
The CCPS is tested upto 1kV at a PRR of 25Hz. The
traces were obtained with Agilent make DSO 9064A and
Tektronix make TPS2024 digitizing oscilloscope. The
voltage measurements were taken with Tektronix make
P5100, 100X probe and Pearson CT. The stability of
capacitor voltage was recorded with Agilent make
34401A multimeter and was found to be ±0.015% as
shown in figure 7.
Figure 6
Conclusion
A high frequency capacitor charging power supply has
been designed assembled tested up to full power in
laboratory. A twin full bridge series resonant topology
using IGBT switches was used. The bridge was driven
with fixed frequency variable phase shift using
commercial phase shift IC UCC3895 and Powerex make
IGBT driver M57962CL-01.The CCPS has been tested
for charging 150uf energy storage capacitor at 1kV at
Pulse repetition rate of 25 PPS.
References
[1] N.Mohan ,T.M.Undeland,W.P.Robbins, Power
Electronics Converters, Applications, John Wiley and
Sons, 2001, pp249-264
[2] B.E.Strickland, M. Garbi , F. Cathell, S.Eckhouse and
M. Nelms, “2kJ/s, 25kV High Frequency Capacitor
Charging Power Supply using Mosfet Switches”,
IEEE tran 1990, pp 531-534
[3] A.C.Lippincott,"A capacitor charging power supply
using a series resonant topology, constant on-Time
/variable Frequency Control, and Zero -Current
Switching", IEEE trans on Industrial Electronics,
Vol.38, No.6 ,Dec1991
[4] J.S.Oh,S.D.Jang,Y.G.Son,M.H.Cho,W.Namkung,
"Development OF Capacitor-Charging Power Supply
For A Smart Modulator",Proc. Of the second Asian
Particle Accel. Conf,Beijing, China,2001
[5] Y.Kelkar, Y.Raikwar,” A Zero Voltage Switching ,
Variable Frequency Capacitor Charging Power
Supply Using Series Resonant Topology” Inpac
2005, pp 457-458