IEEE PEDS 2005
Switched Capacitor Snubber-Assisted Zero Current Soft Switching PWM High Frequency Inverter with Two-Lossless Inductive Snubbers Manal H. Hashem Nabil A. Ahmed Sophia University Tokyo, Japan Email:
[email protected]
Eiji Hiraki, Tarek Ahmed
Khairy Fathi, Hyun-Woo Lee, Mutsuo Nakaoka Electric Energy Saving Research Centre Kyungnam University, Masan Korea
Yamaguchi University Yamaguchi, Japan
[email protected] [email protected]
Abstract - In this paper, a novel type of auxiliary switched capacitor assisted edge resonant soft switching PWM series load resonant high frequency inverter with two auxiliary edge resonant lossless inductor snubbers is proposed for small consumer induction heating appliances. The operation principle
of this high frequency inverter is described using the switching mode equivalent circuits. This newly developed multi resonant high frequency inverter using trench gate lGBTs can regulate its high frequency output AC power under a principle of constant frequency edge-resonant soft switching commutation by asymmetrical PWM control scheme. The high frequency power regulation and actual power conversion efficiency characteristics of consumer brand-new IH products that use the proposed soft switching PWM series load resonant high frequency inverter are evaluated. The practical effectiveness and operating performances of the newly proposed soft switching inverter are discussed as compared with the conventional soft switching high frequency inverters based on simulation and experimental results from an application point of view. Index Terms: Single-ended push-pull inverter, high frequency power conversion, series capacitor compensator load resonant circuitry, auxiliary lossless snubbing inductor, auxiliary switched capacitor, zero current soft switching, consumer induction heating. 1. INTRODUCTION In recent years, the consumer comparative study power electronics relating to high frequency electromagnetic eddy current based induction heating (IH) becomes more suitable and acceptable for consumer food cooking and processing appliances, hot water producer, super heated steamer and fixing roller in copy and printer machines [1-3]. In general, consumer IH equipments for effective power and energy applications in home and business use not only meets the practical demands of safety, cost effectiveness and cleanliness, but also has excellent advantages of very high thennal conversion efficiency, rapid and direct local focusing heating, high power density, high reliability, non-acoustic environmental and low electromagnetic noise [4]-[8]. The consumer IH appliances are based upon the eddy current joules heat on Faraday's electromagnetic induction law and supply high frequency AC power to a variety of IH loads,
which may consist of planer (pancake) or cylindrical type
working coil with electromagnetic eddy current based heated
materials. Consumer IH appliances operating at high frequency ranges from 20 kHz to several MHz need a cost effective product, high efficiency and high power density of high frequency inverters. Among various types of the high frequency inverter topologies, there are full bridge, half bridge, single-ended push-pull (SEPP), centre tap push-pull and boost-half bridge circuit configurations, in which voltage source type lossless snubber inductor assisted zero current soft switching (ZCS) SEPP series load resonant topologies, voltage source type zero voltage soft switching (ZVS) SEPP resonant inverter and voltage source ZVZCS-SEEP multi resonant inverter topologies [4]-[8]. These high frequency soft soft f The shave inverter switching inverterS have the advantages of simple configuration, high efficiency, low cost and wide soft commutation operating ranges, which are indispensable for high frequency operation. The voltage source type ZCS high frequency inverter and its modifications match the practical operating requirements mentioned previously. However, previously developed ZCS high frequency inverters with PWM control scheme could not able to regulate its output
song
high
power under constant frequency PWM control strategy [8]. In this paper, a novel circuit topology of voltage source multi resonant ZCS-SEPP high frequency inverter with constant frequency PWM control strategy using active auxiliary quasi-resonant lossless inductor sunbbers and switched capacitor snubber is newly proposed for new generation cost effective consumer IH food cooking and processing applications, which additionally includes practical outstanding features. The operating principle of the proposed high frequency inverter topology incorporating ZCS-PWM control scheme for light AC power regulation and its actual efficiency characteristics for PWM control strategy are illustrated and evaluated on the basis of simulation and practical experimental results and the effectiveness of this proposed high frequency ZGS inverter using IGBTs are substantially proved for cost effective consumer IH appliances.
0-7803-9296-5/05/$20.00 © 2005 IEEE
198
11. PROPOSED VOLTAGE SOURCE ZCS- SEPP PWM HIGH FREQUENCY INVERTER A. Circuit Configuration
and Q3. Then, the main switch Q2 is turned on after turning off the auxiliary switch Q3 by a dead time of Td,. The main switch Q, is again switched on after a dead time Td2 as another period starts as depicted in Fig. 2. By adjusting the constant frequency asymmetrical PWM control duty cycle, which is defined as the sum of the conduction time Ton, of the main active switch Q and conduction time T,33 of the auxiliary power switch Q3 to the total switching period operating time T of the high switching, the proposed high frequency ZCS-PWM inverter can regulate its high frequency output power continuously under a condition of soft switching. The conduction time To", of the main active switch Q, can be controlled while keeping the conduction time T,33 of the auxiliary active switch Q3, the overlapping time To and the dead time Td, constants. As a control variable, the duty cycle D is defined as
Fig. I shows the newly developed multi-resonant ZCSSEPP PWM high-frequency inverter circuit using the latest trench gate IGBTs and operating with constant frequency PWM control strategy. This voltage-fed ZCS PWM high frequency inverter circuit consists of two main switches of reverse conducting IGBTs Q,(SW,/Di) and Q2(SW2/D2), a single auxiliary switch Q3(SW3/D3) in series with auxiliary edge-resonant switched capacitor Cr as an active snubber in parallel with Ql and Ls,, two ZCS-assisted lossless inductor snubbers Ls, and Ls2 connected in series with the main switches Q, and Q2, power factor compensated series load resonant capacitor C3, and highly inductive IH load represented by its R0 and L. series inductive equivalent circuit
D = (Ton + TdI)/T
model. The proposed voltage source ZCS-PWM SEPP high frequency inverter is configured by a few circuit components and power semiconductor devices as three controlled active switches are used.
Cr
Q1(SW1ID)
JL ; is.
'swI IJid i;
Q3(SW3/D3) Vs
s
~ Vsw|J
m
.....
IS0IJ I'i
Q2(SW2ID2)
-..........
...
R0 -
Lo
(I)
The proposed voltage source series load resonant ZCSSEPP PWM high frequency inverter with two lossless inductor snubbers and a single switched capacitor can not only be controlled by the constant frequency asymmetrical PWM technique for high power settings, but also it can be controlled by a constant high frequency pulse density modulation (PDM) technique at low power settings. In addition, by using a dual mode hybrid control of asymmetrical PWM and PDM at a iSWconstant high frequency, soft switching operating range can be effectively expanded from high power to low power settings.
Inducfion
HeatedVGw
0Load
LS2~S cS
Fig. 1. Multi-resonant ZCS- SEPP PWM high frequency inverter.
Toni
VGsw3
_
I
i
B. High Frequency A C Power Control Scheme The high frequency AC output power of the proposed inverter circuit, which is delivered to the IH load as IH cooking heater and rice cooker, can be continuously regulated by a constant frequency asymmetrical PWM (duty cycle) control scheme under a condition of zero current soft switching commutation mode. The proposed gate voltage pulse timing PWM sequences for the active power switches Q], Q2, in series with lossless inductor snubber, and the auxiliary power switch Q3, in series with lossless capacitor snubber, are shown schematically in Fig. 2. The main active power switch Q, is firstly switched on during period of time Ton, and before the main switch Q, is turned off by a time of T. the auxiliary switch Q3 is turned on for a period T,,3 inserting a an overlapping time of T, between the switches Q,
k
III.
( )HTdi Ton3)
Ton
iP
Tt2~
Ton2
Fig. 2. Proposed PWM gate pulse timing sequences. PRINCIPLE OF SOFT SWITCHING OPERATION
The switching operation mode commutation transitions and their corresponding equivalent circuits of the proposed zero current soft switching high frequency inverter in steady state during one switching cycle are shown in Fig. 3. The current operating waveforms and the relevant operating modes of this inverter in steady state are illustrated in Fig. 4 for a duty cycle D = 0.34. This multi resonant high frequency soft switching inverter circuit includes eleven operating switching modes as shown in Fig. 3. The operation principle of the proposed zero
199
current soft switching inverter circuit will be explained in the mode transition for the switch SW2 of Q2. The current and following by using the corresponding switching mode voltage operating waveforms of each element and the relevant equivalent circuits, operating modes in steady state during one switching period of The zero current soft switching operation for high frequency this are illustrated in Fig. 4. power two main switches and the auxiliary power switch in the proposed high frequency power inverter circuit can be completely achieved under the PWM gate pulse timing sequence pattem shown in Fig. 2. At the:beginning of each switching cycle, the high side plain power switch SW, of Q, is now conducting and high frequency AC -effective power is 4r * supplied to the IH load. After the switch current iSw) through SW, of Q, naturally commutates to the switch anti-parallel diode Di of Q, by quasi-resonance due to ZCS-assisted high TT ii e side inductor snubber Ls,, in series with the switch Ql, 1 together with the auxiliary series inductive load resonant tuned 4 capacitor Cs, the auxiliary active power switch SW3 of Q3 iS turned on and the main power switch SW, of Ql is turned off. m As a result, a ZCS commutation at a turn-off switching mode transition can be achieved by the arbitrarily timing processing Mode 9 D 4 1t when turning off the main power switch SW, of Ql. At this mode, since an auxiliary resonant current iSW3 flows through the switch SW3 of Q3 and increases softly, a ZCS commutation at a turn-on switching mode transition can be achieved for SW3 of Q3. Then, after iSW3 is commutated to the anti-parallel i od7 diode D3 of Q3 by the resonance formed by Cr, R0-L0 inductive t load circuitry and power factor series load compensated capacitor C3, a ZCS soft switching commutation at a tum-off switching mode transition can be performed by turning off SW3 of Q3. While the auxiliary power switch SW3 of Q3 iS code) conducting, the voltage VQ2 across the low side main switch Fig. 3. Operating mode transitions and equivalent circuits SW2 of Q2 decreases toward zero. Before the low side main at steady state during one switching cycle. switch SW2 of Q2 is turned on as soon as the diode D2 Of Q2 becomes reverse biasing state and begins to conduct naturally. 5 5 8 9 10 11 6 f 2 3 While the diode D2 continues conducting, the current flowing through D2 Of Q2 is naturally commutated to SW2 of Q2Therefore, a complete ZVS and ZCS (ZVZCS) hybrid commutation transition can be actually achieved for SW2 of Q2. On the other hand, after the current isw2 through the low side main switch SW2 of Q2 is naturally commutated to D2 of 120 4no Q2 with the aid of low side ZCS-assisted inductor snubber Ls2, the high inductive induction heating load Ro-Lo and load ¢ 2(X) power factor compensation series load resonant tuned .60. -120v. . ..4 capacitor C3, ZCS commutation at a turn-off switching mode transition can be performed by turning off the switch SW2 Of < ,H 3( > . Q2. While the diode D2 of Q2 is conducting, the current iD2 . _ flowing through D2 is commutated to the switch SW, of Q( by - 48-'*~~~~~~~~~~~~ 4 6X turning on the switch SW, of Q, when a second switching - 3i) _ cycle starts. At this mode, a ZCS turn-on switching H commutation can be realized with the aid of ZCS-assisted -,..,2X)) inductor snubber Ls,. The operation modes of the proposed high frequency zero current soft switching inverter shown in 6'X¶ -300 Fig. I are divided into eleven operating modes during each _40 .. switching cycle as shown in Figs. 3 and 4. .3 _ The proposed edge resonant ZCS PWM inverter offers a _8() _'_____i__6__ complete ZCS for all the main and auxiliary switches and 10swdiv Fig. 4. Voltage and current waveforms during one switching cycle achieves ZVZCS hybrid commutation at turn-on switching operating modes for a duty cycle of 0.34. -
1
-
200
IV. EXPERIMENTAL EVALUATIONS AND DISCUSSIONS A. Design Specifications and Circuit Parameters An experimental setup assembly implementation by using _ trench gate reverse conducting IGBTs with low saturation voltage is proposed to validate the steady state performance evaluations of the proposed zero current soft switching high frequency inverter circuit. The design specifications and 0 circuit parameters used in the experimental breadboard setup are respectively indicated in Tablel. The ZCS-PWM high frequency inverter circuit proposed here for . . is. and .designed .....................(a) Voltage and current wavefonTis of Q, (SW,/D,) consumer IH cooking heater.. in. .home business under vsw, 1250V/dlvl, isw, 140A/divl, t applications. An enamel pan has a bottom diameter of 18 cm is used for the IH load as a heated object. The high frequency IH load consists of enamel pan, ceramic spacer as top plate and a planner working coil composed of litz wire assembly. The circuit parameters of this high frequency ZCS-PWM inverter are determined by considering the operating condition of zero current soft switching commutation condition and the required high frequency AC output power ranges. Table 1. Design specifications and circuit constants. o
...
1lO10s/divl
Item
Symbol
Value
DC Source Voltage
Vs
282.8 V
Switching Frequency
fs
(b) Voltage and current waveforms of Q2 (SW2/D2)
20 kHz
under vSw2 1250V/dlvl, isw2 140A/divl, t I 1J0s/divi.
Inductance of ZCS-assisted Inductor
LS,
2.09 pH
Inductance of ZCS-assisted Inductor
LS2
2.01 MH
Capacitance of Auxiliary Quasi-resonant
Cr
324 nF
Cs
0.802 gF
Ro
2.54 nl 57.96 LH
Capacitor
Capacitance of Power Factor
Compensation Series Tuned Capacitor Enamel Pan
Load Resistance Load Inductance
L0
0
(c) Voltage and cuerent wavefonns of Q3 (SW3/D3)
under ViW3
B. Experimental Results The steady state measured operating voltage and current waveforms for specified duty cycle D = 0.34 under an input DC power of 2.7 kW are represented in Fig. 5. As it can been noticed in this figure, all the main power switches Ql, Q2 and the auxiliary active power switch Q3 can operate under a principle of ZCS-PWM commutation operation. In particular, it can be recognized that a complete ZVZCS hybrid commutation at turn-on switching mode transition can be performed for the SW2 of Q2, because SW2 is turned on during a conduction period of the diode D2 of Q2. Since the gate pulse voltage signal is given to the auxiliary power switch SW, of Q, during the conduction period of its anti-parallel diode DI of Q,, the ZCS commutation at a turn-on switching mode transition can be achieved for the switch SW, of Q,. In spite of
1250V/div], iSW3 140A/dlvl, t IIOps/divi.
0
(d) Output voltage and curTent wavefonns under v0 1250V/divj, io 14OA/divi, t 1lOps/divl. Fig.5 Measured voltage and current waveforms in case of D=0.34
201
the additional auxiliary switch SW3 of Q3, a high efficiency power conversion can be achieved in the proposed high frequency zero current soft switching PWM inverter circuit depicted in Fig. I due to the inherent principle of soft switching operation in all the active and auxiliary power switches.
100 .
L,[..
J.
t 95c.'
C. High FrequencyACPowerRegulation Characterbitics The input power or high frequency AC output power vs. duty cycle characteristic for the proposed ZCS-PWM high frequency SEPP inverter, which is based on duty cycle PWM control scheme is depicted Fig. 6. The solid line shows the simulation results and the dotted line gives the measured experimental ones. A good agreement between the experimental and the simulation results is evident as shown in Fig. 6. In the proposed zero current soft switching high frequency inverter circuit, its input power or the high frequency AC output power can be regulated approximately from a low value of 0.4 kW to 2.6 kW under a principle of zero current soft switching commutation. It is noted that the soft switching operating range becomes relatively large in the proposed voltage source ZCS-PWM SEPP high frequency inverter.
a
.Z
,
I
'
c - -r n- - -- t-- ;4~ -r-r -n - - -t-- -I- -r-L .LLLJ _
9 90
J
,,
I LLe
L:
L1,1..-
85 0.15
-
0.2
0.25
0.3
0.35
DutyFactor D Fig. 7. Actual eficiency vs. duty factor characteristics.
Although the proposed high frequency inverter operating under a principle of asymmetrical PWM control or duty cycle control can achieve a complete soft switching commutation operation in the high output power settings, it becomes a partially hard switching commutation operation in a certain low power settings and its actual efficiency might be D. Actual Effciency Characteristics substantially reduced. However, this high frequency inverter The measured actual power can still operate under a considerable high efficiency power vS. efficency conversion. For example, in the case of specific duty cycle D voltage source type ZCS cycle characteristic of the proposed version = 0. 17, under a condition of the minimum low output power PWM high frequency inverter for consumer lH cooking heater is shown in Fig. 7. Under the rated output condition, the setting requirement of duty cycle PWM control scheme, the actual power conversion efficiency of the proposed high measured actual efficiency of this zero current soft switching inverter using the newly developed trench gate IGBTs is frequency inverter can kept to be more 86% in average as estimated to be about 94% in total system, since a zero current depicted in Fig. 7. soft switching commutation operation can be completely In addition, the zero current soft switching operating range achieved for this high fiequency inverter. of the proposed high frequency inverter can be actually extended by the use of high frequency pulse density scheme under the low power modulation (PDM) control Soft switchin area due to PWM settings or by the use of dual mode implemnentation of PWM and PDM selective control. Therefore, zero current the soft operation can be completely realized over all the iswitching iil , t ! --F--F-i--2__, ~t--2--F--F-2--- / regulation ranges including PDM in low power __ output power conditions and PWM in bigb power settings. > . |t || X t t || setting
-t,-,,-,,,,3 I
4
|| t l t } i iI lit/; { s
{
II
---,1~-.-Ir--¢--l-~l-;; I--r--'--n-- --X--r-----'--
0o
i
_ i , ,!
l.L
-
,I:/i b
_ --l----l-j-t--.
u
0.15
I
I
0.2
[
~-,--
/ t
I
1
- iSimulation value t,, ,0 * Experimental value, . I o .1 ,- 'f 0.35 0.3 0.25 .
I
I
{.
Duty Factor D
.
Fig. 6. Input power vs. duty factor characteristics.
E. Comparative Actual Efficiency Characteristics For more evaluation, the proposed soft switching ZCSPWM series capacitor compensated load resonant SEPP high frequency inverter shown in Fig.l is compared with the
developed voltage source ~~~~~~~previously snubber assisted ZVS-PWM SEPP
lossless capacitor high frequency inverter designed for IH cooker shown in Fig. 8, which has narrow soft switching operation performance under a constant frequency
PWM control scheme. The actual efficiency vs. the input power regulation characteristics of the proposed high frequency ZCS-PWM and ZVS-PWM type inverters are comparatively illustrated in Fig. 9. Although the high frequency power regulation
202
characteristics for both high frequency inverters mentioned above are based on a constant frequency the asymmetrical PWM control strategy, it has to be noted that the soft switching commutated operation schemes are different for both high frequency inverters.
Jl V l s
4'
eI(SNVD)
Induction _ Heated Load
l V5..
range of the newly proposed voltage source type ZCS-PWM SEPP high frequency inverter power conversion circuit is much wider than that of the previously developed one in spite of the new pulse modulation control scheme based on selectively controlled PWM. Since the output power of the newly proposed zero current soft switching high frequency inverter can be also efficiently regulated by introducing a simple PDM control scheme in low power setting range or .dual mode implementation of PWM and PDM control strategies for low power setting range, a relatively high actual operating efficiency can be achieved under a lower power setting condition of high frequency output power less than 0.4 kW.
_ v. CONCLUSIONS In this paper, a new topology of active auxiliary quasiT C, Comp,SeniesaTned resonant snubber-assisted voltage source type ZCS-PWM Series Tn high frequency series load resonant inverter using _ ^ Capacitor-SEPP IGBTs, with is composed of an active auxiliary switched L Capacitor snubber capacitor and two lossless snubber inductors has been Lossless proposed and developed originally for consumer IH cooker, Capacitor heated steamer, super heated steamer appliances and IH heat Snubber rollers. The high frequency operation principle, switching Fig. 8. Conventional ZVS-PWM SEPP high frequency inverter. mode transitions and the operating characteristics of the PWM controlled soft switching high frequency inverter have been illustrated and evaluated on the basis of simulation and Soft switching operating experimental results. For next generation consumer IH range ofproposed inverter range .fproposed inverter products, the practical effectiveness of the newly-proposed Soft switching operating range voltage source type ZCS-PWM SEPP high frequency multi of conventinal inverter resonant inverter using the latest trench gate IGBTs have been OC Ioo proved on the basis of the simulation and experimental results by producing an actual breadboard prototype. A wide soft r---------___ --___ --___ --________. ---g-______ switching commutation operation range of the newly proposed high frequency ZCS-PWM SEPP inverter has been obtained as .compared with the previously developed voltage source type ZVS-PWM SEPP inverter. The high frequency power _____ _____ _____regulation strategy of this high frequency inverter could be *., 80 4 Proposed ZC-PWM-SEPP efficiently supplied to the consumer high frequency IH heater from full power to relatively small power HF inverter cooking Conventional ZVS-PWM-SEPP _ 3 70 / settings. Applying a dual mode pulse modulation control ConventHF inverter strategy of the asymmetrical PWM in the higher power setting T
vJ
V2)
Power Factor
C.
-----------------------------------
60) 0
HFI
-
0.5
1
1.5
2
2.5
rand the PDM in the lower power setting, the output power of
3
Input Power [kWJ Fig. 9. Comparative actual power conversion efficiency vs. input power
characteristics.
The actual power conversion efficiency of the proposed voltage source ZCS-PWM high frequency inverter circuit is much higher than that of the previously developed ZVS-PWM one for lower input power or lower output power setting ranges and the power conversion efficiency is almost the for higher input power power or or higher higher output output power power ranges. ranges. same forsame higher input
this soft switching pulse modulated multi resonant high frequency inverter could be regulated under a condition of expanded stable soft switching operation ranges as compared with previously developed ZVS-PWM high frequency inverter. Therefore, the newly proposed dual mode ZCS PWM/PDM high frequency inverter could actually achieve higher efficiency, high performance and wider soft switching
operatingcranges.
REFERENCES [1] B. K. Lee, J. W. Jung, B. S. Suh, and D. S. Hyun, "A New Half-Bridge Inverter Topology with Active Auxiliary Resonant Circuit Using Insulated Gate Bipolar Transistors for Induction Heating Appliances,"
This is due to the reason of the conduction power loss of the added auxiliary power switch Q3. The soft switching operation
Proceedings of IEEE Power Electronics Specialists Conference (PESC), Vol.2, pp. 1232-1237, June, 1999.
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[2] H. Terai, H. Sadakata, H. Omori, H. Yamashita, and M. Nakaoka, "High Frequency Soft Switching Inverter for Fluid-Heating Appliance Using Induction Eddy Current-based Involuted Type Heat," Proceedings of IEEE Power Electronics Specialists Conference, Vol 4, pp. 1874-1878, Cairns, Australia, June, 2002. [3] H. Terai, T. Miyauchi, 1. Hirota, H. Omori, Mamun A. Al, and M. Nakaoka, "A Novel Time Ratio Controlled High Frequency Soft Switching Inverter using 4th Generation IGBTs," Proceedings of IEEE Power Electronics Specialists Conference, (PESC), Vol. 4, pp. 18681873, Vancouver, Canada, June, 2001. [4] Laknath Gamage, Tarek Ahmed, Hisayuki Sugimura, Srawouth Chandhaket and Mutsuo Nakoka," Series Load Resonant Phase Shifted ZVS-PWM High frequency Inverter with a Single Auxiliary Edge Resonant AC Load Side Snubber for Induction Heating Super Heated Steamer", Proceedings of 2003 International Conference on Power Electronics and Drive Systems (PEDS), Vol. 1, pp. 30-37, Singapore, November,2003. [5] H. Terai, 1. Hirota, T. Miyauchi. H. Omori, K. Ogura, Y. Hirota, and M. Nakaoka, "Comparative Performance Evaluations of IGBTs and MCT in Single-Ended Quasi-Resonant Zero Voltage Soft Switching Inverter," Proceedings of IEEE Power Electronics Specialists Conference, (PESC), pp. 2178-2182, Vancouver, Canada, June, 2001.
H. Tanaka, M. Kaneda, M. Ishitobi, E. Hiraki, and M. Nakaoka, "Electromagnetic Induction based Continuous Fluid Heating Appliance using Soft Switching PWM High Frequency Inverter," Proceedings of IEEE-IAS (Industry Application Society), International Appliance Technical Conference, (IATC), pp. 11-20, USA. May, 2000. [7] Haruo Terai, Hideki Sadakata, Hideki Omori, Hidekazu Yamashita, Mutsuo Nakaoka :"High Frequency Soft Switching Inverter for FluidHeating Appliance Using Induction Eddy Current-based Involuted Type Heat", Proceedings of IEEE Power Electronics Specialists Conference (PESC), Vol.4, pp. 1874-1878 Cairns, Australia, June,2002. [8] H. Kifune, Y. Hatanaka, and M. Nakaoka, "Latest Development of a Novel Fixed Frequency Power Controlled Soft Switching PWM High frequency Inverter with a Single Auxiliary Switch for Induction Heating Super Heated Steam Generator," Transactions on The Institute of Electrical Installations Engineers-Japan, Transactions on IEEJ-IA (Industry Applications), Vol.22, No.10, part D, pp. 797-804, October, [6]
2002.
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