What are the Common Design Parameters Engineers Should Consider When Selecting Solid-State Relays?

The problems with solid-state relays (SSRs) in most factories often stem from the improper choice of design parameters. The four important design parameters to consider when choosing an SSR are thermal management, switching type selection, current rating interpretation, and overvoltage protection. This article will cover these four design parameters in depth and explain how Littelfuse’s SSR products and their variants help achieve them in an optimal manner. Ultimately, the article demonstrates that Littelfuse’s SSR exhibits superior endurance through testing.

SSR switching types for different load applications

Heating systems can sometimes create unexpected electromagnetic interference, leading to failed compliance testing. Motor control applications sometimes have slow response times. Both problems usually have the same simple cause. Engineers chose the wrong SSR switching type for their application.

Different types of electrical loads need different switching approaches. Resistive loads, such as heating elements, work best when the electrical current starts flowing smoothly from zero. This approach prevents voltage transients and electromagnetic noise.

Inductive loads like motors are different. Motors require a prompt switching response, regardless of the position of the AC waveform. This is due to the inherent phase relationship between current and voltage in motors, a feature of inductive circuits.

The electrical characteristics of these different loads create completely different switching requirements. Using the wrong switching type can cause the problems engineers encounter in their systems. Figure 1 illustrates the phenomenon of zero-cross turn-on and random turn-on, which are suitable for the resistive and inductive loads, respectively.

Figure 1: Voltage waveforms showing conduction timing (green areas) for different switching modes. Zero-cross switching minimizes transients, while instantaneous switching provides an immediate response for time-critical applications. (Image source: Littelfuse)

This mismatch creates multiple problems. Voltage transients damage sensitive electronics, and electromagnetic interference necessitates expensive redesigns due to compliance issues. The equipment lifespan shortens significantly, making system performance unpredictable.

Most SSR manufacturers do not help solve this problem. They offer generic switching options with very little application guidance. This means engineers have to figure out complex load compatibility on their own. They end up using trial-and-error approaches to find what works. This delays projects and increases costs.

Littelfuse delivers application-matched switching technology engineered specifically for load characteristics using IXYS semiconductors and Direct Bonding Technology. Zero-cross switching models like SRP1-CBAZH-050NW-N and SRP1-CRAZH-050TC-N eliminate electrical transients by switching accurately at AC voltage zero crossings. These models are well-suited for controlling heating systems up to 24 kW at 600 VAC with minimal electromagnetic interference.

Figure 2: From left to right, Littelfuse’s SRP1-CR, SRP1-CB, and SRP1-CB…F SSRs. (Image source: Littelfuse)

For motor and inductive applications requiring immediate response, instantaneous switching models, including SRP1-CBARH-050NW-N and SRP1-CRARH-050TC-N, activate immediately upon control signal receipt. They handle challenging motor starting characteristics for high-power industrial automation. This application-specific engineering approach ensures reliable performance from initial installation. Figure 2 shows the different variants of Littelfuse’s SSRs.

Current rating guidelines and safety margins

Why do engineers consistently undersize SSRs despite following manufacturer datasheets? There is a disconnect between laboratory specifications and real-world operating conditions.

Current ratings seem straightforward at first. But then, engineers discover problems. Heating elements draw 1.4 times their nominal current during cold starts, and ambient temperatures may exceed the +40°C rating basis. This scenario requires substantial derating. Also, inadequate wire sizing further reduces current capacity. These factors create a complex specification environment. Undersized components fail prematurely. Oversized units waste money and panel space.

Most SSR suppliers compound this problem by providing basic current ratings with minimal application context. Engineers receive datasheet numbers without understanding the operating assumptions, safety margins, or real-world derating factors. This challenge forces interpretation through costly trial-and-error methods, which delay projects and often result in component failures that could have been prevented with proper guidance from the outset.

Figure 3: Littelfuse’s SSR design guidelines showing a 20% derating factor for heating applications. Power values represent maximum safe heater wattage for each SSR rating at standard AC voltages. (Image source: Littelfuse)

Littelfuse provides detailed current rating guidance (Figure 3) through clear specifications, which eliminates guesswork.

• 10 A models, like the SRP1-CRAZL-010TC-N, safely handle 8 A heater currents, enabling applications ranging from 960 W to 4.8 kW while providing integrated Transient Voltage Suppressor (TVS) protection for electrical environments.
• 25 A versions, such as the SRP1-CBAZL-025NW-N, manage 20 A loads, supporting 2.4 kW to 12.0 kW systems with zero-cross switching for heating applications.
• 50 A units control 40 A applications, powering equipment ranging from 4.8 kW to 24.0 kW.

Each specification includes conservative utilization factors of 75-80% and detailed temperature derating data, which demonstrate that intelligent thermal and electrical stress management leads to an extended service life.

Protection from voltage spikes and electrical transients

Electrical transients occur frequently in industrial environments. Such instances include lightning surges through power lines and back-EMF generation during motor switching operations. Utility grid disturbances also create voltage spikes exceeding 1200 V. Even though each event lasts only a few microseconds, it can damage SSRs and other equipment linked to them. Over time, the cumulative damage from many smaller transients can break down parts, ultimately stopping production.

The conventional approach requires external surge protection devices, resulting in additional panel space, complex wiring, and careful coordination among multiple protection levels. Many SSR suppliers offer basic units with no integrated protection, forcing engineers to design separate surge suppression systems. But external protectors introduce failure points through additional connections and may not respond quickly enough due to parasitic inductance and response delays.

Figure 4: Internal functional blocks showing optocoupler isolation, trigger timing control (zero-cross or instantaneous), and anti-parallel thyristor output configuration for bidirectional AC switching. (Image source: Littelfuse)

Littelfuse provides integrated protection through the SRP1-CR series, incorporating SMBJ Series TVS diodes directly within the SSR housing. Figure 4 illustrates the internal functional blocks showing the optocoupler isolation and trigger timing control that enable this integrated protection approach. This component-level protection responds within nanoseconds, clamping voltage spikes between 900-1200 V_PK before damage occurs.

Models like the SRP1-CRAZH-050TC-N for heating systems and the SRP1-CRARH-050TC-N for motor control provide built-in overvoltage protection optimized for their specific applications. These are ideal for electrically harsh environments with variable frequency drives where back-EMF transients are common threats.

The integrated design eliminates external components while providing protection positioned exactly where needed in the circuit. This approach demonstrates improved reliability compared to unprotected alternatives, providing complete protection against electrical transients.

Heat dissipation and temperature control solutions

While most engineers focus on electrical specifications, thermal design determines the actual lifespan of SSRs. Heat generation during operation seems manageable until junction temperatures exceed safe limits. Semiconductor degradation begins silently, resulting in inconsistent performance.

The challenge starts small, where most applications operate above the standard +40°C rating basis, requiring current derating that specifications mention but do not emphasize. Add thermal interface inconsistencies from messy paste application, inadequate heatsink sizing, and poor ambient airflow. What appears to be a simple thermal management task becomes a complex engineering challenge with significant cost implications.

Littelfuse provides integrated thermal management through the SRP1 series, incorporating every aspect of thermal control into a complete solution. Pre-attached thermal pads eliminate installation variables while ensuring consistent heat transfer without messy compounds. IXYS semiconductor technology and Direct Bonding Technology provide improved heat dissipation characteristics compared to standard components. Detailed thermal derating curves enable precise heatsink selection for any operating condition.

Figure 5: Load current limitations based on ambient temperature and heatsink thermal resistance (°C/W). Essential for preventing thermal failures in high-temperature industrial applications. (Image source: Littelfuse)

Figure 5 illustrates the load current vs. ambient temperature curve for different thermal scenarios. 50 A models, like the SRP1-CBAZH-050NW-N and the SRP1-CRAZH-050TC-N, maintain full current capacity up to +50°C with proper 0.7°C/W heatsinking. They still deliver 35 A capacity at 1.5°C/W heatsinking at +40°C ambient. This makes them well-suited for applications like controlling heaters in high-temperature industrial environments.

Test results and performance validation data

Independent comparative testing validates Littelfuse's performance claims. When subjected to identical 750,000-cycle endurance testing at 2x rated current, the Littelfuse SRP1 series significantly outperformed three major competitors (Figure 6). While Littelfuse units completed the full test cycle, competitors failed at 200K, 130K, and 60K cycles, respectively. Competitor 3 experienced catastrophic semiconductor explosions posing safety risks.

Figure 6: Visual comparison of SSR internal damage after endurance testing, showing top cover removed and detailed failure modes. (Image source: Littelfuse)

Post-failure analysis revealed thermal fatigue damage in competitor units, demonstrating the effectiveness of Littelfuse's IXYS semiconductor technology, Direct Bonding Technology, and thermal management. This real-world validation proves that Littelfuse's integrated four-pillar approach delivers measurably improved reliability. The result makes the SRP1 series the clear choice for critical industrial applications while meeting cЯUus, CE, and RoHS compliance standards.

Conclusion

Littelfuse SRP1 series SSRs address the four engineering challenges that lead to industrial SSR failures. Application-matched switching types eliminate electromagnetic interference, and conservative safety margins prevent under-sizing failures. Integrated overvoltage protection handles electrical transients, while the advanced thermal management extends service life. Real-world testing validated superior performance, achieving 750,000 cycles compared to competitor failures at 200,000 cycles or less. This engineering approach ensures reliable operation from installation through years of demanding industrial service.