Boost-half-bridge edge resonant soft switching PWM high-frequency inverter for consumer induction heating appliances N.A. Ahmed and M. Nakaoka Abstract: A novel soft-switching PWM utility frequency AC to high-frequency AC power conversion circuit, incorporating boost-half-bridge inverter topology, which is more suitable and acceptable for cost effective consumer induction heating applications, is presented. The operating principle and the operation modes are described using equivalent circuits with the operating voltage and current waveforms. The operating performances are illustrated and evaluated, including the power regulation and power conversion efficiency against duty cycle characteristics based on the power dissipation as compared with those of the previously developed high-frequency inverter. The practical effectiveness of the power converter is substantially proved, based on experimental results from a practical design example.
1
Introduction
With the tremendous advances of power semiconductor switching devices, electromagnetic induction-current-based directly heated energy processing products and applications using solid-state high-frequency power conversion circuits, inverters, cyclo-inverters and cyclo-converter have attracted special interest for consumer food cooking and processing applications and hot water producers. Recently, costeffective induction heating (IH) appliances using highfrequency inverters have been rapidly developed as utility frequency AC to high-frequency AC power conversion systems for consumer power and energy applications in home and business use. IH equipment using high-frequency inverter topologies has the practical advantages of safety, cost effectiveness, energy saving, clean environment, very high thermal conversion efficiency, rapid and direct local focusing heating process, high power density, high reliability, environmental non-acoustic and low electromagnetic noise [1–8]. These unique advantages are practically gained in accordance with the great progress of power semiconductor switching devices, digital and analogue control devices, and high-frequency soft switching inverter circuits. In the above technological situations, high-frequency soft switching inverter topologies are indispensable for consumer IH appliances. These high-frequency soft switching inverters must have the advantages of simple configuration, high efficiency, low cost and wide soft commutation operating ranges, which are necessary for high-frequency operation. The voltage-source-type ZCS high-frequency inverter and its modifications match the practical operating requirements
mentioned previously. However, this type of high-frequency inverter cannot regulate its output power under constant frequency PWM control strategy and has low power conversion efficiency [2, 8]. Integrated one stage configuration of high-frequency power conversion can be established by combining both boost converter and half-bridge inverter in one conversion stage. In this paper, a novel prototype of a boost-half-bridge one-stage high-frequency soft switching PWM inverter, which converts the utility frequency AC power into highfrequency AC power with voltage boosting, is presented. It is a one-stage high-frequency inverter, composed of a single-phase diode bridge rectifier, non-smoothing filter, boost-half-bridge type zero voltage soft switching PWM high-frequency inverter, and induction heated load with planar type litz wire working coil assembly. The operating principle and the operation modes are presented using the switching mode equivalent circuits and voltage and current operating waveforms. The performance of this high-frequency inverter using the latest IGBTs is illustrated, which includes high-frequency power regulation and actual efficiency characteristics, unity utility side power factor in addition to zero voltage soft switching operation ranges under a pulse width modulation (PWM) control scheme. In addition, a dual mode control scheme of this high-frequency inverter based on asymmetrical PWM and pulse density modulation (PDM) control scheme is also discussed in this paper in order to extend the soft switching operation ranges and to improve the power conversion efficiency. 2
r The Institution of Engineering and Technology 2006 IEE Proceedings online no. 20060086 doi:10.1049/ip-epa:20060086 Paper first received 3rd March and in final revised form 24th May 2006 N.A. Ahmed is with the Department of Electronic and Electrical Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan M. Nakaoka is with the Electric Energy Saving Research Center, Kyungnam University, 449 Wolyoung-Dong, Masan, 631-701, Korea E-mail:
[email protected]
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Induction heating cooking appliance
Figure 1 shows the schematic configuration of a home and business use IH cooking and processing appliance. The solid-state high-frequency inverter circuit delivers highfrequency power to the planar-working coil with a mutual coupling secondary circuit of electromagnetic eddy-currentbased heated materials. These electromagnetic induction eddy currents flow directly through the pan or vessel. In accordance with Faraday’s electromagnetic induction law, a IEE Proc.-Electr. Power Appl., Vol. 153, No. 6, November 2006
Pan, Kettle etc
Top Plate
parallel to the IH working coil L1. It also includes the voltage boosted (charge-up) block composed of the boost inductor L1b and active power switch Q1(SW1/D1). As can be seen from the circuit configuration of the proposed topology, the switching block Q1(SW1/D1) shares and performs the operation of both single-phase boost chopper converter and high-frequency ZVS high-frequency
Cs
Pan
Qs
High Frequency Magnetic Flux Ds
Working Coil
SWs
L2 Q1
DB C3
L1
C2
C1 D1
High Frequency Inverter
SW1
a Qs(SWs/Ds) SWs Ds
Fig. 1
Cs
Schematic configuration of IH cooking appliance L2
L1
L1b
DB
large amount of thermal heating is produced with high conversion efficiency. In the case of multi-burner type highfrequency IH equipment, the output power of each burner has to be controlled under the same constant frequency, because of beat sound caused from difference frequencies of the operating inverters. To alleviate power dissipation of the working coil, it is necessary to block lower frequency current components that are not effective for induction heating. In particular, the power dissipation reduction due to lower frequency coil current components and an improved soft-switching high-frequency inverter operating under the condition of constant frequency pulse modulation should be developed and considered.
SW1 C3
D1
C2
C1
C2b
Q1(SW1/D1)
b
Fig. 2 a Previously developed high-frequency soft switching power converter b Proposed high-frequency soft switching power converter
3 Boost active clamp one stage high-frequency inverter
3.1
Circuit description
Figure 2a shows the previously developed soft switching PWM power converter using an active clamping circuit, which is composed of the active switch Qs and the capacitor Cs. The function of the clamping circuit is to reduce the working voltage of the power semiconductor device Q1 and enable variable power constant frequency VPCF operation. This power converter has some limits such as it cannot boost its output voltage. In addition, the current of the working coil has the components of utility frequency, which are not useful for IH and increase the losses of the working coil and semiconductor power devices SW1 and SWs. Incorporating a one-stage configuration of a highfrequency soft switching power converter can be established by combining the function of the boost converter and the half-bridge inverter in one conversion stage. Figure 2b shows the novel circuit configuration of the proposed onestage soft switching PWM power converter, incorporating only two switches for boost chopper and half-bridge zero voltage soft switching (ZVS) high-frequency PWM inverter. The boost-half-bridge one-stage high-frequency inverter circuit topology includes two active power switch blocks Q1(SW1/D1), Qs(SWs/Ds), divided series capacitors Cs and C2b and inductive load compensating capacitor C1 in IEE Proc.-Electr. Power Appl., Vol. 153, No. 6, November 2006
Fig. 3
Operating mode transient, voltage and current waveforms 933
PWM inverter. In addition, the divided series capacitors Cs and C2b are used to block the lower frequency current components flowing through the IH working coil, which is assembled from litz wire.
3.2 Principles of operation and switching modes Figure 3 shows the operating voltage and current waveforms and the operation modes of the proposed one-stage high-frequency power converter during one switching cycle. The corresponding equivalent circuits are shown in Fig. 4. The operating modes include six modes during one switching period, which will be simply explained in the following: Mode 1 (SW1: on, D1: off, SWs: off and Ds: off) The switching mode equivalent circuit is shown in Fig. 4a. In this mode, there are two current loops in the equivalent
Fig. 4
circuit. The magnetic energy is stored in the boosting inductor L1b through the loop C2-L1b-Q1-C2, while the energy is delivered to the induction heated load through C2b-L1-Q1-C2b. Mode 2 (SW1: off, D1: off, SWs: off and Ds: off) The equivalent circuit of mode 2 is shown in Fig. 4b. In mode 2, the resonant energy is stored in C1 through the two loops composed of L1b-C1-C2b-C2-L1b and L1-C1-L1. Mode 3 (SW1: off, D1: off, SWs: off and Ds: on) The energy is stored in Cs through the loop composed of L1b-Cs-C2b-C2-L1b and the energy is delivered to the IH load through the loop composed of Cs-L1-Ds-Cs. The equivalent circuit of mode 3 is shown in Fig. 4c. Mode 4 (SW1: off, D1: off, SWs: on and Ds: off) The equivalent circuit of mode 4 is shown in Fig. 4d. In mode 4, the energy is delivered to the IH load through the loop composed of Cs-Qs-L1-Cs and the energy is stored in
Operating mode transitions and equivalent circuits of proposed power converter
a Mode 1 b Mode 2 c Mode 3 d Mode 4 e Mode 5 f Mode 6 934
IEE Proc.-Electr. Power Appl., Vol. 153, No. 6, November 2006
the capacitor C2b through the loop composed of L1b-L1C2b-C2. Mode 5 (SW1: off, D1: off, SWs: off and Ds: off) The equivalent circuit of mode 5 is shown in Fig. 4e. During this operating mode, the energy is transferred to the IH load working coil L1 through the loop composed of L1-C1-L1 and the energy is stored in the capacitor C2b through the loop composed of L1b-L1-C2b-C2 as in the previous mode (mode 4). Mode 6 (SW1: off, D1: on, SWs: off and Ds: off) The mode equivalent circuit is shown in Fig. 4f. In mode 6, the energy stored in the IH working coil L1 is transferred into the capacitor C2b through the loop composed of L1-C2b-L1 and the energy is stored in the capacitor C2b through the loop composed of L1b-L1-C2b-C2, as in mode 5.
4 Practical implementation and experimental results
4.1
Design specifications
A 3.5 kW prototype of the proposed boost half-bridge soft switching PWM high-frequency power converter has been practically implemented using 4th generation IGBTs rated at 60 A, 950 V, model GT60M322, produced by Toshiba Semiconductor Co. Ltd. as power switching devices Q1 and Qs. A photograph of this IGBT is shown in Fig. 5. Table 1 gives the design specifications and circuit parameters of the
Fig. 6 coil
Photograph of developed power converter and its working
a Printed circuit board b Air-cooled litz wire working coil assembly, L1
newly proposed high-frequency inverter with boost-halfbridge topology.
4.2 Fig. 5
Photograph of GT60M322 IGBT
Table 1: Design specifications and circuit parameters Parameter
Symbol
Value
Rated output power
P
3 kW
AC input voltage
Vi
200 V
Working coil inductance
L1
58 mH
Iron pan resistance
RL
2.5 O
Charge-up boost inductor
L1b
500 mH
Filter inductor
L2
200 mH
Filter capacitors
C2
2 mF
C3
2 mF
Load compensating capacitor
C1
0.21 mF
Divided series capacitor
Cs
3 mF
C2b
4 mF
IEE Proc.-Electr. Power Appl., Vol. 153, No. 6, November 2006
Implementation of printed circuit board
Figure 6a shows the overall appearance of the newly proposed one-stage high-frequency power converter inverter for IH cooking heater with voltage boosting function, and Fig. 6b shows the exterior view of the litz wire assembled IH working coil. The power converter circuit and the control circuit are built on the same printed circuit board.
4.3 Measured voltage and current waveforms Figure 7 shows the measured operating current and voltage waveforms of the power switches Q1, Qs and IH working coil L1. It is easy to see that the power switches Q1 and Qs can operate under ZVS mode transition. Consequently, this operation reduces the switching losses and can allow high energy conversion efficiency. 5
5.1
Operating characteristics
High-frequency power regulation
Figure 8 shows the input power regulation against duty cycle characteristics and ZVS operation ranges of the newly 935
3500
Input power (W)
3000 2500 2000 1500 1000 Not ZVS
500
range
Newly Developed Previously Developed
0 12.5
Fig. 8
25 37.5 50 Duty cycle D = Ton / To
62.5
75
Input power against duty cycle characteristic
93.5 93.0
Efficiency(%)
92.5 92.0 91.5 91.0 90.5
Newly developed Previously developed
90.0 89.5 0
Fig. 9
500
1000 1500 2000 2500 Input power (W)
3000
3500
Conversion efficiency against input power characteristics
the previously developed high-frequency inverter. Both high-frequency inverters could not operate with ZVS mode in the area of duty cycle below 22%, where a pulse density modulation (PDM) control technique can be used for the proposed power converter to extend its soft switching operation range.
5.2
Fig. 7 Switching waveforms of voltage and current for Q1, Qs and L1 Current axis: 40 A/div.; voltage axis: 200 V/div.; time axis: 10 ms/div.
proposed and previously developed high-frequency inverter. Both high-frequency inverters work at 200 V utility frequency AC voltage source and can regulate their power continuously using an asymmetrical PWM control scheme with a constant switching frequency. Although the newly proposed one-stage power converter topology has significantly improved performances, it does not have the advantage of extending the soft switching operation range. It is recognised that the ZVS operation range of the newly proposed high-frequency inverter is similar to that of 936
Actual conversion efficiency
Figure 9 shows the characteristics of the power conversion efficiency against the input power of both high-frequency inverter circuits. The efficiency of the newly proposed onestage high-frequency inverter is below the previously developed inverter efficiency under the condition of input power in the range of 1 kW or less. This results from adding additional circuit components as the boost chopper inductor L1b. However, for high power setting of more than 1 kW, the proposed converter has high power conversion efficiency as compared to the previously developed one, because the saving in switching losses of the semiconductor switching devices Q1 and Qs due to the soft switching operation exceeds the losses in the boost inductor L1b.
5.3 Zero voltage soft switching commutation and power dissipation analysis Figure 10 shows the voltage and current waveforms of the power switches Q1 and Qs at the turn-off switching IEE Proc.-Electr. Power Appl., Vol. 153, No. 6, November 2006
Table 2: Comparison of power dissipations at 3 kW input power Components and devices
Power dissipations (W) Previously developed
Q1 (SW1/D1)
65
49
Qs (SWs/Ds)
30
21
L1
60
53
L2
25
18
L1b
–
30
DB (Diode bridge rectifier)
28
23
Capacitors C1, C2, C3, Cs, C2b
15
15
223
209
Total losses
Fig. 10 and Qs
Newly proposed
Switching waveforms at turn-off transitions of switches Q1
a Turn-off voltage and current waveforms of Q1 Current axis: 20 A/div.; voltage axis: 100 V/div.; time axis: 400 ns/div. b Turn-off voltage and current waveforms of Qs Current axis: 20 A/div.; voltage axis: 100 V/div.; time axis: 400 ns/div.
transition. It is clear that the voltage waveform rises slowly owing to the effect of the load resonant capacitor C1, while the current waveform declines sharply. As a result, turn-off power dissipation in power switches Q1 and Qs in the newly proposed power converter is dramatically decreased as compared to that of the previously developed converter. Turn-off power dissipation in the newly proposed boosthalf bridge one-stage high-frequency converter is computed to be 20 W for Q1 and 8 W for Qs at rated power of 3 kW. The maximum working voltage across Q1 or Qs is computed as 700 V. Therefore, the power switches Q1 and Qs are selected to have 950 V rating. Table 2 gives the total power loss comparison at the condition of rated power. The IGBTs losses are improved by 25% owing to a reduction in the operating voltage. The working coil losses are improved by 12% by eliminating the DC and low-frequency current components. When the proposed one-stage high-frequency power converter and the previously developed one are operating under the condition of the maximum input power of 3 kW, the power dissipation of the newly proposed one-stage converter is effectively estimated as about 6% lower than IEE Proc.-Electr. Power Appl., Vol. 153, No. 6, November 2006
Fig. 11
Utility AC voltage and current waveforms
Current axis: 10 A/div.; voltage axis: 200 V/div.; time axis: 2 ms/div.
the previously developed power conversion circuit. This is because of the elimination of the power loss in DC and lower-frequency working coil current components. The working voltage across Q1 as well as that across Qs can be lowered in this boost-half-bridge one-stage circuit topology and consequently the peak voltages of Q1 and Qs are both reduced.
5.4
Unity utility input power factor
Figure 11 shows the utility AC-side voltage and input current waveforms of this boost-half-bridge active clamp one-stage soft switching PWM high-frequency inverter. The input current and utility AC-side voltage become in phase, in other words, unity power factor with sinewave current can be obtained. Therefore, although the switch of the highfrequency inverter and the switch of boost chopper converter are shared, the proposed boost-half-bridge onestage power converter can operate as a boost power factor correction (PFC) converter. Figure 12 shows the built-in IHCH KZ-32A multi-burner IH cooking appliance, which uses the proposed inverter produced by Matsushita Electric Industrial Co. Ltd., Osaka, Japan. 937
developed high-frequency power converter with active clamping circuit and without the boosting function. For further future work, the boost-half-bridge one-stage high-frequency power converter using the promising power switching devices as emitter switched bipolar transistors (ESBTs) and silcon carbide junction field effect transistors (SiC-JFETs) will be evaluated and discussed in order to improve the overall power conversion efficiency. 7
Fig. 12 Built-in IHCH KZ-32A multi-burner IH cooking appliance uses proposed one-stage soft switching high-frequency power converter
6
Conclusions
In this paper, a novel circuit topology of utility frequency AC to high-frequency AC power converter employing a boost-half-bridge one-stage soft switching PWM highfrequency inverter has been proposed for consumer induction heating appliances. The new one-stage highfrequency IH inverter using boosted voltage function can eliminate the DC and low-frequency components of the working coil current and reduce the power dissipation of the circuit components. The operating principle, the operation modes and its unique features have been presented and discussed based on experimental and simulation results. The steady-state operating performances, which include high-frequency AC power regulation, and power conversion efficiency based on power loss analysis, have been experimentally illustrated as compared with a previously
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References
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