Download A Frequency Controlled LCL - T Resonant Converter for H

A FREQUENCY CONTROLLED LCL - T RESONANT CONVERTER FOR
H- ION SOURCE
V.K. Gauttam, A. Kasliwal, R. Banwari, T.G. Pandit, A.C. Thakurta
Power Supplies and Industrial Accelerator Division, Raja Ramanna Centre for Advanced
Technology, Indore 452 013, India
L1
Abstract
-
An H ion source is being developed at RRCAT, Indore.
An LCL-T resonant power converter with variable
400 V
AC
frequency control is proposed which is utilized to develop
50Hz
a -20 kV/ 100 mA high voltage (HV) power supply for
extraction of H ̄ ions. The LCL-T resonant topology offers
many advantages like gainful utilization of the
transformer parasitics as a part of resonant network and
low circulating current. The power converter is operated
with variable frequency control and above resonance to
get well known zero-voltage switching (ZVS) advantages
for full bridge semiconductor switches in full load range.
The converter energizes the symmetrical CockcroftWalton (CW) based HV generator to achieve required
high voltage. The CW circuit is an attractive solution for
HV generation since it has features like low stored energy
and low output ripple. The HV power supply is operated
in constant current (CC) mode with closed loop control
and soft start of the power supply is achieved by
sweeping the switching frequency from 40 kHz to defined
operating point. Design parameters, simulation results and
experimental results of the power converter are presented
in this paper.
INTRODUCTION
High frequency resonant converters are widely used in
many applications like high voltage (HV) power supplies,
induction heating, power factor correction etc. owing to
their features of small size, zero voltage switching (ZVS)
at above resonant frequency (operating in lagging power
factor mode), lower switching losses, high efficiency, low
electromagnetic interference [1]. Recently high frequency
resonant converter with three or more resonant elements
are more attractive solution because of their distinct
advantage of having load independent operation. An Hion source is being developed at RRCAT, Indore and it
requires a high voltage power supply (HVPS) for
extraction of H- ions. The power supply consists of two
stages. First one is high frequency resonant power
converter and second one is a balanced symmetrical
Cockroft-Walton (CW) scheme based HV generator.
L2
generator should be energized by a high frequency sine
wave source to achieve required +load regulation, +ripple
Lo
G
and low stored Genergy.
The
LCL
– T resonant tank circuit
D
D
excited by
an
H
bridge
voltage
S
S
V source inverter
V(VSI)
Co
C
with variable frequency control is chosen as power circuit
G
topology. The G LCL-T
resonant
tank circuit has the
D
D
advantage that itS can absorb
the - leakage inductance
of
S2
transformer as a part of resonant network and output can
be controlled even at no load by sweeping the switching
frequency. The basic schematic of LCL-T resonant circuit
is shown in Fig. 1.
3
1
1
3
1
3
i
4
2
4
L1
D1
Vdc
-
C2
Pri
2
4
+
C1
L2
D3
S1
S3
C
Cdc
D4
RL
L
O
A
D
D2
S4
S2
Figure 1: VSI with LCL-T resonant topology
The resonant frequencies f01 and f02 of the LCL-T
resonant tank can be defined by complex input impedance
(Z). These frequencies can be found by calculating those
frequencies which result in either infinite or zero input
impedance [2].
Z→∞
f01 =
Z →0
f02
1
2π√C. L2
=
(1)
1
2π√C.L1 L2 ⁄(L1 +L2 )
𝑍𝑛
L
𝑍𝑛 = √ , Q =
C
𝑅𝐿
(2)
(3)
ANALYSIS AND DESIGN PARAMETERS
The voltage gain vs. frequency plot of LCL-T with
resistive load at different Q is shown in Fig. 2 which
FREQUENCY CONTROLLED LCL- T
RESONANT CONVERTER
The high frequency power converter energizes the
symmetrical CW based HV generator which consists of
multiplier circuits connected in cascade. The HV
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Figure 2: Plot of voltage gain vs. frequency.
C'1
L1
L2
+
~
Vi
+
C
V0 (-20 kV)
VPri
VSense
-
-
L
O
A
D
ISense
Figure 3: Circuit diagram of HV power supply with LCL-T resonant topology.
shows two resonant frequencies at 18.38 kHz (f01) and 26
kHz (f02) from light load (Q=9) to rated full load (Q=0.5)
respectively. The operating region is shown in Fig. 2
which is from operating point B (10 % of rated load) to A
(full load) and switching frequency varies from 36 kHz to
26 kHz respectively. The depicted operation region shows
above resonance operation of the converter from light
load to full load. This ensures the soft switching (ZVS)
for all four IGBT switches of H - bridge VSI because of
lagging power factor in all loading conditions. The
designed parameters are shown below in table 1.
input over current and arc protection are implemented and
tested successfully.
Table 1: Design Parameters
Sr. No.
Design Parameter
Value
1
Power (Po)
2 kW
2
Voltage (Vo)
- 20 kV DC
3
Current (Io)
100 mA
4
L1=L2=L
250 µH
5
C
0.3 µF
6
f01
18.38 kHz
7
f02
26.0 kHz
The power circuit diagram of HV power supply with
LCL-T is shown in Fig. 3 in which VSI is operated with
fixed 50 % duty cycle. The diagonal devices are switched
simultaneously to generate square wave voltage which
excites the LCL tank circuit. The sinusoidal output of the
resonant tank energizes the centre tapped step up HV
transformer (1:20:20). The HV transformer then feeds the
HV generator to generate required high voltage.
EXPERIMENTAL RESULTS
-
A 100 mA -20 kV H ion extraction power supply has
been designed, developed and commissioned in Ion
Source Laboratory (ISL) at RRCAT, Indore as per
designed specifications shown in table 1. The soft start of
the power supply is implemented by sweeping the
switching frequency from 40 kHz (point C in Figure 2) to
operating point defined by closed loop control. The power
supply is controlled in constant current mode and various
protections like output over current, output over voltage,
Figure 3: Experimental waveform of voltage across
and current through the IGBT of VSI
Experimental waveform of voltage across and current
through the IGBT switch is shown in Fig. 3 which shows
the soft switching (ZVS) at turn on. The parasitic output
capacitance of the IGBT is used as a lossless snubber
capacitor to reduce the turn off losses. The HV power
supply which consists of the power converter and HV
generator is operated in closed loop control while taking
the output current as a control parameter. The voltage gain
of the resonant network is controlled by varying the
switching frequency which in turn controls the output
voltage and hence the output current. The HV generator
which is basically a symmetrical CW circuit consists of
three doubler multiplier stacks. The measured value of
voltage build up ratio (first stack to top) is 3.2 at no load.
CONCLUSION
The frequency controlled LCL –T resonant converter is
well suited for HV power supplies with variable load and
the power supply can be operated either in constant
current mode (CC) or constant voltage mode (CV) by
sweeping the switching frequency in the preferred
operating region. Furthermore, the transformer parasitics
can be integrated as part of resonant network in LCL –T
resonant topology and ZVS at turn on for IGBT switches
can be achieved for full load range.
ACKNOWLEDGMENT
Authors are sincerely thankful to Mr. Deepchand, Mr.
Rajesh Nagdeve, Mr. Nathan Singh, Mr. G. H. Ansari and
Mr. S. K. Sonawane for their help in fabrication,
assembly, testing and commissioning of the HV power
supply.
REFERENCES
[1] Mangesh Borage, K. V. Nagesh, M. S. Bhatia and Sunil
Tiwari, “Design of LCL – T resonant converter including
the effect of transformer winding capacitance,” IEEE
Trans. Ind. Electron., vol. 56, no. 5, May 2009, pp. 1420–
1427.
[2] S. Dieckerhoff, M. J. Ryan and R. W. De Doncker, “Design
of an IGBT – based LCL-resonant inverter for high frequency induction heating,” in Proc. IEEE Ind.
Application Conference, vol. 3, 1999, pp. 2039–2045.
[3] R. P. Severns, “Topologies for three element resonant
converters,” IEEE Trans. Power Electron., vol. PE-7, Jan.
1992, pp. 89-97.
[4] M. Borage, S. Tiwari, and S. Kotaiah, “LCL-T resonant
converter with clamp diodes: A novel constant-current
power supply with inherent constant-voltage limit,” IEEE
Trans. Ind. Electron., vol. 54, no. 2, Apr. 2007, pp. 741–
746.
[5] A. K. S. Bhat, “Analysis and design of LCL-type series
resonant converter,” In Proc. of IEEE International
Telecommunications Energy Conference (INTELEC),
1990, pp. 172 - 178.