Defeating the Challenges of Electromechanical and Solid-State Relays
Electromagnetic relays (EMRs) are broadly used in motor control, automotive, HVAC, valve control, solar inverter and many other industrial applications. Over the last 10 years, solid-state relays (SSRs) have seen fast growth as they begin to replace EMRs. Designers have found that SSRs can address most of the limitations of EMRs. But as is often the case with alternative solutions, SSRs have their own set of tradeoffs that can challenge designers. A third alternative exists: using custom SSRs. Let’s exam the limitations and tradeoffs of each approach.
Electromagnetic relays (EMRs) are broadly used in motor control, automotive, HVAC, valve control, solar inverter and many other industrial applications. Over the last 10 years, solid-state relays (SSRs) have seen fast growth as they begin to replace EMRs. Designers have found that SSRs can address most of the limitations of EMRs. But as is often the case with alternative solutions, SSRs have their own set of tradeoffs that can challenge designers. A third alternative exists: using custom SSRs. Let’s exam the limitations and tradeoffs of each approach.
Fast Growth of SSRs at the Expense of EMRs
Electromechanical relays use a coil that when sufficiently powered can move an armature to switch contacts based on the magnetic flux generated. EMRs have the benefit of truly being off without leakage current when not energized. However, they have many limitations that SSRs can address, which is contributing to SSR market growth. SSRs have no moving parts as they are based on semiconductor technology, which directly contributes to better reliability, much longer lifetime and fast switching speed. As EMRs switch, the contacts generate both acoustical and electrical noise along with arcing, which makes them unsuitable in some applications. EMRs are bulky, often impacting industrial design and placement options on a printed circuit board (PCB). The typically large, through-hole EMRs also increase manufacturing cost versus smaller surface-mount SSRs.
Design Challenges of Optocoupler based SSRs
As designers look to SSRs to address EMR limitations, they are finding a different set of challenges. SSRs offer small board space when used in low-power switching applications. However, higher power switching applications must use larger custom packages to deal with the power dissipation and heat of the integrated FETs. Quite often the SSR user must compromise on FET performance, power or cost as there are limited choices of integrated FETs.
SSRs typically use optocoupler based designs to achieve isolation. These optocoupler designs have inherent LED limitations such as poor reliability and stability across temperature and time. A key optocoupler wear-out mechanism is LED light output. As LEDs age their light output declines, which negatively impacts timing. The degradation in light output grows worse over time with increased temperature and higher currents. Other common issues include unstable input current thresholds and complicated current transfer ratio. Designers are forced to use more current and add external components to address these issues or use alternatives to optocoupler-based isolation. More and more industrial applications such as industrial drives, solar inverters, factory automation and metering are targeting 20+ years of system life so it is important for the designer to carefully consider these effects on the system lifetime.
Alternative Custom SSR Using Optocoupler-Based Isolation
Many system designers prefer to use existing high volume and cost-effective discrete FETs as their performance and thermal characteristics are well understood in contrast to the often unknown integrated FETs of SSRs in non-standard packaging. A custom SSR enables the use of these application optimized FETs instead of the typically compromised FETs that are integrated into SSRs. There is a tradeoff in board space versus low-power switching SSRs, but this tradeoff becomes less important with higher powered SSRs due to heat dissipation challenges of integrated SSRs.
Figure 1 shows a custom SSR based on traditional optocoupler based isolation. A secondary, switch side power supply is usually required with these types of solutions as power is not transferred across the isolation barrier.
Figure 1: Creating a custom SSR with optocoupler-based isolation
Alternative Custom SSR Using CMOS-Based Isolation
Over the last few years, multiple semiconductor suppliers have introduced more advanced CMOS-based isolation products with double-digit growth over traditional optocoupler based isolation. This is especially true in high temperature and high reliability industrial applications. Traditional optocoupler based custom SSRs can address the limitations of integrated FETs but require an additional power supply. Even then, customer SSRs built around an optocoupler cannot address the inherent limitations of using LEDs. Another alternative is now available that enables developers to use their choice of application specific, high volume FETs without the disadvantages of optocoupler based designs as shown in Figure 2.
Figure 2: Custom dc switching SSR with Si8751 CMOS-based isolated FET driver
The Silicon Labs Si875x family features the industry’s first isolated FET drivers designed to transfer power across an integrated CMOS isolation barrier, eliminating the need for isolated secondary switch-side power supplies and reducing system cost and complexity. Since Si875x drivers do not use LEDs or optical components, they provide superior stability over time and temperature with up to 125ËšC automotive operation. A single Si875x can support either dc or ac load switching with one FET required for dc load switching as shown in Figure 2 or two FETs for ac load switching as shown in Figure 3.
Figure 3: Custom ac switching SSR with Si8752 CMOS-based isolated FET driver
Developers have the option to use a CMOS digital input with the Si8751 (Figure 2) or the diode emulation input of the Si8752 (Figure 3). The Si8752 isolated FET driver makes it easy for developers to migrate from optocoupler-based solutions while efficiently generating a nominal 10.3 V gate drive using only 1 mA of input current. Optional miller clamp inputs are implemented that allow the addition of a capacitor to eliminate the possibility of inductive kickback changing the state of the switch when used in applications with high dV/dt present on the FET’s drain. The Si8751 easily interfaces with low-power controllers down to 2.25 V and provides a unique low-power TT mode that provides exceptionally fast turn-on speeds, as fast as 100 µs, while dropping static holding current as much as 90 percent. An optional capacitor is tied to ground using the TT pin to enable this power-saving feature. This approach allows the device to draw more current to initially switch the external FET on quickly yet draw less supply current in the steady state. Total power over time is reduced while maintaining the FET’s fast switching speed.
Summary
Developers face continual challenges to implement next-generation designs with lower system cost, higher performance and better reliability. Offering a unique combination of robust, reliable CMOS-based isolation technology and advanced capability to transfer power across the isolation barrier, new isolated FET drivers now provide a much-needed replacement solution for antiquated EMRs and optocoupler-based SSRs. CMOS-based isolated FET drivers give developers the flexibility to choose a cost-effective FET customized to their application needs, creating an easy migration to state-of-the-art solid-state switching.
