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Energy Efficient Induction Heated Cooking – Range using MCT based Hybrid Resonant Converter

Pradip Kumar Sadhu (1st Author) Associate Professor: Department of Electrical Engg.

Indian School of Mines (under MHRD, Govt of India) Dhanbad, Jharkhand, India

e-mail: [email protected]

Netai Paul (2nd Author) Assistant Professor: Department of Electrical Engg.



Dola Sinha (3rd Author) Junior Research Fellow: Deptt. of Electrical Engg.



Atanu Bandyopadhyay (4th Author) Assistant Professor: Department of Electrical Engg.

Asansol Engineering College Asansol, Burdwan, West Bengal, India


Abstract—This paper describes a converter where both series and parallel resonant circuits are used. In order to minimize losses, switching is made at zero current crossover. The converter described in this paper, has four Mos controlled thyristors in H–bridge configuration. It is used for an induction cooking-range with multi hot zones. The switching frequency is in the band of 20 kHz to 60 kHz i.e. 50 kHz. To optimize the system efficiency, different control strategies are discussed.

Keywords-MCT; Hybrid Resonant; Induction Heating; ZCS; H-Bridge

I. INTRODUCTION Modern devices like Mos controlled thyristors (MCT)

make it easier to apply induction-heating technique in kitchen ranges for domestic use [1, 2, 4]. Induction heating technique in kitchen ranges is becoming a strong alternative to gas, normal electrical hotplates and microwave oven. As compared to dielectric heating as in microwave oven, the proposed technique of induction heating is more convenient and cost effective [3, 5]. For example, in microwave oven, food is cooked in a special non-metallic bowel inside a closed chamber, while the proposed method would permit cooking using normal house-hold metal utensils like steel pan, pressure-cooker, iron bowl etc leading to a lot of savings in utensil cost. The proposed topology also possesses an unique advantage for Indian style of cooking, in which for majority of recipe, it starts with frying of foodstuff in vegetable oil in a metal pan [4, 8]. This art of cooking is quite possible in the proposed scheme while microwave oven does not permit frying or even handling the substances at the time of cooking. Besides, the proposed induction heating system is absolutely safe from shock hazard during cooking [3, 5, 6]. The induction-heating system is also extremely rugged because of absence of red-hot temperature in the induction coil resulting in no deterioration or aging of coil [7, 8]. Besides, there is no conduction loss during transfer of heat from source to the cooking pan in induction-heating

system [4, 8]. It is possible to get an efficiency of about 85 % to 89 % for each heating system. The induction heated cooking range was built and tested with and without cooking-pots.

II. SELECTION OF MCT AS SUITABLE SEMICONDUCTOR SWITCH

Wherever In the present scheme, the selection of MCTs over other conventional semiconductor switches (like Thyristors, BJTs, Power MOSFETs, IGBTs etc) has been guided by the following favorable conditions in respect of the former.

� A low forward voltage drop during conduction like normal thyristor. The normal thyristor is not a self turn off device but MCT is turned off by positive voltage pulse. So it has no requirement of commutation circuit.

� Fast turn-on time, typically, typically 0.4 � sec and fast turn-off time, typically 1.2 � sec for an MCT of 500 V, 300 A.

� Low switching losses compared to other semiconductor switches.

� High power handling capacity like normal thyristors. � High gate impedance which allows simpler design of

drive circuit. � MCT consumes very small power from control circuit for

switching-on, since it is a voltage pulse driven switch.

III. THE PRESENT SCHEME The resonant converter consists of four semiconductor

switches (MCT’s) one for each hotplate with, switching at 50 kHz. The converter is a combination of both series and parallel resonant circuits where the switching is made at zero current cross over (ZCS). The converter for two cooking zones is shown in Fig. 1. It consists of two parallel resonant circuits which represents two cooking zones. The

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two zones have diameters and output power levels of 14 cm for 1100 W and 18 cm for 1800 W respectively.

Different control strategies have been discussed to show that an optimum concerning efficiency occurs when switching around the zero crossing of the parallel resonance circuit voltage. With the converter implemented in an induction heating system, the total system efficiency (i.e. from the mains to the load) of around 85 % has been achieved. Discussions regarding detection of missing cooking pot and short circuit are included in later part of the paper.

An H-bridge with its load impedance is shown In Fig. 2. The load can be of series type, that is ZS, or it can be a parallel type like ZP. An advantage of the series circuit is that both zero current and zero voltage switching are possible. However, the full resonant current must pass through the switches resulting in ON losses. Another disadvantage is that the supply voltage must be reduced which means that a DC / DC converter must be used. Depending on the converter there will be a reactive power consumption or more complexity. In a parallel load, ZP there would be lower ON losses in the switches but turn-on / turn-off losses would be more as the switching takes place at high voltage and current.

Therefore, by using both series and parallel combined circuit, a hybrid converter can be used to reduce the losses in the switches. Fig. 3 shows a resonant converter system for one hot-plate. The mode of operation basically consists of interaction between two resonant circuits where the energy is transferred from the series resonant circuit (consisting of CR and L1) to the parallel resonant circuit (consisting of CR, RL and LR). By turning on one of the transistor pairs S1, S4 or S2, S3 a resonant current starts flowing through L1 to CR and when this circuit current is zero the transistors are switched off. Hereafter the series resonant circuit is disconnected and the energy transferred to CR is now dissipated as heat in RL by the current flowing in the parallel resonant circuit. RL is mainly an equivalent resistance for the magnetic loss mechanism in the induction heating system, and secondly it represents the ohmic resistance of the parallel resonant circuit components. Illustration of the currents in the series and parallel circuit are shown in Fig. 4.

IV. BRIDGE CONFIGURATION An important issue is the choice of a bridge

configuration. Two alternative bridge configurations to the full bridge configuration shown in Fig. 2 and 3 are shown in Fig. 5. The single switch configuration saves driver circuits and semiconductor switches. In the single switch configuration the current from L1 is only transferred to the parallel resonant circuit once per period of the current in the parallel resonant circuit. This means that a DC displacement of the parallel resonant current occurs equal to the mean value of the current through L1. A DC component would lower the efficiency of the system because it only produces ohmic losses and will not produce magnetic losses in the pots. Therefore the single switch configuration is rejected.

The half-bridge configuration is considered to give no savings compared to the full-bridge configuration due to the need of two extra capacitors. The half-bridge configuration will reduce the voltage over the parallel resonant circuit. A larger CR is demanded. The resonant current IR will be larger which is a disadvantage concerning the efficiency. Because switching loss will be more when current IR will be larger.

The full-bridge configuration shown in Fig. 3 is considered to give the highest efficiency when costs are considered compared to the single-switch and half-bridge configurations and is therefore chosen for this application

V. SWITCH CONFIGURATION It is important to keep the number of switches as low as

possible. A lower number of switches means lower price and a lower circuit complexity. In a system with two cooking zones, it is tempting to use some of the switches to transfer energy to both zones. Then savings in both drivers and switches can be made. The zones are represented by the components LR and RL.

The four main switches (S1, S2, S3, S4) have to be of larger current carrying capacity in order to deliver the same energy as in Fig. 6, where eight switches are used. As it is very unlikely to run both zones simultaneously, it still becomes necessary to over dimension the parallel resonant circuit components. Circuit of Fig. 6 becomes the choice of full bridge hybrid inverter for the two zones cooking range

VI. PARALLEL RESONANT CIRCUIT The sizes of the components in the parallel resonant

circuit are dimensioned by using the following formulas, which are valid at the natural resonant frequency that is when the Q-values is infinite. The Q-value is the relation between the reactive and active energy in the system.

A. Resonant frequency Initial test on the system showed an increase in efficiency

of 1.2 %, with an increase in frequency from 16 kHz to 33 kHz. On the basis of this and considering the obtainable speed of control circuitry and the fact that pets, specially dogs can hear up to 35 kHz, the resonant frequency was chosen at 50 kHz.

B. DC – Link A DC – link capacitor has not been used in order to

make savings and to avoid problems with high current at startup. The mean power related to the power at maximum DC – link voltage is expressed as:

2max4�

�� PP

In order to protect diodes and switches varistors and zener-diodes are used.

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C. Rectifier bridge

In the rectifier bridge, fast recovery diodes are used to prevent the DC-link current IL1 from going negative making zero current switching more difficult to obtain. This is necessary due to the missing DC-link capacitor.

D. DC – link coil L1 The size of L1 is chosen so that the natural resonant

frequency of the series resonant circuit is higher than the frequency of the parallel resonant circuit in order to ensure zero current switching. The efficiency of the system was found to be dependent on the size of L1 as well as the instance where the energy is fed into the parallel resonant circuit. L1 is therefore constructed so that the inductance can be varied and optimum value of L1 has been found experimentally.

E. Switches and freewheeling diodes MCT was chosen for this application due to lower

conduction losses combined with simple drive circuitry. The freewheeling diodes must only carry the reverse recovery current of the rectifier diodes and since fast recovery diodes are used in the rectifier bridge there are only little demands to the power capability of these diodes. The diodes must be able to withstand the same reverse voltage as the MCT’s.

VII. CONTROL This section describes how the system is controlled

regarding transfer of energy to the parallel resonant circuit, safety aspects during startup and how a missing cooking pot is detected.

A. Startup A safe startup is ensured made by measuring the DC –

link voltage and only starting the system when the voltage is below a certain voltage where the currents are safe, 100 V for example.

B. Missing cooking pot detection When the pot is removed the Q-value increases very

much. The current in the parallel resonant circuit does not change significantly, but the DC-link current becomes very small. This is shown in Fig. 7. A signal for detecting a missing cooking pot is therefore made by measuring the DC-link current. A circuit for this purpose was made and the resulting output of this circuit is shown in respectively Fig. 8 and 9 with and without cooking pot. From Fig. 8 and 9 it is seen that a missing cooking pot can be detected when the signal goes below 0.8 V.

C. Transfer of energy to the parallel resonant circuit The instance for the transfer of energy to the parallel

resonant circuit is important for the efficiency of the system. Initial tests shows that the best instance for starting the energy transfer is when the voltage over the resonant circuit is zero. The result was that the efficiency was increased from 75 % to 77 % when feeding the energy when the voltage UCR

passes zero. The tests were made with reduced voltage, where the max DC-link voltage was 270 V instead of 565 V under normal conditions. The efficiency was defined as the thermal energy in the load in the pot, i.e. in water was divided by the electrical input energy. Then the losses due to steam and heat radiation were included. The converter efficiency is thus higher than measurements shows.

D. Test of the system A system with one cooking zone was build and tested. In

Fig. 10, 11 and 12 respectively the DC-link current IL1, the parallel resonant current IR and the voltage over the parallel resonant capacitor UCR are shown. From Fig. 12 it is seen that the currents in the parallel resonant circuit looks very sinusoidal and have only a little content of high harmonics. This is an advantage concerning the efficiency because the system was optimized for the fundamental and higher harmonics would probably reduce efficiency due to skin effects and could also cause problems concerning radiation of electromagnetic energy. The DC-link current IL1 and parallel resonant capacitor voltage UCR also looks reasonable as they seem to have only little harmonic content. The DC-link current was optimized to be as wide as possible and still ensuring zero current switching. Making IL1 as wide as possible reduces the conduction losses in the system and this increases the efficiency.

E. Power factor The power factor of the system was measured to be 0.993.

An impression of a high power factor is given in Fig. 13, which shows the DC-link current IL1 and the voltage over the rectifier bridge. From Fig. 13 it is seen that the shape of the current peaks follows the voltage resulting in a high power factor.

F. Efficiency The efficiency of the system reached 85 % using only

the small cooking zone system. The converter efficiency was on this basis found to be 86.5 %.

VIII. CONCLUSION In this paper a resonant converter for an induction

kitchen range with two cooking zones has been described and test results for one zone have been given. The efficiency of the system reached 85.5 % with one cooking zone and can be expected to be higher when both cooking zones are operating. The reason for the high efficiency lies in proper optimization of the energy transfer between the DC-link and the parallel resonant circuit and in designing the system to minimize higher harmonics in the currents. The system has also a high power factor of 0.993. This will be more and more important in the future. It is also an advantage for the end users as this means a lower consumption of energy. Further a control strategy was developed for detecting a missing pot.

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ACKNOWLEDGMENT Authors are thankful to the UNIVERSITY GRANTS

COMMISSION, Bahadurshah Zafar Marg, New Delhi, India for granting financial support under Major Research Project entitled “Simulation of high-frequency mirror inverter for energy efficient induction heated cooking oven using PSPICE” and also grateful to the Under Secretary and Joint Secretary of UGC, India for their active co-operation.

REFERENCES [1] P.K.Sadhu, R.N.Chakrabarti, and S.P.Chowdhury, “A cooking

apparatus using high frequency induction heating,” Patent Number 68/cal/2001, Patent Office, Government of India.

[2] P.K.Sadhu, R.N.Chakrabarti, and S.P.Chowdhury, “An improved inverter circuit arrangement,” Patent Number 69/cal/2001, Patent Office, Government of India.

[3] P. K. Sadhu, S. K. Mukherjee, R.N. Chakrabarti, S. P. Chowdhury, and B. M. Karan, “Microprocessor–based energy efficient sterilization for surgical instrument using a new generation inverter topology,” Journal of Energy, Heat & Mass Transfer, Asia and the Pacific, Vol 23, Number 1, March 2001, pp. 39-53.

[4] P. K. Sadhu, S. K. Mukherjee, R.N. Chakrabarti, S. P. Chowdhury, and B. M. Karan, “A new generation microprocessor – based series resonant inverter for induction heated cooking appliances,” Journal of Indian Institution of Industrial Engineering, Navi Mumbai, Vol XXX, No 9, Sep 2001, pp. 10-15.

[5] P. K. Sadhu, S. K. Mukherjee, R.N. Chakrabarti, S. P. Chowdhury, and B. M. Karan, “A new generation microprocessor based radio–frequency operated induction heating for sterilization & boiler plant,” Journal of IEEMA, Mumbai, Vol XXII, No 2, Feb 2002, pp. 36-48.

[6] P. K. Sadhu, R.N. Chakrabarti, and S. P. Chowdhury, “A new generation fluid heating in non–metallic pipe–line using BJT and IGBT,” Journal of Institution of Engineers (I), Vol 82, March 2002, pp. 273-280.

[7] P. K. Sadhu, S. K. Mukherjee, R.N. Chakrabarti, S. P. Chowdhury, and B. M. Karan, “High efficient contamination free clean heat production,” Journal of Engineering & Material Sciences, National Institute of Science Communication, New Delhi, Vol 9, June 2002, pp. 172-176.

[8] Dr. P.K. Sadhu, Dr. R. N. Chakrabarti, Mrs. N. L. Nath, Naveen.K. Batchu, Smita Kumari, and Kumari Rimjhim, “Analysis of a series resonant superimposed inverter applied to induction heating,” Journal of Institution of Engineers (I), Vol 84, March 2004, pp. 214-217.

Figure 1. Hybrid resonant converter system for two cooking zones.

Figure 2. Converter with resonant load.

Figure 3. Hybrid resonant converter system for one cooking zone

Figure 4. Wave shapes of currents in the series and parallel circuits

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Figure 5. Modified half-bridge and single-switch configuration

Figure 6. Full-bridge circuit for two cooking range

Figure 7. A PSPICE simulation showing currents and voltages when the pot is removed

Figure 8. Signal across the sensor when the cooking pot is present.

Figure 9. Signal across the sensor when the cooking pot is removed

Figure 10. DC – Link current IL1 , at 565 V, 5 A/ div

Figure 11. Parallel resonant current at 565 V , 10 A / div

Figure 12. Parallel resonant voltage at 565 V over rectifier bridge

Figure 13. Full-wave rectified voltage and DC – link current IL1

Figure 14. Photograph of developed induction coil

Figure 15. Photograph of single zone cooking-range set-up

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