Relays have been used as switches since before the transistor was invented. The ability to safely control high-voltage systems from lower-voltage signals, as is the case in isolation resistance monitoring, is necessary for the development of many automotive systems. While the technology of electromechanical relays and contactors has improved over the years, it is still challenging for designers to achieve their goals of lifetime reliability and fast switching speeds, along with low noise, shock vibration and power consumption.
Solid-state relays (SSRs) exhibit performance and cost benefits and are rated for different levels of isolation. SSRs also possess advantages over alternative technologies such as electromechanical relays and solid-state photo-relays.
Figure 1. The TPSI2140-Q1 reduces solution size in high-voltage systems.
Traditional relay switching solutions
Electromechanical relays (EMRs) are common in high-voltage switching applications. EMRs employ the use of electromagnetic forces to mechanically switch contacts on and off. Given their mechanical nature, EMRs feature an incredibly low on-resistance; their contacts are essentially a metal-to-metal connection.
EMRs do have trade-offs, however, when it comes to switching speeds and reliability. Moving parts inside the relay are a limiting factor, and switching speed is typically in the 5 to 15 ms range. Over time and with use, an EMR can experience failures such as arcing, chattering and welding shut.
Unlike EMRs, photo-relays have no moving parts and provide a high isolation voltage. Photo-relays are an improvement over traditional EMRs; but they also have design considerations such as limitations on the achievable power transfer as well as deterioration of the internal LED. Additionally, photo-relays need an external current-limiting resistor and often use additional field-effect transistors (FETs) to manage the LED’s switched state.
Higher-reliability isolation using SSRs
Solid-state relays from TI are available as switches (with integrated FETs) or drivers for controlling external FETs. TI’s TPSI2140-Q1 isolated switch and TPSI3050-Q1 isolated driver feature higher reliability and longevity compared to EMRs, since they do not experience mechanical deterioration over time. SSRs thus enable a ten times higher lifetime reliability than traditional EMRs. These SSRs can also switch in the microsecond range, orders of magnitude faster than EMRs.
Since the TPSI3050-Q1 and TPSI2140-Q1 integrate power and signal transfer across a single isolation barrier, no secondary bias supply is necessary, making it possible to achieve a small solution size. Figure 1 illustrates the use of the TPSI2140-Q1 isolated switch in a high-voltage system, eliminating external components such as a bias supply and external control circuits.
These solid-state relays also offer advantages over traditional photo-relays and optocouplers. The TPSI2140-Q1 and TPSI3050-Q1 achieve better reliability over photo-relays because there is no LED degradation, and no external control circuits are necessary because the logic-level input can drive the system directly.
These solid-state relays provide the highest dielectric strength at the fastest speed, highest operating temperature and lowest system cost. They also enable more reliable switching in a smaller package.
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