Download A Novel High-Frequency Multiphase Crowbarless High

A NOVEL HIGH-FREQUENCY MULTIPHASE CROWBARLESS HIGHVOLTAGE DC POWER SUPPLY
Mangesh Borage#, Alok Singh, Sunil Tiwari and A. C. Thakurta
Raja Ramanna Centre for Advanced Technology, Indore 452013, India
Abstract
A novel topology based on high frequency switching
resonant immittance converters (RICs) is proposed in this
paper. The principle of operation, design, simulation and
experimental results on a -20 kV, 1 A dc prototype power
supply, that uses a three-phase RIC operating with 120o
phase shift involving switching at 25 kHz and a dc-dc
step down converter with energy recovery snubber in the
front-end, is presented.
INTRODUCTION
High voltage (HV) dc power supplies are required for
myriad of applications such as X-ray tubes, electric
discharge machining, corona discharge processes,
electrostatic precipitators, ozone generation, electrostatic
accelerators, electron guns, klystrons, modulators,
vacuum pumps, beam diagnostic devices, low
temperature plasma generation, lasers, pulsed power
supplies, etc. Design of a HV power supplies is complex
because of non-negligible effect of circuit parasitic
components, e.g. high leakage inductance and winding
capacitance associated with the transformer. Besides,
partial discharge and arcing can occur frequently in HV
loads. HV power supplies required for sensitive and
expensive loads like klystrons need to have minimum
energy storage in the output filter as stored energy is
dissipated in the load in case of arcing. Fast closing
switches (spark gap, thyratron, ignitron, light activated
thyristors) operating in parallel to the klystron path
(known as crowbars) are used for protection. To avoid
false firing and reliability issues of crowbars, a crowbar
less power supply is preferred, wherein stored energy in
the power supply is kept less than the maximum energy
handling capability of klystron.
In order to reduce stored energy in the power supply,
the value of output filter capacitor must be reduced. To
achieve this without sacrificing the output voltage ripple,
a way to reduce the output ripple amplitude and increase
the ripple frequency must be built in to the power
converter circuit. This often calls for the use of highfrequency power converter. Use of single power converter
is practically not feasible, particularly in the case of highpower applications, because the required switching
frequency becomes prohibitively high. Instead, many
converters, with their outputs connected either in series
[1] or parallel [2] and operating in phase-staggered
manner, have been proposed for applications demanding a
crowbarless power supply. Alternatively, a technique
known as pulse step modulation, in association with many
low voltage switch-mode converters connected in series,
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has been successfully implemented [3]. In these
techniques, the phase staggered operation is mainly
responsible for reduction of the output filter capacitor and
associated stored energy. However, the power supply,
being a voltage source, is not inherently immune against
arcing and short circuits. In this paper a new approach is
suggested, wherein it is proposed to use phase staggered
current sources in parallel instead of the conventional
arrangement of series-connected voltage sources. The
principle of operation, design, simulation and
experimental results on a -20 kV, 1 A dc prototype power
supply are presented.
THE CURRENT SOURCE
Resonant converters (RCs) have been a potential candidate
in many power electronics applications due to soft switching,
high frequency operation, high efficiency, small size, etc. A
new family of RC, known as resonant immittance converters
(RICs) is recently proposed [4] that can transform a voltage
source into a current source. Salient merits of RIC are:
 Easy parallel operation without any complex control for
equal current sharing
 Modularity and redundancy
 In-phase bridge output voltage and current under all
loading conditions – lowest conduction loss and softswitching is guaranteed
 Inherently short-circuit proof – device current drops to
very small value under load short-circuit condition.
 Constant-frequency operation – simplifies the design of
magnetic and filter components.
 Gainful utilization of transformer parasitic components.
In all 24 RIC topologies with three and four reactive
element, the circuit shown in Fig. 1 is chosen as the basic
building block for the presented power supply, mainly
because it gainfully utilizes the transformer leakage
inductance and winding capacitance into the resonant
network [5]. It is a half bridge converter followed by a
network of reactive elements feeding to a transformer,
rectifier and filter. Defining, o  1
L1C1 , n   o ,
  L2 L1 ,   C2 C1 and Z n  L1 C1 , it has been
shown that the converter operates as a current source
Io
Cdc
S1
D1
L2
Vd
L1
C1
Dc1
1:n
Dr1
+
Vo
C2
Cdc
Dr3
Cf
RL
Tr
S2
D2
Dc2
Dr2
Dr4
Figure 1: Circuit diagram of a half-bridge RIC.
-
when designed with the condition     1 and when it
is operated at ωn=1. The current gain, nI o Z n Vd  ,
under these condition is equal to 8  2 . Addition of two
clamp diodes Dc1 and Dc2, as shown in Fig. 1, impart
inherent constant-voltage characteristics to the circuit [6].
PHASE-STAGGERED, PARALLELCONNECTED CURRENT SOURCES
Phase-staggered operation of power converters
connected in series or parallel on the input and output side
can reduce the ripple amplitude and increase the ripple
frequency, thereby reducing the filter, without increasing
the switching frequency. Since a RIC is inherently a
current source, it lends itself for easy paralleling without
any additional care for equal current sharing, as illustrated
in Fig. 2. For m identical modules operating in parallel at
the same switching frequency but with phase shift θ, the
components of rectified output current are:
4I
 2I 
it t    m m  m

  

h  2, 4,...
1
 cos nt  x 
h  1h  1 x 0,1,2,... m1
(1)
The dc components of individual modules directly add
independent of θ and the total dc current of the paralleled
modules is m times the output current of one module.
However, harmonics can selectively get cancelled in the
total current depending on θ.
CA
SA
1:n
Dr1
Tr
SB
Dr2
Table 1: Specifications and component details
Parameter
Value
Output
-20 kV, 1 A
Ripple and regulation
<0.5 %
Stored energy
<10 J
Input
415 V, 3-ph., 50 Hz
Dr3
Resonant
Immittance
Network
(RIN)
CB
In the three-phase RIC, each half-bridge IGBT operates
at 25 kHz. However, the operation of each half bridge is
phase shifted by 120o. Three identical resonant
immittance networks composed of inductors L1A, L1B, L1C
as well as L2A, L2B, L2C and external capacitors C1A, C1B,
C1C have been used. The capacitors C2A, C2B and C2C
(nearly 70 nF) are in fact the parasitic winding capacitors
of the respective transformers. Similarly, the leakage
inductance (nearly 3 µH) is absorbed as the part of
resonant inductors L2A, L2B, L2C. The step-up transformers
are Y-Y connected and have turns ratio of 1:50. At the
output, three-phase voltage doubler rectifier is used to get
the required high voltage. Three sets of clamping diodes,
Dc1A Dc2A, Dc1B Dc2B and Dc1C Dc2C are used to clamp the
open circuit output voltage passively at the designed
value. Semikron make SKM100GB128D IGBTs are used.
Ultrafast diodes DSEI 2 X31 - 100 are used as the clamp
diodes. In the output rectifier, 40 nos. of MUR4100
diodes are connected in series to realize each of the diode
stack used in high voltage rectifier. Eight capacitors rated
for 0.22 µF/ 5000 V are connected in series to form the
output filter capacitor. This capacitor stores only 5.5 J at 20 kV, enabling the crowbarless operation.
The output voltage sensing is done using compensated
resistive voltage divider and output current is sensed
indirectly by measuring transformer primary current.
Dr4
Lf
Figure 2: Parallel connected RICs.
Db
415V,
3 Ph
Mains
Cf
Cd1
Ds3
Cs1
Rd1
PROTOTYPE POWER SUPPLY
A prototype power supply has been developed using
three-phase RIC with clamp diodes. Major specifications
are summarized in Table 1 and the schematic diagram is
shown in Fig. 3.
In the front-end, three phase diode rectifier and a
damped LC filter generates unregulated dc bus. A dc-dc
step-down converter is used for the control and regulation
of output high voltage, which is accomplished by
changing the output dc voltage using PWM. To reduce the
switching loss in the dc-dc converter, an energy recovery
snubber, composed of only passive components (Ls, Cs1,
Cs2) and diodes (Ds1, Ds2, Ds3) has been implemented [7].
The switching frequency of IGBT in the dc-dc converter
is 25 kHz. The output filter capacitor is divided into two
parts (Cdc) so that it can also perform as the split
capacitors for the subsequent half-bridge converter stages.
Ldc
Sb
Ls
Lf
Cs2
S1B D1B
Rd2
Cdc
Ldc
Dc1A Dc1B Dc1C
S1A D1A
Cdc
Ds2
Ds1
Vdc(+)
Vdc(+)
Cd2
Vdc(-)
Dr1A Dr1B Dr1C
S1C D1C
Cf1
L2A
L1A
C1A
L2B
L1B
C2A TrA
L1C
C2B TrB
C1B
L2C
C1C
C2C TrC
S2A D2A
Vdc(-)
S2B D2B
Cf2
S2C D2C
Dc1A Dc1B Dc1C
Dr2A Dr2B Dr2C
Figure 3: Circuit diagram of the power supply.
HV
Va
Va
Ia
Ia
Vb
Vb
Vc
Vc
A
(b)
B
C
(d)
(c)
3.5
3.0
23V
Arc Energy (J)
@ 15 kV DC
15V
@ 10 kV DC
voltage
current
2.5
2.0
1.5
1.0
0.5
@ 5 kV DC
8.5V
0.0
0
50
100
150
200
time (us)
250
300
(f)
(e)
(g)
(a)
Figure 4: (a) Photograph of the power supply. (b) IGBT bridge output voltage and current waveforms under loaded
condition. (c) IGBT bridge output voltage and current waveforms under output short circuit condition. (d) Transformer
primary current waveforms under output short-circuit condition showing balanced operation and equal current sharing
(e) Output ripple voltage waveforms under different settings. (f) Output voltage and current waveforms under arc
condition. g Calculated arc energy under arc condition.
EXPERIMENTAL RESULTS
Figure 4(a) shows the photograph of the power supply.
It is housed in a 0.6 m X 0.8 m cabinet of 32U height.
Only semiconductors are mounted on water-cooled
heatsink. All magnetic components, except for the high
voltage transformers are air-cooled. The high voltage
transformers, rectifiers and filter capacitor are placed in
an oil tank for insulation and cooling purpose.
The circuit functionality has been confirmed under
various steady-state and transient operating conditions.
Fig. 4(b) shows the waveforms of three IGBT halfbridges and the current in one of the half-bridge under the
loaded condition. It can be seen that voltage waveform is
clean and free from any ringing. This is because of the
soft-switching operation of the IGBTs which is evident
from the voltage and current waveforms, which are nearly
in phase. The similar set of waveforms under output
short-circuit condition is shown in Fig. 4(c). Here it is
important to note that the soft-switching is still
maintained and the bridge output current has dropped to a
very small value as compared to the case when the
converter
was loaded. This demonstrates that the
converter can be safely operated under load short-circuit
conditions. The transformer primary current waveforms
under output short-circuit condition demonstrating
balanced operation and equal current sharing are shown in
Fig. 4(d). The high frequency output ripple under loaded
condition was measured at different output levels as
shown in Fig. 4(e), which is seen to be well within the
design specifications.
The transient performance of the converter was
examined under simulated arcing conditions created by
using sphere-gap. Fig. 4(f) shows the output voltage and
current waveform under the output short-circuit condition
created when the converter was operating at -15 kV
output. A small inductor has been used in series with the
output to limit the peak discharge current of output filter
capacitor within its safe limits. The recorded voltage and
current waveforms are then used to calculate the energy
deposited in the arc. The results shown in Fig. 4(g)
confirm that the experimental arc energy of 3.11 J is very
close to the calculated value of 3.09 J at -15 kV output.
It is possible to make the power supply with positive
output voltage polarity just by changing the way output
rectifier diodes are connected. The number of phase can
be increased with suitable phase shift to further reduce the
output ripple. Also, the same power supply can be
configured to function as a capacitor charging power
supply by making suitable changes in the control circuit.
ACKNOWLEDGMENT
The authors acknowledge the contribution of Shri P.
Renukanath and his team at PSIAD, RRCAT in the
development of high voltage transformers used in the
prototype, and, Shri Vinod Somkuwar for his assistance
during assembly, wiring and testing of the power supply.
REFERENCES
[1] H. Kwon et al., "Klystron power supply for KOMAC,"
APAC'98, http://www.JACoW.org
[2] H. Kozu et al., J. Synchrotron Rad. 5 (1998) p. 374-375.
[3] J. Alex et. al, “A new klystron modulator for XFEL based
on PSM technology," PAC’07, http://www.JACoW.org
[4] M. Borage, et. al, IEEE Trans. Ind. Electron, 58(3) (2011)
p. 971-978.
[5] M. Borage, et. al, IEEE Trans. Ind. Electron, 56(5) (2009)
p. 1420-1427.
[6] M. Borage, et. al, IEEE Trans. Ind. Electron, 54(2) (2007)
p. 741-746.
[7] A. Singh, et. al, Adv. Power. Electron., 2012 (2012).