1. Introduction
In the new era, domestic Induction Heating applications are replacing the traditional electric and gas heating technology. The induction heating system has the inherent benefits of higher conversion efficiency with a lower time constant to achieve the required cleaner heating. Based on the IH application, it requires a high bandwidth inverter ranging from 20 kHz to 100 kHz. A typical IH system has a very high-power handling capacity of up to 500 kW [
1,
2]. In two-power conversion, commercial 50 Hz AC is rectified, filtered, and converted into High-Frequency AC (HFAC). This results in more power losses, and further, the bulky capacitor increases the system time constant with a lesser source side power factor [
3,
4]. Thus, a selective harmonic elimination control technique is used to improve the source side power factor [
5].
Various converter topologies, such as class E, class D, and full-bridge inverter fed topologies, were proposed in [
6,
7] and various power control techniques were used to control the output power [
8,
9]. The choice of the inverter depends primarily on the cost and the selected application. These topologies resulted in more losses due to AC-DC-AC power conversion. Thus, direct AC-AC converter topologies were proposed with a lesser number of components and with DC-link storage requirements [
10,
11]. The IH system’s efficiency depends not only on the topology, but also on the control algorithms used for regulating the output power [
12]. It plays a crucial role in regulating the temperature according to the load requirements. This can be accomplished by adjusting the angle of current or phase angle between voltage and current or the frequency of the supply.
The most widely used method to control the output power is the Pulse Width Modulation (PWM) technique [
13,
14]. This technique is very simple and gives a cost-effective solution for controlling the output power. Hard switching is the key downside to these control systems, which increases switching losses. Thus, this problem is overcome by a variable frequency control scheme with Phase Locked Loop (PLL) for regulating the output power [
15]. PLL helps in tracking the resonant frequency as and when the load varies. However, this control scheme results in acoustic noise and EMI problems in the system. Hence, the converter is manufactured using GaN devices [
16]. The operating frequency is varied from 20 kHz to 500 kHz. The system possesses fast switching transients with high reliability.
Phase Shift Control (PSC) is constant frequency control, in which output power control is performed by adjusting the phase shift of the inverter pulses [
17,
18]. Though this method possesses smooth power control without an EMI problem, the source side power is found to be much lesser. This problem is resolved by using the Asymmetrical Voltage Cancelation (AVC) technique [
19,
20]. In this method, the output voltage waveform is made asymmetric by adjusting the duty cycle of the inverter. This results in the larger dead band either in the positive or negative cycle for the zero crossings of the current. Due to the asymmetric voltage, the DC components are injected into the system. This issue was overcome by Pulse Density Modulation (PDM) control [
21,
22]. The output power is controlled by varying the density of the switching pulses without distributing the switching frequency. This method guarantees reliable and seamless output power control with lesser switching losses.
In a recent scenario, IH systems with multi burners (more than two) are preferred for cooking applications. The selection of appropriate control techniques for feeding power to multiple loads remains a typical problem in real time. A two-load IH system is proposed by Forest F et al. in [
23] by considering one load as a master and the other load as a slave. The resonant frequency of the load was varied by connecting additional capacitors using electro-mechanical switches to control the output power. This resulted in higher costs with a complex control. Sarnago et al. proposed an AC-AC converter for feeding power to two loads. PWM and PDM control techniques were used to control the output power and it was varied from 20% to 80% of the rated power [
7]. The converter was tested with different frequencies which resulted in switching losses. In [
24], a two-output Series Resonant Inverter (SRI) was proposed by Burdio et al. A high-frequency inverter was developed to feed power to two loads with the output power control using the AVC control technique. The main drawback of this scheme was cross interference as two loads were used. Hence, it is necessary to develop a simple control technique for mitigating the interference issue when multiple loads are used.
In the literature, various methods are used to reduce the cross-interference for wireless applications. One among them is by configuring the separate magnetic flux path for each load. This is achieved by designing the secondary receiver with an E-shaped ferromagnetic core [
25]. However, this technique is limited to low-power applications. For high-power applications, a novel control scheme is developed for extracting the maximum power from the source with minimum cross-coupling [
26]. Though the system works satisfactorily, designing a control circuit remains complex for real-time applications. Thus, a multisource fed system with multi-frequency was developed with various transmitters and receivers [
27]. This resulted in lesser magnetic flux linkage and, due to higher number of coils, the associated efficiency was also reduced. To avoid cross-coupling across the loads, frequency bifurcation phenomena were used in wireless power transfer applications [
28,
29]. This concept was used in [
30] for a wireless power transfer system with double, triple, and quadruple coils. The expressions for obtaining the coupling distance were obtained for different coils based on the system.
It is evident from the literature that there is a quest to propose a method to feed power to multiple loads with a single frequency to avoid the cross inference of supply. Hence, this work proposes the frequency bifurcation phenomena for feeding power to multiple loads with an AC-AC converter and controlling the output power with PDM control logic. The proposed control scheme provides a smooth power control with minimum switching losses for a multi-load fed IH system. The most significant contribution of this work is to analyze the performance of the control scheme for the single-stage AC-AC converter topology with a reduced number of semiconductor switches and to control the output power simultaneously and independently. The simulation of the system is tested with a 1 kW power rating in MATLAB Simulink and real-time implementation is carried out using a PIC16F877A microcontroller. The following points are unique with respect to the work:
- ⮚
The control algorithm feeds power to two loads with a single frequency source.
- ⮚
At rated power, the system possesses 5% lower power losses as compared with the existing articles [
31,
32].
- ⮚
The efficiency of the system is greater than 92% for 0% to 100% of the rated power.
- ⮚
As switching frequency is not varied, soft switching is realized for the whole operating range.
The course of this paper is organized as follows:
Section 2 describes the system description of a single-stage AC-AC converter. Simulation and experimental validation of the control technique are performed in
Section 3. The conclusion of the paper is presented in
Section 4.
4. Conclusions
In this work, a single-stage AC-AC series resonant inverter for a cooking application feeding power to multiple loads is proposed using a single supply frequency. The proposed scheme is tested by placing the transmitter coil and work coil at a distance of 3 cm, which produces the dual frequencies at the output side for a 25 kHz source frequency. The bifurcated frequencies, such as 20 kHz and 33 kHz, are used to power load 1 and load 2, respectively. The current spectrum confines that there is no cross-frequency interference across the loads. The output power control is performed by adjusting the duty cycle of the PDM signal. The temperature in both the loads is studied using an FLIR thermal imager for various time instants. The summary of this work is listed as follows:
Output power is controlled from 100% to 0% of the rated power.
The proposed inverter possesses higher efficiency of about 94.7% at 100% DPDM and is greater than 92% for other values of DPDM.
Independent power control is achieved.
The output power can be regulated to any value within the rated power.
At rated power, the system possesses 5% less power losses as compared with the existing articles [
31,
32].
As switching frequency is not varied, soft switching is realized for the whole operating range.
The PDM frequency is chosen as 25 Hz, which results in no noise.
From the test results, it can be concluded that the proposed system produces multiple output frequencies with a single source. Though the tested system is able to handle two loads, the power rating cannot be maintained as uniform due to multi-frequency operations. Thus, a converter topology is required to produce uniform power distribution across all loads. This work can be extended for more loads by varying the distance.