Efficient and Cost-Effective ZCS Direct AC–AC
Resonant Converter for Induction Heating

Resonant Converter for Induction Heating; Domestic induction-heating technology requires specific features such as high output power levels in a reduced enclosure, elevated operating temperature, large load variation, and reduced cost. To fulfill these requirements, classical solutions are based on the combination of a rectifier and a dc-link inverter.  This is a well-balanced solution, but there is still room for efficiency and cost improvements. Unlike previous proposals, this paper proposes a direct ac–ac converter to reduce the component count, reduce cost, improve reliability, and increase efficiency. The proposed converter is a voltage-source series-resonant converter that achieves linear output power control, reducing control complexity. Moreover, the proposed converter achieves soft-switching during both turn-on and turn-off transitions, further improving the efficiency and enabling the selection of low-speed devices with improved conduction properties. A 3.6-kW converter has been designed and implemented, verifying the feasibility of the proposal and the expected performance. 

I. INTRODUCTION

Resonant Converter for Induction Heating

INDUCTION Heating (IH) technology takes advantage of the contactless energy transfer to the pot to obtain faster heating times, improved safety, and higher efficiency than conventional heating methods. Recent advances in power electronics and magnetic component design have allowed the design of high-performance and reliable systems. This paper is focused on the design of a cost-effective and improved-efficiency power electronic converter for the domestic IH application.

In domestic IH systems, the pot is directly heated by the Joule effect caused by the eddy currents circulating through the base of the pot. Thus, a medium-frequency ac current generator, in the range of 15–100 kHz, is required to supply the inductor–pot system. Classical solutions perform this conversion in two stages [see Fig. 1(a)]. First, a full-bridge diode rectifier is used to convert the low-frequency mains voltage into a high ripple dc-link voltage.

A small dc-link capacitor is used to ensure an input power factor (PF) close to one. After that, an inverter stage generates the required high-frequency ac current to supply the inductor–pot system. This stage is the most restrictive due to its elevated requirements in terms of cost and reliability. Several solutions have been already described by other authors. Voltage-source inverters are commonly preferred to current-source inverters due to their reduced input choke.
Depending on the balance between cost and performance, which has to be evaluated in each application, different resonant topologies have been proposed. Low-power low-cost implementations use one switch quasi-resonant (1-SW) topologies, whereas, for high output power applications, the full-bridge converter is the preferred solution. The half-bridge series-resonant inverter has a good balance between cost and performance and is nowadays the most employed technology to implement domestic IH appliances up to 3.6 kW. 

Direct ac–ac conversion is intended to reduce component redundancy by using a single-stage converter [see Fig. 1(b)]. This leads not only to component and cost reduction but also to a potential improvement of efficiency and reliability. Several approaches have been proposed in the past. The first family of direct ac–ac converters proposes the use of four-quadrant switches, formed by the combination of simple switches, typically insulated gate bipolar transistors (IGBTs), MOSFETs, and diodes. These proposals lead to reliable and straightforward implementations. However, the main drawbacks of these solutions are the increased number of switches and control complexity that can compromise the converter cost and efficiency, which are two of the primary targets of this work. A second family of proposals has the advantages of component redundancy to regroup or remove common elements without additional complexity.

Some examples are the direct ac–ac resonant boost converter which only uses two rectifier diodes, or the single-stage active power factor corrector zero voltage switching (ZVS)-pulse width modulation (PWM) converter proposed.
This paper proposes an efficient and cost-effective solution for the domestic IH application based on a resonant direct ac–ac converter. The proposed converter includes the rectifier stage within the inverter, reducing the component count when compared with classical direct ac–ac converters [see Fig. 2(b)]. By using the modulation strategy proposed, soft switching is achieved during both turn-on and turn-off transitions for a wide load range. As a consequence, power devices with optimized conduction parameters can be used, further improving the converter efficiency. Additional advantages of this converter are the linear output power control and the reduced dependence on load variations, which are very common in IH applications.

The remainder of this paper is organized as follows. Section II details the proposed converter and its operation mode. Section III focuses on efficiency analysis, which is one of the key design aspects. Section IV shows the experimental setup and the main results used to validate the analytical results and the feasibility of the proposed converter. Finally, the main conclusions of this paper are drawn in Section V.

II. PROPOSED DIRECT AC–AC CONVERTER

The series-resonant half-bridge inverter [see Fig. 2(a)] is the most used inverter topology for domestic IH due to its well-balanced performance and cost. The resonant tank is composed of the inductor–pot system, modeled as an equivalent resistance Req and inductance Leq [30], [31], and the resonant capacitor Cr =CH +CL. In this topology, the supply voltage is applied to the resonant tank when the high-side switching device SH is activated, whereas the resonant tank is shortcircuited when the low-side switching device SL is activated. The effective voltage applied to the resonant tank is, therefore, a square wave, and the output power and switching conditions are determined by the switching frequency. Usually, frequencies above the resonant frequency are applied to obtain ZVS during the turn-on transition, although hard switching during the turnoff transition still occurs. A lossless snubber capacitor network Csnb is commonly used to reduce the switching losses during the turnoff transition.

The proposed power converter [see Fig. 2(c)] is designed to include the rectifier within the inverter stage, eliminating, therefore, the component redundancy and switch count when compared with classical direct ac–ac solutions [see Fig. 2(b)]. In addition to this, the proposed converter optimizes the switching conditions during both turn-on and turn-off transitions. Mains ac voltage vac is filtered by a small series inductance Ls and a small value capacitor Cs to ensure an input PF close to one and reduced harmonic content. Four fast switching diodes, DH, A, DL, A, DH, B, and DL, B, are used to rectify the mains and to provide a fast unidirectional path to naturally disable the load current when it reaches zero. After that, two complementary activated switching devices, SH and SL, provide the source voltage for the inductor–pot system. As a difference with
classical implementations, resonant capacitors CH and CL are directly connected to the mains voltage. Consequently, the load current cannot be recirculated through the switching devices, ensuring a unidirectional flow and reducing conduction losses. It is important to note that, unlike dc-link inverters, there is no energy storage element in the dc bus and fast diodes are used and embedded within the ac–ac converter. Moreover, zero current switchings (ZCS) are achieved during turn-on and turnoff transitions, and diode conduction is reduced, further increasing the converter efficiency.

The power converter operation modes depending on the mains voltage sign. Six different configuration states are identified and represented in Fig. 3. States I to III correspond to positive mains voltage, whereas states IV to VI correspond to negative mains voltage. Beginning with the positive mains voltage sign, when the high-side switching device SH is activated, the supply

voltage v.ac is applied to the resonant tank (state I), and consequently, load current i.o oscillates

Current io flows through DH, A, and SH. When io reaches zero, it is deactivated by the diode DH, A, and the resonant tank is open-circuited (state II) until the low-side switching device SL is activated. As a result, when both devices are deactivated, the switching devices must withstand not only supply voltage vac but also the remaining resonant tank voltage, i.e., the resonant capacitor peak voltage VˆCr. When SL is activated,
the resonant tank is short-circuited (state III), and consequently, load current flows through SL and diode DL, B. As it has been previously explained, the load current is deactivated when it reaches zero (state II) by DL, B.

When the supply voltage is negative, states IV to VI occur. In-state IV, the rectified value of the supply voltage is applied to the resonant tank through DH, B, and SH. When the resonant current reaches zero, it is deactivated by DH, B (state V). Again, when SL is activated (state VI), the resonant tank is shortcircuited by SL and DL, A, and current flows until io reaches zero (state V).

The main converter waveforms are shown in detail in Fig. 4, where the modulation parameters used to control the converter are also represented. The converter output power is controlled by means of the switching frequency FSW. The maximum output power is obtained at the resonant tank natural frequency. Unlike ZVS resonant converters, the switching frequency is reduced to reduce the output power while keeping ZCS lossless switching. Using previously obtained results from [29], an analytical model of the proposed converter can be obtained. The output current i.o is

 where the switching frequency is always lower than the natural frequency: FSW, max = ωn/2π. An important conclusion is that the output power variation is linear with switching frequency, simplifying the output power control.

 III. EFFICIENCY ANALYSIS

Resonant Converter for Induction Heating

One of the main design constraints for domestic IH converters is the efficiency that determines not only the environmental impact but also the converter performance and reliability. In this section, power converter losses are deeply analyzed.

 A. Conduction Losses Conduction losses are determined by the current through the devices and their conduction parameters, i.e., ON-state voltage (v.on for IGBTs and v.ak for diodes) and ON-state resistance (ron and r.ak for IGBTs and diodes, respectively). As a consequence,
Taking into account that the output power in a switching period can also be expressed as

B. Switching Losses
The proposed converter operates under ZCS for both turnon and turn-off transitions. However, nonidealities of switching devices lead to additional switching losses (see Fig. 5). First, IGBTs present two switching losses. The first one is related to the voltage fall time t.f,v during the switching-on transition [see Fig. 5(a)]. This switching energy can be calculated by

As a summary, the main converter ratings are shown in Table I, whereas a summary of converter losses is shown in Table II.

IV. EXPERIMENTAL RESULTS

A. Design Procedure In order to test the proposed converter and verify the analytical expressions, a prototype for domestic IH purposes has been designed and implemented (see Fig. 6). It is compared with

V. CONCLUSION

Resonant Converter for Induction Heating

In this paper, a direct ac–ac converter has been proposed for a domestic IH application. The main benefits of the proposed converter are the reduced number of components and increased efficiency. Moreover, linear output power control is achieved, reducing the control complexity. The proposed converter has been deeply analyzed, and analytical expressions have been provided for the output current, output power, power losses, and efficiency. The proposed converter operates under ZCS conditions during both turn-on and turn-off transitions. This allows
the designer to select power devices with optimized conduction parameters, further increasing the converter efficiency. The performance of the proposed converter has been verified by means of a 3.6-kW prototype designed for the domestic IH application. In a conclusion, a direct ac–ac converter is proposed as a cost-effective and easy-control design for the domestic IH application.