How do solid state isolators combine with MOSFETs or IGBTs to optimize SSRs?

Solid-state relays (SSRs) are crucial components in a diverse range of control and power switching applications, spanning white goods, heating, ventilation, and air conditioning (HVAC) equipment, industrial process control, aerospace, and medical systems. Solid-state isolators utilize coreless transformer technology to provide isolation between the high-voltage and low-voltage sides of an SSR.

CT-based solid-state isolators (SSIs) include a transmitter, a modular section, and a receiver, or demodulator section. Each section contains a coil, and the two coils are separated by a silicon dioxide (SiO2) dielectric isolation barrier (Figure 1). Magnetic coupling is used to transmit the signal between the two coils. This technology is compatible with standard CMOS processing and can be employed in discrete isolators or integrated gate driver ICs.

Figure 1. Example of a CT as used in a discrete SSI showing the SiO2 dielectric between the coils (right). (Image: Toshiba)

Combined with one or more power switches, SSIs can be used to create custom SSRs. For example, two N-channel MOSFETs can be driven with an SSI and used to control the 24 Vac power in an HVAC system.

The two MOSFETs support positive and negative current flow during the ON time (Figure 2a). During the MOSFET OFF time, the current flow is blocked by the reverse-bias body diodes (Figure 2b). Adding a pair of diodes (not shown in Figure 2) completes the rectification function and provides DC power to the load.

Figure 2. Turn ON and turn OFF current flows using two N-channel MOSFETs in an SSR. (Image: Texas Instruments)

SSR design considerations

While the basic topology of SSRs is simple, there are numerous additional design and performance considerations. The CMOS compatibility of CT-based SSIs simplifies the integration of protective functions like current sensing for over-current protection and temperature sensors for thermal protection. The ability of a CT-based SSI to directly deliver the gate drive power needed by MOSFETs and IGBTs eliminates the need for a separate power supply on the isolated side, simplifying SSR design.

The design should be tailored to the load type and characteristics. Is the load resistive and, therefore, simple to design for? If it’s capacitive, the SSR must be able to handle high inrush currents and may require a current-limiting resistor or positive temperature coefficient thermistor. If the load is inductive, an RC snubber circuit may be needed to protect the SSR from voltage spikes.

The design must consider the voltage and current requirements of the load being controlled. Environmental factors like elevated operating temperatures may require derating the SSR current. Heatsinking and adequate airflow can also be needed.

The SSR input must be designed to handle the input signal type. Is it AC or DC? The control signal strength delivered across the isolation barrier must be adequate to reliably trigger the power semiconductor switch.

In addition to providing galvanic isolation between the low-voltage control and high-voltage load/output side of an SSR, CT-based SSIs minimize the transfer of noise from the high-voltage output back to the sensitive control circuitry on the input. That can be especially important in high-frequency switching applications like driving silicon carbide (SiC) MOSFETs.

Driving SiC MOSFETs

SiC MOSFETs can be utilized in high-voltage and high-power SSRs for electric vehicles, as well as in industrial and military applications. Those MOSFETs generally require high-current gate drivers, especially for high-speed switching applications. SSIs with CTs can support the drive requirements of SiC MOSFETs, enabling high-power and high-voltage SSRs.

SiC MOSFETs require drive voltages up to 20 V, compared to 10 to 15 V for silicon MOSFETs and IGBTs. In addition, SiC MOSFETs require rapid charging and discharging of the input capacitance and gate charge to support high-frequency power control.

Additionally, sufficient drive power is required to minimize high-frequency switching losses and achieve the high efficiency for which SiC MOSFETs are known. CT-based gate drivers can deliver efficient drive for SiC MOSFETs, support isolation to protect system operation, and can directly interface with a microcontroller to simplify control (Figure 3).

Figure 3. Simplified high-power SSR circuit using a CT isolated driver and external microcontroller, and SiC MOSFET. (Image: Infineon)

Summary

CT-based SSIs can be used with a variety of power semiconductor devices, such as silicon MOSFETs, IGBTs, and TRIACs, as well as SiC MOSFETs, to create custom SSRs. The power-handling ability and features of those SSRs can be tailored to meet the specific needs of a wide range of applications and operating environments.