ISSN: 2277-6370
Global Journal of Advanced Engineering Technologies, Vol3, Issue1-2014
DESIGNING A HIGH EFFICIENCY IGBT SERIES
RESONANT INVERTER USING FUZZY LOGIC
CONTROLLER
1, 2
P.Sathishkumar1, R.Anitha2
Assistant Professor, Department of EEE, Maharaja Institute of Technology, Coimbatore, India
Abstract:-This paper analyzes a high-power (50 kW) high
frequency (150 kHz) voltage-fed inverter with a seriesresonant load circuit for industrial induction heating
applications, which is characterized by a full bridge
inverter made of insulated-gate bipolar transistor and a
power control based on pulse density modulation (PDM).
This power control strategy allows the inverter to work
close to the resonance frequency for all output- power
levels. In this situation, zero-voltage switching and zerocurrent switching conditions are performed, and the
switching losses are minimized. Results are verified
experimentally using a prototype for induction hardening
applications.
Index Terms:- Induction heating, pulse density
modulation(PDM) control, series-resonant inverter (SRI).
I.INTRODUCTION
Voltage or current fed inverters have been developed for
induction heating applications such as melting, forging
and surface hardening. A voltage-source inverter is a cost
effective solution, however, it doesn’t have the ability to
control the output power by itself, so that the output
power of such an inverter has to be controlled by
adjusting the dc input voltage. A thyristor bridge rectifier
having input inductors and a dc link capacitor has been
conventionally used as a variable dc- voltage power
supply. This causes some problems in size and cost. In
order to overcome these problems, inverter frequency
control or phase-shift control are normally used to
regulate the output power and use a diode bridge rectifier
like a dc voltage source. These power control schemes,
however, may result in an increase of switching losses
and electromagnetic noise because it is impossible for
switching devices to be always turned on and off at zero
current. Therefore, in high frequency induction heating
applications only MOSFET inverters can be used.
Nevertheless, IGBT’s are preferred in high power
industrial applications (availability, cost, etc.) and it will
only be possible if a low losses power control scheme is
found. This paper describes an induction heating system
of 50 kW, 150 kHz for industrial applications. The
induction system consists of a three-phase diode
rectifier, a single-phase voltage-source inverter using four
IGBT’s, and a series resonant circuit with a matching
transformer.
The working frequency is automatically adjusted close to
the resonance frequency in order to allow ZCS inverter
operation for any load condition. Exactly speaking, the
inverter performs as a quasi-ZCS because the transistors are
always turned off at almost zero current. The output power
control based on PDM maintains this condition in a wide
range of output power. The blanking time of the inverter
transistors is designed to maintain ZVS mode. With this
circuit an important improvement of the inverter efficiency is
expected in high frequency working conditions.
II. INDUCTION HEATING PRINCIPLE
Many practical work-pieces in induction heating
applications have cylindrical form and are heated by
being placed inside of coils with one or more turns.
The magnetic field induced in the coil, when it is fed
with an alternate current, causes eddy currents in the
work-piece and these give rise to the heating effect.
Theoretical analysis demonstrates that most of the
heat, generated by eddy currents in the work piece, is
concentrated in a peripheral layer of thickness given by
ߜ=ට
ఘ
గఓ௙
(1)
Where µ and ρ are the magnetic permeability and
electrical resistivity of the work-piece respectively and f
is the applied frequency. The induction heating load
(heating coil and work-piece) can be modeled by means
of a series combination of its equivalent resistance RL
and inductance LL. These parameters depend on several
variables including the shape of the heating coil, the
spacing between the work- piece and coil, the work-piece
temperature, its electrical conductivity and magnetic
permeability, and the frequency.
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ISSN: 2277-6370
Global Journal of Advanced Engineering Technologies, Vol3, Issue1-2014
III.SYSTEM CONFIGURATION
Fig. 1 shows the typical system configuration of a series
generator for induction heating. The output-power stage
consists of a single-phase voltage-source inverter using
four IGBTs. The output of the inverter is connected to a
series- resonant circuit with a matching transformer. The
dc power supply for the inverter is a three-phase diode
bridge rectifier connected to a 400-V 50-Hz power line.
The working frequency is 150 kHz, the maximum rms
value of the output voltage is 450 V, and the maximum
output power is 50 kW. Water-cooled load is used. The
output current is limited by power losses in order to
ensure the inverter reliability. The values of the main
components of the circuit are shown in Fig.1.
T is the period of the PDM sequence, Ton is the time
where the inverter is “running”.
t =
Q
(pf o )
(3)
where Q and ݂௢ are the quality factor and the
resonant frequency of the load circuit respectively,
and
æ 2ö
Pmax = ç
÷Vd I max cos q
èp ø
(4)
Where θ is the phase shift between output voltage and
current
Figure 1: System configuration
IV. PULSE DENSITY MODULATION
Figure2 shows the equivalent circuit of the voltage
source series resonant PDM inverter with its different
switching modes. A conventional voltage source seriesresonant inverter takes alternate mode I and mode II in
Fig. 2 (a) and (b) to produce a square-wave ac voltage
state. In addition to modes I and II, the PDM inverter
introduces mode III and mode IV to produce a zero
voltage state at its output terminals as show in Fig. 2 (c)
and (d). During mode III or mode IV, a gate turn-on
signal is provided to either lower or upper leg IGBT’s
respectively. As a result, both, one IGBT and a diode
connected in anti-parallel to the other IGBT, remain
turned on. Fig. 3 illustrates the principle of the PDMbased power control. The PDM inverter frequently
repeats “run and stop” in accordance with a control
sequence to adjust its average output voltage. The
inverter output power [6] is given by
éT t é1 - e P = Pmax ê on + ê
T
ê T T êë 1 - e - t
ë
Ton
t
T ù
ùæ - T
- ö
T
úç e - e t ÷ú
øúû
úûè
Figure 2: Switching modes in PDM. (a) Mode I. (b)
Mode II. (c) Mode III. (d) Mode IV
Figure 3: Switching pattern in PDM.
on
(2)
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Global Journal of Advanced Engineering Technologies, Vol3, Issue1-2014
ISSN: 2277-6370
V.CONTROL CIRCUIT
Figure 4 shows a block diagram of the control circuit
developed for the PDM inverter. The control circuit is
divided into the following three parts. (i)A FLC
circuit for phase control b e t w e e n i o and vo. It
m a i n t a i n s t h e s w i t c h i n g frequency close to the
resonance frequency in order to achieve ZCS condition.
(ii)It consists of a PDM circuit for output power closed
loop control.
Figure5: Experimental waveforms with pulse density ¼
Figure 4: Block diagram of the control circuit
Comparison of the reference of the output power Pref and
the actual average value Po generates a control value that
is compared with the absolute value of the output current
to obtain a synchronized logic signal that controls the
sequence of the PDM circuit. It consists of an actual
output power sensor, a power reference, a proportional
and integral Controller (PI), an absolute value circuit
(ABS), an analog Comparator and a flip-flop based
circuit for synchronization and a combinational logic
circuit.(iii) A blanking time generator. It obtains the
IGBT drive signal (Q1, Q2, Q3 and Q4) in order to
achieve ZVS operation
VI.EXPERIMENTAL RESULTS
The 50 kW, 150 kHz prototype described in section III
is being tested in order to meet the industrial application
requests. Fig. 5 and 6 show experimental waveforms of
the inverter output current and voltage, io and vo. Figure
5 corresponds to the case of a pulse density of 25%.
The dc input power of the inverter is 4 kW.
Figure 6 shows experimental waveforms during
operation at a pulse density of 75%. Now, the dc input
power of the inverter is 31 kW
Figure 6: Experimental waveforms with pulse density ¾
Fig. 7 illustrates experimental measures of power losses
of each IGBT module versus total output power of the
inverter working at 150 kHz for two output power
control techniques: frequency modulation (FM) and the
proposed control strategy (PDM). Induction heating
applications and especially the induction hardening
processes require a repetitive sequence of the inverter
switch on and switch off. This type of inverter work
implies the existence of a limited power cycling
capability of power devices; the temperature excursion
of the junction of IGBT modules depicts the quality of
its solders and bond wires. In order to obtain a reliable
operation of the inverter after one million of power
cycles with enough safety margin the junction
temperature excursion must be limited to 36 K. Dashed
horizontal line in Fig. 7 shows the module power
losses level that implies this junction temperature
increment for a thermal resistance, junction to case of
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Global Journal of Advanced Engineering Technologies, Vol3, Issue1-2014
Rth
JC=0.05 K/W supposing invariable the case
temperature for time intervals of few seconds.
Figure 7: Module power losses using Frequency Modulation
(FM) and Pulse Density Modulation (PDM) control
techniques.
Using this practical power limitation criterion, the
reliability of the inverter can be assured. In these
conditions, the inverter output power obtained with PDM
is more than 1.4 times bigger than the achieved with
frequency modulation control method.
ISSN: 2277-6370
REFERENCES
[1]E J. Davies and P Simpson, “Induction Heating
Handbook”, McCraw- Hill Book Company (UK) Limited,
1979.
[2]P. P. Roy, S. R. Doradla, and S. Deb, "Analysis of the
series resonant converter using a frequency domain
model", in IEEE/PESC Rec.,1991, pp. 482-489.
[3]J.M. Espí, E.J. Dede, J.Jordán, E. Navarro, S. Casans.
"The New Controlled Sources Method to Synthesize
Large-signal Circuits of Resonant Inverters". ISIE'99
International Symposium on Industrial Electronics.Bled,
Eslovenia 1999. pp. 345-350.
[4]L. Grajales, J. A. Sabate, K R. Wang, W. A. Tabisz,
and F. C. Lee, "Design of a 10 kW, 500 kHz phase-shift
controlled series-resonant inverter for induction heating,"
in IEEE/IAS Annu. Meet. 1993, pp, 843- 849.
[5] M Kamli, S. Yamamoto, and M. Abe, “An improved
method for the determination of induction heating loads
parameters,” in IEEE Ind. Applicat. Soc. Conf Rec., Aug.
1992, pp. 196-200.
[6] H. Fujita and H. Akagi, “Pulse-density-modulated
power control of a 4 kW 450 kHz voltage-source inverter
for induction melting applications,” in IEEE Trans.
Industry Appli., vol IA-32, no.2, pp. 279-286, 1996.
VII.CONCLUSION
This paper has proposed a voltage source series- resonant
PDM inverter for industrial applications of the induction
heating. This power control strategy allows that the
inverter works close to the resonance frequency for all
output power levels. In this situation zero-voltage
switching (ZVS) and zero-current switching (ZCS)
conditions are performed and the switching losses are
minimized. Therefore IGBT transistors can be used for an
optimum design of the power stage. A 50 kW, 150 kHz
PDM inverter prototype with IGBT is being tested
successfully in order to meet the industrial application
requests
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