International Journal on Advanced Electrical and Computer Engineering (IJAECE)
______________________________________________________________________________________________
LLC Resonant inverter based Induction Heating Application using Asymmetrical voltage cancellation technique
1Susan Thomas, 2V.S Kirthika Devi
1,2Dept of EEE, Amrita School of Engineering, Bangalore, Karnataka Email: [email protected], 2[email protected] Abstract— Resonant inverters are widely used in
Induction Heating applications. A full bridge topology is investigated in this paper. The control technique used here is asymmetrical voltage cancellation control technique (AVC). Operating frequency is automatically adjusted with (AVC) technique to maintain a small constant lagging phase angle under load parameter variation during heating process. The output power is controlled using (AVC) technique. The validity of the proposed method is verified using MATLAB/Simulink and the operating frequency is around 53 to 55 kHz.
Keyword: Asymmetrical voltage cancellation (AVC)
I. INTRODUCTION
The use of power electronics equipments for consumers is increasing day by day, and hence there arises a need for the efficiency of these power conversion circuits to be increased. LLC resonant inverter has greater advantages including the zero-voltage switching (ZVS) in operating load ranges and zero-current switching (ZCS) for specific condition in case of diode rectifiers.
The popularity of the LLC resonant converter among other switching inverters is that it has less components and high power density.
Induction heating systems are developed using electromagnetic induction. Electromagnetic induction is the production of electromotive force across a conductor when it is exposed to a varying magnetic field which was first discovered by Michael Faraday.
Induction heating is a well known technique which is used to produce high temperature for applications like brazing, steel melting, and surface hardening. In each application, an appropriate frequency should be used depending on the geometry of the work piece and its skin depth. Induction heating technique requires high frequency current supply that can induce a high frequency eddy current in the work piece so as to produce the heating effect [12]. The main advantage of induction heating is that it does not harm the materials since there is no physical contact. The risk of unpredictable electrical hazards is also avoided.
Current fed and voltage fed inverters is the most commonly used varieties among inverters.
Semiconductor switching devices are the major risks in
PWM based converters. Recent developments in switching schemes and control methods have made voltage-source resonant inverter to be widely used in induction heating application that require output-power control capability.
The output power can be controlled by varying the switching frequency while the inverter is operating under zero voltage switching scheme (ZVS) [1].
The asymmetrical voltage-cancellation (AVC) is proposed in [2] and [3] where the voltage cancellation for conventional fixed-frequency control strategies is described.
AVC is implemented in a full bridge series-resonant inverter [4], [10]. The series resonant inverter requires an output transformer for the purpose of matching the output power to the load. Most of the induction heating applications like cooking purposes require accurate power control over wide range of power. Zero voltage switching (ZVS) can ensure high efficiency. By using this technique in fixed frequency and optimum duty cycle for ZVS operation, it is difficult to control the output power due to variation of parameters in the resonant load during the process of heating. In high temperature applications, a high current must flow in the surface of the metal for heating effect. One of the demerits of using series resonant inverter is that it may need a transformer for matching the output power and high current in the induction coil.
LLC configuration can offer a better performance than the series resonant [5]. In [6], a high efficiency LLC resonant inverter is used for induction heating using asymmetrical voltage cancellation control.
Realization of lagging leg switches are obtained, and the output current of the LLC converter makes up only a very tiny portion of the total output current. In this technique, the rectification loss is reduced and improves the conversion efficiency [7], [12]. Under light load condition, the converter efficiency got reduced due to high switching frequency.
A PWM based control strategy for an LLC converter for which the advantages include improving the stability of the system and also reduces the switching frequency in
light load applications [8]. The tank parameters are optimized to reduce the power loss.
ZVS series resonant converter for high input voltage application is used [9] where fixed switching frequency with duty cycle control is used to regulate the output voltage. Switching frequency is lower than the series resonant frequency.
VD C L R
C Cb
S2
S1 S3
S4 D1
D2
D3
D4
Ls Is
Io
Fig. 1. Full Bridge Resonant LLC resonant inverter
Fig. 2. Equivalent circuit
II. FULL-BRIDGE LLC RESONANT INVERTER
A. Circuit Description
Fig. 1 shows the LLC resonant inverter configuration for induction heating application. It consists of four switches with anti-parallel diodes, a resonant capacitor C, a matching inductor (Ls) and an induction coil that comprises of a resistance (R) and inductance (L) which are connected in series. An equivalent circuit of the full- bridge LLC inverter system is shown in Fig. 2 where the input voltage is viewed as an asymmetrical ac voltage supplied to the system.
B. Modes of Operation
There are five modes of operation present within one switching cycle. The corresponding modes of operation are illustrated in Fig. 3 to Fig. 7. The analysis is as follows:
1) Mode 1 (t0−t1): During the time when switches S2 and S3 are off, at t = t0, switches S1 and S4 receive positive gating signals and a negative input current (is) is flowing through diodes D1 and D4.
2) Mode 2 (t1−t2): In this mode, at the instant t1, as soon as the anti-parallel diodes D1 and D4 are turned off, switches S1 and S4 conduct and zero voltage switching(ZVS) operation is achieved. During this mode, the positive input current (is) is flowing.
3) Mode 3 (t2−t3): At t2, as in mode 1, S1 and S4 are turned off and the anti-parallel diodes D2 and D3 conduct by the positive input current (is).
4) Mode 4 (t3−t4): At t = t3, when the diodes D2 and D3 are turned off, the switches S2 and S3 conduct and ZVS condition is attained. Negative input current (is) flows in this mode.
5) Mode 5 (t4−t5): At t = t4, When the switch S3 conducts, the switch S2 is turned off and the diode D1 of S1 conducts. The ZVS condition of S2 is achieved.
VDC
L R
C Cb
S2
S1 S3
S4 D1
D2
D3
D4
Ls
Is
Io
Fig. 3. Mode 1 operation
VDC
L R
C Cb
S2
S1 S3
S4 D1
D2
D3
D4
Ls Is
Io
Fig. 4. Modes 2 operation
VD C L R
C Cb
S2
S1 S3
S4
D1
D2
D3
D4
Ls Is
Io
Fig. 5. Mode 3 operation
VD C L R
C Cb
S2
S1 S3
S4
D1
D2
D3
D4
Ls Is
Io
Fig. 6. Mode 4 operation
VDC L R
C Cb
S2
S1 S3
S4
D1
D2
D3
D4
Ls
Is
Io
Fig. 7. Mode 5 operation
III. CIRCUIT ANALYSIS
A. Detailed Analysis
Fig. 8. Typical waveform
The relationship between the load voltage (i.e. the capacitor voltage vc) and inverter output voltage (vo) can be calculated as,
) )
)(
(( j L j C R j L j L R j L L
j R v
v
S S
o c
(1) The inverter is designed to operate in such a way that the switching frequency (ω) is greater than the resonant frequency (ωo) so as to maximize the output power. The resonant frequency of the system in Fig. 2 is,
C L L
L L
S S
O
(2)The load voltage is given as,
1 2
C v L L
L L RL j L L v L
S S S
S
c
(3) We can represent the fundamental voltage v1 by the following Fourier series,
2 ( 1 ) cos ( 180 )
n
n
b
nV
m n(4)
sin ( 180 )
n
n a
nV
m(5)
Where α is the shifted angle of the switch S2 as in Fig. 8.
The amplitude of the fundamental voltage v1 is given by,
3 cos(180 )
) 180 ( sin2
1
Vm
V
(6) The average output power P can be obtained as,
1
2
Re ( )
v
oZ
coilj
oP
(7)
2 2
2 2 2
) )) 180 cos(
3 ( ) 180 (
2 (sin
S m
L L R
P V
(8)
Since the output power depends on the shifted angle (α), it can be easily controlled by adjusting the value of α.
IV. CONTROL STRATEGY
A. Control Description
Fig. 9. Control block diagram
Fig. 10. Generation of asymmetrical gate drive signals The controller consists of a current sensor, phase detector, and a voltage-controlled oscillator (VCO), as shown in Fig 9, (which represents the control block diagram for generating the gate signals as shown in Fig 10). The 4046 phase-locked loop IC is used for
frequency control at slightly higher than the resonant frequency. In typical voltage-fed inverter, the gate drive signal is in phase with the asymmetrical inverter output voltage Vo. Therefore, we can use the gate drive signal instead of the load-voltage pulse for phase detection.
The current signal io is compared with the voltage signal in order to detect the phase difference. A RC low-pass filter is present which filters the output signal of the digital phase detector to get an average value which is proportional to the phase difference at the load. The generation of asymmetrical gate drive signals for power control is shown in Fig 10. The Pcontrol signal is compared with the ramp signal from the 4046 IC to generate the gate signal G2. If Pcontrol signal is greater than the ramp signal, then G2 is set to high. Otherwise, it is set to low. In this way, α is dependent on the Pcontrol signal. The gate signal G3 is always off from π to 2π. The G1 and G4 signals are the inverse of G3 signal. Ramp signal is generated from the phase detector. Therefore, its frequency is automatically adjusted so as to track the resonant frequency and turns on as ZVS operation is obtained.
A limiter is used for phase-protection where the Vphase signal (a dc signal proportional to the phase), is put through a limiter. This is to allow an operation in the desired frequency range for the ZVS mode. If the phase lies in the region that is out of the predetermined limits, an active signal is sent to the transistor Q, thereby turning it on and grounding all gate signals S1 to S4. Hence the inverter is turned off.
V. RESULTS
In this paper, complete simulation has been performed on a full-bridge LLC resonant inverter circuit with asymmetrical voltage cancellation control technique. The following parameters are used: Vdc = 42.148V, L = 4μH, R = 0.1Ω, α = 60º, Ls = 24μH, C=3μF and Cb=9μF. The inverter operates at a frequency of 54 kHz. The closed loop simulation results showing control of output power to the load, the voltage and current waveforms are showed in Fig no. 11 and 12. Fig. 13 shows the switching states of the switches and the corresponding voltage Vo.
Fig. 11. Voltage Vo and Is of the inverter circuit
Fig. 12. Voltage Vc and current Io
Fig. 13. Switching states of the switches S1, S2, S3 & S4 respectively and voltage V0
VI. CONCLUSION
Once the work piece temperature increases, the impedance of the induction-coil changes in such a way that the resonant frequency increases. The phase-locked loop control then increases the switching frequency of the inverter in order to track for the resonant frequency.
This is to ensure the ZVS operation.
Closed loop simulation of the LLC resonant inverter circuit is verified through MATLAB/Simulink. The proposed AVC technique was found to have better stability over conventional controllers.
REFERENCES
[1] P Viriya, S Sittichok and K Matsuse “Analysis of
high frequency induction cooker with variable frequency control” in Proc PCC Osaka, vol . 3, pp 1502-1507 Apr 2002
[2] J.M Burdio, L. A. Barragán, F. Monterde, D.
Navarro, and J. Acero “Asymmetrical voltage- cancellation control for full-bridge series resonant inverters” IEEE Trans power Electronics,. vol 19,no.2 , pp. 461-469, Mar 2004.
[3] J. I Ar t i ga s , I U r r i z a , J Ac e r o , L A B a r r a ga n, D. Navarro, and J. M. Burdío ,
“Power measurement by output-current integration in series resonant inverters”, IEEE Trans. Ind Electron vol 56,no . 2, pp 559- 567,Feb 2009.
[4] S. Chudjuarjeen and C. Koompai, “Asymmetrical control with phase lock loop for induction cooking appliances,” in Proc. ECTI-CON, pp.
1013–1016,2008.
[5] J. M. Espí Huerta, E. J. Dede García Santamaría, R. García Gil, and J. Castelló Moreno, “Design of the L-LC resonant inverter for induction heating based on its equivalent SRI,” IEEE Trans. Ind. Electron,. vol. 54, no. 6, pp. 3178–
3187, Dec. 2007.
[6] Saichol Chudjuarjeen, Anawach Sangswang and Chayant Koompai, “ An Improved LLC Resonant Inverter for Induction-Heating Applications With Asymmetrical Control”, IEEE Trans. Ind.
Electron., vol. 58, no. 7, pp. 2915–2925, July 2011
[7] Mengyuan Yu, Deshang Sha, Xiaozhong Liao,“Hybrid phase shifted full bridge and LLC half bridge DC-DC converter for low–voltage and high-current output application,” Power Electronics, IET, vol. 7, Iss.7, pp. 1832–1840, Dec. 2013.
[8] Haiyan Pan, Chao He, Farooq Ajmal, Henglin Chen, Guozhu Chen ,Pulse- width modulation control stratergy for high efficiency LLC resonant converter with ligh load application”
Power Electronics, IET, vol. 7, Iss.11, pp. 2887–
2894, May. 2014.
[9] Bor-Ren Lin, Bo- Ren Hou, “Analysis and implementation of a zero-voltage switching pulse-width modulation resonant converter”
Power Electronics, IET, vol. 7, Iss.1, pp. 148–
156+, May. 2014.
[10] M. K. Kazimierczuk and C. Wu, “Frequency- controlled series-resonant converter with synchronous rectifier,” IEEE Trans. Aerosp.
Electron. Syst., vol. 33, no. 3, pp. 939–948, Jul.
1997.
[11] K. Harada, A. Katsuki, M. Fujiwara, H.
Nakajima and H. Matsushita, “Resonant converter controlled by variable capacitance devices,” in Proc. Power Electron. Spec. Conf., Jun. 1990, pp. 273–280.
[12] M. Enokizono and H. Tanabe, “Numerical analysis of high-frequency induction heating including temperature dependence of material characteristics,” IEEE Trans. Magn., vol. 31, no.
4, pp. 2438–2444, Jul. 1995.
