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version are current fed and voltage fed converters. The voltage
fed resonant inverter has been used in the designed system
because voltage source inverter VSI drives outperform cur-
rent source inverter CSI drives in terms of efficiency. This is
because we employ Insulated Gate Bipolar Transistor IGBT
switching devices in VSI drives, which are more efficient
than the Gate Turn Off Thyristor GTO or Symmetrical Gate
Commutated Thyristor SGCT devices used in CSI versions.
Another advantage of VSI over CSI, is that the VSI has a
small size and its output voltage waveform does not depend
on the type of load.
Induction heating system needed two principles to be in
the designer consideration, first one is keeping the inverter in cess in dynamic mode. The sophisticated programming logic
a resonant functioning state during the heating process, the
control circuit must be able to track frequency automatically
with the load variations [11–13]. This control circuit must
respond faster and more accurate while increasing the out-
put frequency of the inverter [14]. The second principle is
achieving the soft switching conditions, the output voltage and
inverter current must be in phase to avoid switching losses.
There is two control methods commonly used for these
purposes, self-oscillating drivers and phase locked loop (PLL)
[15]. The PLL has been utilized in this paper for induction
heating system because it is more reliable and accurate spe-
cially when implemented and analyzed with a software based
PLL algorithm. Many studies and research work used these
control methods for induction heating applications [16–19].
M. Ali, R. Srinivasu, and T. R. Jyothsna [20], conducted a
series resonant half-bridge inverter with a series parallel con- improved to make the inverter operated at the resonant fre-
nection of capacitors for an induction heating system. They
used PLL control unit to keep a unity power factor regard-
less to load changes. The performance of IH system with
power rated of 6kW had been tested on frequency (50-150
kHz). They ignored the impact of overlap period and snubber
components to get an information about the inverter output
currents and voltages.
M. Roy and M. Sengupta [21], proposed 2kW, IH system
with 10 kHz resonant frequency. They used digital-PLL based
on FPGA to track the resonant frequency. The IH system
consisted of CSI-fed IH coil linked to an H-bridge of four
IGBTs with series diodes. At resonance state, the phase shift
between voltage and current is ideally zero. The track of reso-
nant frequency had been achieved by adjusting the switching
frequency until the phase-shift is zero.
A. Namadmalan and J. S. Moghani [22], presented IH sys-
tem with a mechanism for adjustable self-oscillating switching
in current source parallel resonant inverters. To validate the
performance of that tuning system, a IH system consisted of
general-purpose ( full bridge ) CSI inverter with an operat-
ing frequency of 25-100 kHz had been used. A comparison
had been done between the conventional technologies such as
Al-Jrew, Mahmood & Ali
phase-locked loop (PLL) with the self oscillating method , the
latest methodology resulted in reduced voltage stress in the
inverter. The used switching approach tracked the changing of
resonance frequency. The self-oscillating switching method
offered the capability to tune the phase error, where the phase
error was important for parallel operating and zone control IH
systems.
G. D. Goranov, N. D. Madzharov, and I. O. Kandov [23],
worked on 20 kW IH system with a full bridge bipolar power
supply inverter. Digital PLL control method had been con-
ducted. They used digital format for the input and output
parameters to get an easy adjustment during the heating pro-
device called (coolrinner2 XILINX) had used for this purpose.
P. Herasymenko et al. [24], introduced control method for
IH application included a software phase-locked loop for 10
kW IH system with a resonant voltage source power supply
inverter. The software PLL control used to adjust the dead
time between the inverter switches devices and regulate the
fluctuation of load current amplitude and produced a phase
shift between inverter output voltage and current.
H. O¨ zbay, A. Karafil, and S. O¨ ncu¨, in [25] discussed 250
W IH system with a sliding mode controlled PLL using a
series resonant full bridge inverter. PLLs were utilized in IH
applications to obtain zero voltage and zero current switch-
ing mode. Sliding mode control used with PLL to avoid the
slow frequency tracking and make the controlling more re-
liable. Pulse density modulation PDM control method was
quency because of significant decreasing in switching losses
and electromagnetic noise . The operating frequency of the
IH system range between ( 35-42 kHz).
P. Herasymenko [26], developed a 2.5 kW IH system of
series resonant full bridge inverter with a digital control sys-
tem based on STM32 and a software PLL controller. The used
PLL system lead to phase synchronization between the output
voltage and current of the series resonant inverter. The experi-
mental studies done in that research work used the software
PLL with several control methods, including phase shift con-
trol, pulse-density modulation control, and PS-PDM control.
The range of frequencies used to test the system between ( 25
to 50 kHz). They claimed that the used PLL STM32-based
system in that research was appropriate for induction heat-
ing application with simultaneously dual-frequency output
current produced from two inverters.
The next section will take an overview about the con-
ducted five level power supply inverter topology for this in-
duction heating application. Section III discuss the PWM
technique used for multilevel inverter. Section IV includes
the description of the mathematical model used to design the
PLL control unit. Section V, details a description of the block