The era of solid-state relays and solid-state circuit breakers is coming

Electromechanical relays and circuit breakers are a well-established technology that is used in a wide variety of applications, but they still have some inherent drawbacks. Although semiconductor-based solid-state technology can solve these problems, it presents a number of design challenges. So what is the right solution?

Currently, most high-voltage relays and circuit breakers are electromechanical.

In addition to the fact that people have been accustomed to using relays and circuit breakers for hundreds of years, the engineering field also generally believes that semiconductor technology is not suitable for high-voltage switches. However, the development of power semiconductor technology in recent years is changing this phenomenon and changing people's inherent impressions, which will be introduced in this article.

Electromechanical vs. Solid State

To this end, it is necessary to review what electromechanical relays and circuit breakers are, as well as the development status of solid-state relays and circuit breakers.

The noise generated by electromechanical relays and circuit breakers comes from their physical properties. Electromagnetic attraction/release of fast-moving metal contacts generates audio and electrical noise. Among them, the action of the mechanical contact is a failure point, with a limited lifespan. Fatigue mainly occurs on the surface of the contact. This is because the high voltage may generate an arc, causing the air gap to break before the contacts are fully in contact, generating arc gap voltage and arc gap current, and generating high temperature. The same phenomenon will occur when the contacts are open, or even more serious.

It should be noted here that when the contacts are connected and disconnected, AC or DC voltage will appear on the contacts. When connecting and disconnecting AC voltage, it is not a zero voltage switch, and arcing may occur every time the relay is operated. Eventually, the contacts will degrade rapidly, the resistance between the contacts will increase, and even fuse together. This is why manufacturers of electromechanical relays and circuit breakers give service life in product manuals.

When using solid-state relays to switch AC power, zero voltage switching is usually performed, which ensures that the device operates at the lowest voltage. When switching DC voltage and current, the fast characteristics of solid-state switches can be used to quickly break to avoid excessive inrush current, which ultimately makes the relay or circuit breaker more reliable throughout its service life, and its life may be longer than that of electromechanical relays/circuit breakers.

Engineers prefer electromechanical relays/circuit breakers for very simple and good reasons: cost, performance and functionality. It is true that solid-state relays/circuit breakers are higher in cost than electromechanical relays/circuit breakers, but when you factor in the lifespan as well as the cost of maintenance, upkeep and operation, solid-state relays/circuit breakers are more cost-effective.

In terms of performance, the most important parameter is the power loss caused by the resistance of the contact switch. For electromechanical contacts, the contact resistance is low when new, but over time, the contact resistance will inevitably increase and the loss will also increase. When using solid-state solutions, the power loss is directly related to the on-resistance. The on-resistance of the new generation of power semiconductors has been greatly improved and will not change during the service life. Ideally, the conduction loss and semiconductor cost can be expressed by the quality factor (i.e. the area on-resistance, RDS(on)*A), which is the focus of semiconductor manufacturers and the advantage of Infineon through the CoolMOS technology platform.

Figure 1: Superjunction MOSFET RDS(on)*A advantage over withstand voltage

Another issue is safety. Solid-state switches have no moving parts and can switch much faster than electromechanical switches, which is an advantage, but there is also a disadvantage: there is no physical break between the input and output, and in many human-machine interfaces, safety regulations require electrical insulation between high-voltage inputs and outputs.

Electrical insulation is a disadvantage of solid-state technology, so the concept of hybrid circuit breakers or relays came into being, that is, using solid-state devices to switch high voltage, and then electromechanical relays as mechanical breakpoints to switch at zero voltage and zero current. At this time, small-sized electromechanical switches can be selected.

Of course, there are many applications that do not require electrical insulation, but the existing regulations for circuit breakers are still based on electromechanical devices and do not fully consider the characteristics and superior performance of solid-state devices. Standards for solid-state switches are being developed, and they may be less stringent in terms of electrical insulation requirements for certain specific applications.

The rise of the superjunction MOSFET

Solid-state switching is achieved with power semiconductors, with silicon being the most widely used material to date. For AC switching, especially in zero voltage switching applications, triacs and silicon-controlled rectifiers (SCRs) are preferred, MOSFETs are typically used to switch DC voltages, and IGBTs can be used for both AC and DC switching.

However, these power semiconductor switches all have losses due to on-resistance, which generates heat and may require a heat sink, ultimately increasing system space and bill of materials.

Figure 2: The size of the solid-state relay is greatly reduced

Super junction MOSFET can effectively reduce the on-resistance RDS(on). Since the 1990s, Infineon has always been a leader in super junction MOSFET and has continuously developed this technology. Compared with other MOSFET structures, it has obvious advantages, especially in terms of on-resistance RDS(on)*A calculated by area, which correspondingly reduces losses and has lower costs. It can also be used in applications with higher voltages and currents under natural cooling conditions.

With CoolMOS 7 technology, Infineon has become the leader in RDS(on)*A. In addition, Infineon has released a new technology, CoolMOS S7, which provides lower RDS(on)*A and optimizes the reduction of on-resistance by appropriately increasing switching losses. In solid-state relay and circuit breaker applications, this fully meets the performance requirements of the device, which does not require high-frequency switching.

Conclusion

There are many advantages to using solid-state/hybrid relays and circuit breakers. They significantly speed up the opening and breaking time, eliminate arcing and noise existing in electromechanical relays or circuit breakers, and essentially have better reliability and predictability, and longer service life. Solutions such as Infineon CoolMOS 7, silicon carbide MOSFETs and SSI series solid-state isolators are solving some of the shortcomings that often affect applications.

Infineon's latest super junction MOSFET platform has made a major breakthrough in the design of solid-state relays and smart circuit breakers. It achieves an unprecedented RDS(on)*A quality factor to meet the needs of designers and their end markets. More importantly, solid-state relays are much smaller than electromechanical relays, saving installation space.

Superjunction MOSFET is just one of Infineon's product families that meets the need for innovation in the power supply sector. Thanks to the development of CoolMOS 7, silicon carbide MOSFET, SSI series solid-state isolators and other technologies, solid-state relays and circuit breakers are becoming more and more feasible. Infineon has a long history of innovation and will continue to develop and deliver solutions that achieve more benefits at a lower cost.