Despite significant advancements in medium-power industrial solid-state relay (SSR) solutions, challenges persist in home appliance induction heating. This process involves a two-stage power conversion: rectifying AC voltage through a diode bridge, then feeding the DC bus voltage (VBUS) into inverters for high-frequency current to induction coils. Conventional induction setups with multiple loads face limitations due to separate inverters for individual power needs. The loads-multiplexing technique uses time-multiplexed Single-Pole, Double-Throw (SPDT) Bi-Directional Switches (BDS) for parallel operation of diverse loads (Leq,1, Leq,2, etc.) resonating with capacitor banks, as seen in Figure 1.

Fig 1 Schematic of an induction heating system

Electro-Mechanical Relays (EMRs) are cost-efficient for low on-state resistance (5-10 mΩ) power sub-circuit blocks. However, their use is limited by voltages/currents exceeding 400 VAC/20~30 A. These challenges highlight the urgency of semiconductor-based designs to optimize system efficiency. The transition from EMRs to advanced SiC/GaN-based devices promises enhanced performance, voltage/current handling, and power multiplexing, propelling SSR solutions for medium-power industrial applications, particularly in induction heating for modern home appliances.

Ⅰ. Bi-Directional Switches (BDS) in Solid State Relays

Some of the BDS solutions that are in the voltage and current ranges of interest are as follows:
A. Single device: Standard TRIACs offer a Built-In Snubber (BDS) with low on-state losses (around 1.15 V at 30 A for an 800 V device). But they have limitations in turn-off behavior for commutating on-state current (dI/dt)C. Snubberless TRIACs have improved compatibility with 50-60 Hz applications but still fall short for specific needs.

B. Multiple power devices: Currently, reverse-blocking IGBTs (RB-IGBTs) are the sole commercially available devices with monolithic integration of modified IGBT structures and reverse-blocking capabilities. Operating at 600 V, these RB-IGBTs offer saturation voltages (VCE(sat)) of approximately 1.5 V at 30 A. Alternatives in the 650 V range include trench IGBTs paired with anti-parallel diodes for low VCE(sat) (<1 V at 30 A), reverse conducting IGBTs (RC-IGBTs) with embedded diodes (VCE(sat) ~1.25 V at 30 A), and Silicon (Si) based planar MOSFETs or Si SuperJunction (SJ) MOSFETs, known for lower on-resistance and conduction losses than IGBTs. Emerging Silicon Carbide (SiC) devices and Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) are gaining traction, offering smaller chip sizes, lower on-resistance, and higher power densities. The choice of device depends on specific application needs, with the potential for monolithic GaN BDS as a promising future option.

Ⅱ. Implementing BDS Solutions

Figure 2 depicts the common structure shared by the analyzed test vehicles. They rely on two primary inputs: a control signal (VIN = 0 V / 5 V) and an external power supply (VAC). These inputs feed into the BDS (Bridge Diode Switch) power terminals, which are electrically isolated from the inputs. The operation of these test vehicles varies based on factors like the number of power devices, their arrangement in the BDS, and their driving requirements (e.g., voltage/current control, normally-on/normally-off modes). Consequently, unique driving circuitry is designed for each solution.

Fig 2 Block diagram of the test vehicles.

The study investigates thirteen implementations using various semiconductor technologies. TRIACs, considered as a reference, are controlled using standard IGBT drivers, despite having higher control losses and power supply needs (1 W). Cascodes, IGBTs, and MOSFETs-based BDSs exhibit low control losses with only two 200 mW power supplies. However, RB-IGBT BDS demands individual drivers and power sources for each device, increasing control complexity, losses, and costs. SiC MOS FETs require non-standard power sources, elevating control complexity and costs. Different voltage power supplies are employed to bias gate opto-coupled drivers, producing a gate control signal (VCON) that enables or disables the power stage devices. Overvoltage protection for gate and power stages is integrated, along with a zero-current detection system to ensure safe operation.

GaN HEMTs and SiC JFETs-based BDSs feature low control losses (two 50 and 100 mW power supplies) but require specific gate drivers and opto-couplers due to unique driving voltages. GaN HEMTs offer monolithic integration capabilities, but both technologies require additional safety measures like TVS for overvoltage protection. SiC BJTs necessitate substantial gate currents, leading to high control losses (10 W power supply) and the need for separate opto-couplers and drivers.

Ⅲ. Characterization and Results

In the analyzed application framework, conduction losses are predominant due to low switching frequencies of the BDSs. Static I-V curves are obtained using a TEK 371A curve tracer for each BDS test vehicle in forward and blocking modes, utilizing bare die areas measured with a Sonoscan GEN-5 Scanning Acoustic Microscope. These curves provide static current densities (JD) and JD-VBDS profiles. All solutions considered are suitable for BDS implementation, but bipolar devices exhibit a slight offset voltage, potentially causing minor AC current/voltage distortion at the BDS in the zero-crossing region and increased power losses. Conversely, HEMTs, MOSFETs, and JFETs offer symmetrical conduction in the first and third quadrants.

Fig 3 Schematic of the test setup.

In a functional BDS, as IBDS approaches zero and VBUS begins, parasitic RLC elements may induce oscillations. These oscillations, determined by device output capacitances (Coss), vary in amplitude and frequency, explaining slower devices' lower oscillation frequencies. The SiC BJT+SiC Schottky Diode BDS experiences minimal voltage oscillations during blocking but exhibits a current peak due to BJT reverse recovery. Integration into a single chip could reduce this peak. RB-IGBTs and SiC/GaN-based BDS exhibit good dynamics, with GaN HEMTs offering excellent performance, making them a cost-effective mid-term replacement for EMRs.

Ⅳ. Conclusion

An experiment demonstrated the feasibility of advanced power semiconductor devices as alternatives to EMRs in home appliances. Various solutions met the necessary I-V static characteristics for BDS functionality, with SiC JFETs, SiC MOSFETs, and GaN HEMTs-based BDSs outperforming others due to their minimal conduction voltage drops. SiC JFETs showed even greater potential if more switches were available. GaN HEMTs emerged as the top EMR replacements, primarily due to their superior performance, especially when considering potential monolithic integration.