Chapter-3 CHOICE OF HIGH FREQUENCY INVERTERS AND SEMICONDUCTOR SWITCHES This chapter is based on the published articles, 1. Nitai Pal, Pradip Kumar Sadhu, Dola Sinha and Atanu Bandyopadhyay, Selection of Power Semiconductor Switches a Tool to Reduce Switching & Conduction Losses of High Frequency Hybrid Resonant Inverter fed Induction Cooker, International Journal of Computer and Electrical Engineering, Vol. 3, No. 2, April, 2011, pp. 265-270. 2. Nitai Pal, Pradip Kumar Sadhu, Dola Sinha, Atanu Bandyopadhyay, Selection of Pan Material - a Tool to Improve Output Heating Response of Hybrid Resonant Inverter Fed Four Zones Induction Cooker, Journal of Energy, Heat and Mass Transfer, Asia and the Pacific, vol. 33, p.p.-169-185, 2011. 3. P. K. Sadhu, N. Pal and Dola Sinha, An Energy Efficient MCT based H.F. Inverter for Operating CFL from Solar PV Charged Batteries, Journal of IEEMA, Mumbai; Volume 1, No 11, July 2010, P.P. 84-88.
Choice of High Frequency Inverters and Semiconductor Switches 27 In this chapter, working principles of a few important types of inverters have been explored and their performances have been compared qualitatively. Finally, two inverter topologies are highlighted, out of which hybrid inverter is chosen for high power multi-zone induction cooker and the mirror inverter is suited for low to medium power single / multi-zone induction cooker. In the present chapter, the performances of different semiconductor switches are also verified through P- SPICE software and real time experimental model. 3.1 Introduction An inverter (Sen, 1998) is an electrical device that converts electricity derived from a DC source to AC and can be used to drive an AC appliance. Some industrial applications of inverters are for speed adjustment of AC drives, induction heating, stand by air-craft power supplies, Uninterrupted Power Supplies (UPS) for computers, high voltage DC transmission etc. Several kinds of inverters are available in the literature, such as Resonant, Pulse width modulated, Push-pull, Fly-back, etc. The resonant inverter consists of resonant circuit and it delivers maximum power to the load at resonant frequency. Resonant inverters are preferred in low power high frequency induction heating appliances like domestic induction cooker. Resonant inverters are of two types; i.e. series and parallel. Series resonant inverters offer reduced switching losses for the power-devices and possess attractive possibilities for high frequency operations. Moreover, higher efficiency, lightweight, overall simplicity in terms of inverter control, protection and maintainability have made it very attractive. In most of the inverter applications the control of output voltage and harmonic reduction are equally important. Different methods of voltage control can be classified into the following three broad categories: Control of the DC voltage fed to the inverter. Control of the output voltage delivered by the inverter. Control of voltage within the inverter using time ratio control techniques.
Choice of High Frequency Inverters and Semiconductor Switches 28 Inclusion of filter will obviously cause some additional power loss and will reduce the overall efficiency. Therefore, it is better to shape the output waveform by switching in a way such that the filter requirements are less. There are basically two techniques to shape the waveform of the output voltage. The first method is to perform switching in such a way so as to selectively eliminate or minimize undesirable harmonics. The second method is to shape the voltage waveform in such a way that the spectrum of harmonic frequencies be totally shifted in the direction of higher frequencies. In this case, the requirement of filter elements is considerably smaller in size and less cost. The second method is more general and often favoured for most of the applications. To reduce switching losses of power semiconductor devices in the proposed induction-heating home appliances, it is necessary to implement zero current switching (ZCS) or zero voltage switching (ZVS). The switches of a ZCS resonant converter turn ON and OFF at zero current. The resonant circuit consists of semiconductor power switch, inductor and capacitor. When the switch current is zero, there will be a current flowing through the internal capacitance due to finite slope of the switch voltage at turn-off. This flow of current will cause power dissipation in the switch and thus imposes limit on higher switching frequency. The switches of a ZVS resonant converter turn ON and OFF at zero voltage. The capacitor is connected in parallel with the switch to achieve Zerovoltage switching. The ZVS converters are used only for constant-load applications. The resonant converter topologies implement either ZVS or ZCS to cherish the advantages of the above features. Thus, the switching frequencies can be increased without increasing the power loss and as a result, the overall power conversion efficiencies can be improved. In the domestic induction cooker, the size, shape and material of cooking pot or pan are varied. As a result, the overall load of the inverter changes. So, ZCS is suitable for domestic induction cooker application. Different types of resonant inverters are discussed below for domestic induction cooker.
Choice of High Frequency Inverters and Semiconductor Switches 29 3.2. Series Resonant Inverter Series resonant inverters are widely used for induction heating over the wide range of high frequency from 4 khz to 500 khz. For self commutation, an under damped resonant circuit is essential. A capacitor is required for under-damped condition and it can be connected in series or parallel with the load. For forced commutation, a reverse voltage must appear across the Silicon Controlled Rectifier (SCR). This reverse voltage can be obtained from a charging circuit consisting of an inductor and a capacitor which are called the commutating component. They are connected to the load so that the overall circuit becomes under damped and zero current is obtained. In modern days, IGBTs, MOSFETs, B-SITHs, SITHs are preferred than SCRs as they offer self turn-off characteristics. Fig. 3.1 shows a basic series resonant inverter circuit. Q 1 and Q 2 are two IGBTs operating in the series resonant circuit. Q 1 turns ON when a gate voltage is applied to G1. M N G 1 L Q 1 O P Vi R G 2 C T Q 2 S Q Fig. 3.1: Basic Series Resonant Inverter Circuit. The capacitor C must be charged through the path MNOPQST to the voltage Vi. In the next time slot Q 1 and Q 2 remain OFF as no pulse is applied to the respective gate. The capacitor C maintains its voltage at Vi. Q 2 turns ON when a gate pulse applied to G 2. The capacitor C then discharges through the path POSQP. The current through inductor L changes its direction depending upon the gate signal. This type of inverter faces a drawback in high frequency operation. The capacitor
Choice of High Frequency Inverters and Semiconductor Switches 30 C charges from the source through the inductor L in the positive half cycle and then discharges through the inductor in the negative half cycle. This difference in current amplitude deing charging and discharging, makes it unsuitable for high frequency current generation. To overcome this problem half bridge series resonant inverter is used. 3.2.1. Half bridge series resonant inverter Fig. 3.2 depicts a half bridge series resonant inverter circuit. In the absence of any signal to IGBTs Q 1 and Q 2, capacitors C 1 and C 2 are changed to a voltage of V i /2 each. The gate pulse is then applied to the gate of Q 1 to turn-on Q 1. The capacitor C 1 discharges through the path NOPTN. At the same time C 2 charges through the path XNOPTSYX. The discharging current of C 1 and charging current of C 2 simultaneously flows from P to T. In the next slot of gate pulse Q 1 and Q 2 remain OFF and both the capacitor gets charged of V i /2 each. Gate pulse is then applied to the gate of Q 2 to turn-on Q 2. The capacitor C 2 discharges through the path TPQST. At the same time C 1 charges through the path XNTPQSYX. The discharging current of C 2 and charging current of C 1 simultaneously flows from T to P. The problem of unequal charging and discharging current through inductor L have been solved by half bridge series resonant inverter circuit. As this type of inverter operates at radio frequency range, some auxiliary circuits and equipments are required to minimize the switching losses occurring at high frequencies. N O X Q1 G 1 Vi C 1 T L R P C2 Q2 G2 Y S Q Fig. 3.2: Half bridge series resonant inverter circuit.
Choice of High Frequency Inverters and Semiconductor Switches 31 3.2.2. Full bridge series resonant circuit Full-bridge circuit is normally used for higher output power. A basic full bridge basic circuit is shown in the Fig. 3.3. Series R-L-C load is connected in conventional position of the circuit. In this circuit, four solid state switches are used, out of which two switches are triggered simultaneously. One anti-parallel diode is connected with each switch which allows the current to flow when the main switch is turned OFF. Q 1 Q3 G1 D1 G3 D3 Vi L R C Q2 Q4 G2 D2 G4 D4 Fig. 3.3: Full bridge series resonant inverter. The load circuit is on when switches Q 1 and Q 4 are triggered simultaneously at t=0. The current equation of resonant circuit is V i V te 1 ( c ) sin Rt 2 L r r L, when V c is the initial voltage of the capacitor. The current flows for a half cycle of the resonant frequency and become zero at tr t and both switches Q 1 and Q 4 are turned off. When Q 1 and Q 4 stop 2 r conducting and switch Q 2 and Q 3 are not yet turned ON the current through the load reverses and are now carried by D 1 and D 4, the anti-parallel diodes which connected with the respective switches. The voltage drops across diodes appear as a reverse bias voltage across switch Q 1 and Q 4. If duration of the reverse bias voltage is more than the switch turn-off time then switch Q 1 and Q 4 get commutated naturally and therefore, commutation circuit is not required. This
Choice of High Frequency Inverters and Semiconductor Switches 32 method of commutation is called load commutation and used in high frequency inverter for induction heating. 3.2.3. Mirror inverter Fig. 3.4 shows the mirror inverter circuit (Sadhu et al., 2010, Patent Number 244527, Government of India) with necessary harmonic filter. In the high frequency mirror inverter circuit a single point MN is stretched. The non smooth DC voltage is available across A and B points in the circuit. The high frequency AC pulses are fed to the gates of two IGBTs alternatively from the control circuit. IGBT-1 is turned ON when a pulse is applied to the gate G 1. The charge of capacitor C 1 discharges through the path QRMNQ as well as C 2 charges through the same path AQRMNOBA. So, the current flows in the short circuit bar from M to N. In the next half of operation, the gate pulse is applied in G 2 and turns ON the IGBT-2. Before ignition on G 2, gate pulse of G 1 is withdrawn to avoid simultaneous conduction and the short circuit between DC source terminals. After the ignition of IGBT-2 the capacitor C 2 discharges through the path NMPON loop and C 1 charges through the same path AQNMPOBA. Thus the current flows in the short circuit bar from M to N. By these gate pulses alternating current of radio frequency like 38.512 khz is generated in MN bar and this alternating current will flow through the induction heating coil, which will generate alternating magnetic flux. Cooking vessel Choke A C1 Vessel support Q R IGBT1 Rsn1 220 V, 50 Hz 1-Ph R X C X Bridge rectifier C N C2 IGBT2 M Rsn2 Harmonic filter B O P Signal for detection of cooking vessel Fig.3.4: Mirror inverter circuit for induction cooker.
Choice of High Frequency Inverters and Semiconductor Switches 33 3.2.4. Hybrid Inverter The hybrid resonant inverter (Pradeep, 1984) as shown in Fig. 3.5 consists of four semiconductor switches (MCT s) for each heating-range. The switching frequency of present scheme lies between 25 to 35 khz. The inverter is a combination of both series and parallel resonant circuits where the switching is made at ZCS. The inverter for two cooking zones consists of two parallel resonant circuits. The two zones have diameters and output power levels of 14 cm for 1100 W and 18 cm for 1800 W respectively. L1 LF S1 L R S3 R L S2 C R S4 Fig. 3.5: Hybrid resonant inverter circuit. Fig. 3.6 represents the hybrid resonant inverter system with series parallel combination of resonant load. Hybrid inverter for four cooking zones is shown in Fig. 3.7. The four zones have diameters and output power levels of 10 cm for 800 W, 14 cm for 1100 W, 18 cm for 1800 W and 22 cm for 2200 W respectively. Different diameters of induction coils have been chosen for different diameters of flat bed utensils. For getting maximum efficiency of the system, the coil diameter and the diameter of the utensils must be equal. Otherwise leakage flux reduces the overall efficiency of the system. Different control strategies show that a concerning efficiency occurs when switching around the zero crossing of the parallel resonance circuit. With the inverter implemented in an induction heating system, the total system efficiency (i.e. from the mains to the load) of around 88.2 % has been achieved.
Choice of High Frequency Inverters and Semiconductor Switches 34 I dc S1 S3 Z S Z S2 S4 Z P Fig. 3.6: Hybrid inverter with resonant load. U LINE Fig.3.7: Hybrid inverter for four cooking zones. The load can be of series type like Z S or parallel type like Z P (refer to Fig. 3.6). The advantage of series circuit is that both zero current and zero voltage switching are possible (Jana et al., 2004; and Pal et al., 2006). However, the full resonant current flows through the switches causes turn-on losses. Another disadvantage is that the supply voltage must be reduced with the help of a DC/DC converter. In
Choice of High Frequency Inverters and Semiconductor Switches 35 case of parallel load, on-load switching losses are lower but turn-on and turn OFF losses would be more as the switching takes place at high voltage and current. Therefore, using both series and parallel combined circuit hybrid inverter can be used to reduce the switching losses. The operation of this type of inverter consists of interaction between two resonant circuits where the energy is transferred from the series resonant circuit to the parallel resonant circuit. Turning on one of the transistor pairs S1, S4 or S2, S3, a resonant current starts flowing through L 1 to C R and when the circuit current is zero the transistors are switched off. The series resonant circuit is disconnected and energy transferred to C R is dissipated as heat in R L by the current flowing in the parallel resonant circuit. R L is mainly an equivalent resistance for the magnetic loss mechanism in the induction heating system and it represents the ohmic resistance of the parallel resonant circuit component. Therefore, hybrid inverter is better for high power multi-zone cooking and mirror inverter is better for low to medium power single zone cooking system. 3.3. Choice of semiconductor switches at high frequency hybrid resonant inverter-fed domestic induction cooker Various devices such as power MOSFET, thyristor etc., are applicable to high frequency and high power induction heating. The recent breaks through the development of Insulated Gate Bipolar Transistor (IGBT) technology have made it to be the obvious choice. Moreover, IGBT offers low ohmic resistance and requires very little power for gate drive. The advantages of using IGBT are in reduced electro-dynamic stresses, considerable reduction of EMI noise level, snubber less arrangement, high efficiency, lighter weight and lower cost. Among all the semiconductor switches, IGBTs found more suitable (1 khz to 60 khz) for domestic induction cooker as it has low turn-on ohmic resistance and in-built gate isolation. Also, it requires low power for gate drive. An IGBT combines the advantages of BJT and MOSFET. An IGBT has high input impedance like MOSFET and low on state conduction losses like BJT. However, there is no second breakdown problem as with BJTs. An IGBT is turned on applying a positive gate voltage. It requires a very simple driver circuit. It has
Choice of High Frequency Inverters and Semiconductor Switches 36 lower switching and conduction losses while sharing many of the appealing features of power MOSFETs such as, ease of gate drive, peak current capability and ruggedness. An IGBT is inherently faster than a BJT. However, the switching speed of IGBT is inferior to that of MOSFET. One experiment has been carried out among the different semiconductor switches by using PSPICE software as well as real time experiment to find the suitable semiconductor switch for domestic induction cooker. 3.4. PSPICE simulation The circuit diagram used for the simulation work has been constructed using PSPICE software as shown in the Fig. 3.8. The single zone hybrid resonant inverter is used here. The parameters of inverter have been shown in Table-3.1. A bridge rectifier is modeled using four diodes of 1N6392 type. In the circuit, four same (1N6392) type of anti parallel diodes are used across the semiconductor switches. Total time period is taken as 26µsec where IGBT on time is 12µsec and off time is 14µsec. For high frequency hybrid resonant inverter four IGBTs of APT30G100 BN type are used. L2 L1 1 2 D1 1N6392 1 2 D2 1N6392 Z1 1 2 D5 1N6392 Z3 1 2 D7 1N6392 VAMPL = 220V FREQ = 50Hz V1 C1 V3 R1 C2 L3 V5 1 2 D3 1N6392 1 2 D4 1N6392 APT30G100BN 0 Z2 1 2 D6 1N6392 Z4 1 2 D8 1N6392 0 0 V2 V4 0 0 Fig. 3.8: Circuit diagram for PSPICE Simulation of hybrid resonant inverter using IGBT.
Choice of High Frequency Inverters and Semiconductor Switches 37 Table 3.1: Input parameters for experiment. Filter circuit components Heating coil parameters (reflected values): L1=50uH, L2=100µH, C1=5µF L3=140µH, R1=0.12Ω Parallel capacitors (C 2 ) = 0.2µF Switching frequency = 30 khz Firstly, the inverter circuit is simulated with IGBT as power switch. Thereafter, IGBTs has been replaced by GTO and MOSFET. In each case, coil current waveform and voltage across it have been recorded and investigated. Finally, a real-time experiment has been carried out to validate the experimental result. With the selected circuit parameters and configuration, following waveforms have been obtained using PSPICE software and real-time experimental model using IGBT, GTO and MOSFET (refer to Figs. 3.9, 3.10 and 3.11), respectively. Using IGBT in the hybrid resonant inverter circuit, it is observed in Fig. 3.9 (a) and (b) that magnitude of current through the coil has almost equal in both positive and negative halves. Such a peak to peak symmetrical current produces more heat. Hence, heating effect becomes very prominent for the same operating frequency range. Further, it is observed that the real voltage across heating coil and current through it, are almost identical to the PSPICE simulated result. (a)
Choice of High Frequency Inverters and Semiconductor Switches 38 (b) Fig. 3.9: Coil current and coil voltage using IGBT by (a) PSPICE software and (b) real time experimental model. (a)
Choice of High Frequency Inverters and Semiconductor Switches 39 (b) Figure 3.10: Coil current and coil voltage using GTO by (a) PSPICE software and (b) real time experimental model. (a)
Choice of High Frequency Inverters and Semiconductor Switches 40 (b) Figure 3.11: Coil current and coil voltage using MOSFET by (a) PSPICE software and (b) real time experimental model. In Fig. 3.10, the real voltage across heating coil and current through it, are indistinguishable to the PSPICE simulated result. But, current through the coil using GTO in the hybrid resonant inverter circuit, doesn t have equal positive and negative peaks. Hence, rms value of the current will be less. So heating effect will also be less compared to IGBT. Furthermore, GTO-based topology requires the arrangement of sending a negative gate pulse through the gate terminal of GTO in order to turn it off, which enhances the circuit complexity. Fig. 3.11 depicts that using MOSFET in the hybrid resonant inverter circuit, the real voltage across heating coil and current through it, are almost same to the PSPICE simulated result. Here also, coil current doesn t have equal positive and negative peaks. Hence, rms value of the current will be less. So heating effect will also be less compared to IGBT.
Choice of High Frequency Inverters and Semiconductor Switches 41 3.5. Summary: After comparison of the wave-forms of PSPICE simulation and real time experiment, it is quite obvious that the selection of IGBT as a power semiconductor switch in high frequency hybrid resonant inverter is advantageous for induction heating purposes for frequency below 50 khz and highly acceptable. IGBT offers highest rms value of coil current among all the probable configurations using different power semiconductor switches. For a frequency range of above 50 khz, MOSFET will be a better option due to its low switching and conduction losses. ******************