Since their introduction over three decades ago, solid state relays (SSRs) have displaced electromagnetic relays (EMRs) for switching applications demanding ultra-reliable, arc-free, low-power operation. Additional advantages of SSRs include noiseless operation and compatibility with digital control circuits.
However, in demanding home, commercial and medical applications—particularly those where adherence to international electromagnetic compatibility (EMC) standards such as IEC 60947-4-3 is required—careful relay selection is needed to ensure electromagnetic interference (EMI) generated by the relay is minimized. Some products can produce voltage spikes and risk non-compliance with EMC standards.
This article will explain the advantages and drawbacks of SSRs and the applications for which they are best suited. The article will then look at the key parts of the relay that can cause troublesome emissions before introducing a range of low-noise SSRs from Sensata Technologies that designers can use for EMI sensitive commercial, home, and medical applications.
EMRs versus SSRs
Because it is exposed to the full circuit current when closed, using a switch to turn a high-power circuit on and off is impractical. The switch arcs dangerously during operation and overheats in operation. The solution is to use a low-power circuit, turned on and off by a conventional switch, to trigger the high-power circuit.
Among the advantages of this arrangement are cost and space reductions due to a reduction in the length of heavy wiring needed for the high-power circuit. These advantages are due to the fact that the relay can be placed close to the load, and thinner wires can be used to connect to the low-power switch. That switch is typically located in a position more convenient for the user. In addition, the low-power circuit can be galvanically isolated from the high-power circuit. Examples where relays are employed include commercial ovens, household appliances and medical equipment.
Traditional EMRs use a coil that is energized by the low-power circuit to create a magnetic field which then closes the (normally open) contacts. The EMRs can switch an AC or DC load up to their maximum rating. Their contact resistance reduces as the load increases, reducing power dissipation and eliminating the need for a heatsink (Figure 1).
Figure 1: EMRs connect AC power to the load when the switch in the low power circuit is closed and energizes the coil which in turn closes the contacts. (Image source: Digi-Key Electronics)
The key benefits of EMRs are low cost and guaranteed isolation at any applied voltage below the device’s dielectric rating. Isolation is particularly important when the high-power circuit has to be completely on or off with no danger of user injury from leakage currents. EMRs are also a good option if large surge currents or spike voltages are anticipated in the AC supply.
The key downsides to EMRs are the potential for EMI and wear. Because arcing can occur when the contacts open and close, the relay can generate appreciable EMI. Generally, the levels are low and well-designed EMRs incorporate shielding to mitigate any emissions, but care is needed for applications used in areas close to EMI-sensitive equipment.
Because EMRs are mechanical devices, even the best designed and manufactured products will eventually wear out. In most cases it is the coil that fails first, leaving the device in a fail-safe condition because the contacts are normally open (NO), leaving the low-power circuits isolated from the high-power circuits. That said, modern EMRs are very reliable and it is often the case that the equipment powered by the relay wears out first.
SSRs have come into their own as the control circuits used to switch high-power applications have migrated to digital electronics. As the name suggests, SSRs are semiconductor-based devices and as such are well suited to supervision by microcontroller-based digital circuits, particularly for applications featuring high switching speed.
SSRs address the key drawbacks of EMRs. Because there are no moving parts, SSRs don’t wear out. The devices typically perform for tens of millions of cycles, but when they do fail it is usually in an “on” position which might have safety implications. SSRs generate no arcs when opening or closing which not only makes them suitable for use in hazardous environments, but also eliminates the source of much of the EMI that can plague EMRs. They are also mechanically silent, function over a wide range of input voltages, and consume little power even at high voltages. The changeover from EMRs to SSRs has accelerated as the price of the latter continues to drop.
The key downsides of SSRs derive from their basis as a semiconductor circuit. For example, when “on”, there is substantial resistance, causing power dissipation of tens of watts with resultant heat build-up. The thermal challenges are usually such that the designer must include a substantial heatsink which increases the size and weight of the solution. SSRs are also affected by ambient heat and therefore must be derated if used at elevated temperatures. The internal circuit resistance can also generate a voltage drop which could cause problems for the load if it is sensitive to changes in the supply voltage. While in the “off” state, SSRs exhibit some leakage current. At high voltages this can be undesirable or even a safety challenge. In addition, many SSRs require a minimum load to operate properly.
Basics of SSR operation
The output switch is the key part of the SSR. For an AC output relay, the output can be controlled by a triac or back-to-back silicon-controlled rectifiers (SCRs). The key advantage of the SCR solution is a fast dv/dt characteristic, particularly when the relay is turned “off”.
For example, when an SSR with a triac controlling the output switches off, dv/dt can be as slow as 5 to 10 volts/millisecond (volts/ms). The slow dv/dt characteristic can be a problem because if di/dt for the decreasing current (and/or dv/dt for the reapplied voltage) is too shallow, the triac may conduct after the AC supply crosses the zero current/voltage point. Such an event destabilizes the output and can increase EMI.
In comparison, SCRs have a dv/dt of around 500 volts/microsecond (volts/µs) and will not conduct after the zero crossing point. Another advantage of an SSR with SCRs is better heat dissipation as the components are spread over a wider area than a single triac. The rest of this article will describe SSRs with a back-to-back SCR output stage.
A basic SSR using SCRs is shown in Figure 2. AC output SSRs are typically powered by the AC line. When S1 (controlled by the input circuit) is closed, the respective gates of SCR1 and SCR2 are connected and current from the AC supply flows through either R1 or R2 and into the gate of whichever SCR is forward biased. This turns the SCR “on” and the relay conducts, powering the load. For each half cycle of the AC supply the SCRs conduct alternately and current is supplied to the load. When S1 is opened, whichever SCR is “on” continues to conduct until the AC current reaches zero when the SCR turns “off”. At this point the other SCR no longer receives any gate current, the relay opens, and power to the load is removed.
Figure 2: Basic layout of a relay using back-to-back SCRs. S1 is formed by the low-power input circuit. (Image source: Sensata-Crydom)
Modern SSRs typically rely on an optocoupler to provide the isolation between the low- and high-power circuits. The two key options for the designer are to use an LED/optotransistor-based optocoupler or a device combining an LED and optotriac. An optotransistor requires less control current, saves space, and does give the designer more opportunity to configure the control circuit characteristics. The key advantage of the triac approach is lower cost. An optotriac controlled relay schematic is shown in Figure 3.
circuits in SSR” alt=”How to Apply Specialized Low-Noise Solid State Relays to Limit EMI and Meet Critical Standards”>Figure 3: In this SSR, isolation between low- and high-power circuits is via an optocoupler based on an optotriac. (Image source: Sensata-Crydom)
(For more information on how to select an SSR, see the Digi-Key technical article, “How to Safely and Efficiently Switch Current or Voltage Using SSRs”.)
SSRs for low EMI environments
Selecting an SSR with SCR-controlled output is a good option for EMI sensitive applications because the devices have inherent low-noise characteristics. For especially sensitive applications, such as those which call for the use of switching products that comply with the IEC 60947-4-3 standard, ultra-low-noise products should be selected. SSRs that switch on only when the AC voltage crosses the zero voltage point—regardless of when the input is activated—are a good option for these applications.
These so-called zero-crossing devices eliminate inrush current and voltage spikes that can result when turning on high-power circuits while the AC output is mid-cycle. This in turn lowers the incidence of EMI. Designers should note that while zero-crossing SSRs are particularly suited for resistive loads such as heaters, they are not suited for highly inductive loads. A better choice for these applications are so-called random-switching SSRs. These switch the instant the input switch is activated, rather than waiting for the AC supply to reach zero.
Sensata Technologies, which offers the Sensata-Crydom brand of SSRs, has recently introduced three products into its LN Series AC output low-noise range SSRs. The LND4425 can supply 25 amperes (A) to the output, while the LND4450 supplies 50 A and the LND4475 supplies 75 A. The devices require a minimum load current of 100 milliampsrms (mArms) for stable operation, are supplied in the “hockey puck” form factor and weigh around 75 grams (g) (Figure 4). All three solutions feature a 48 to 528 volt AC output and operate from a control voltage of 4.8 to 32 volts DC. They feature built-in input/output overvoltage protection, and their dielectric strength from input to output is 3500 voltsrms.
Figure 4: Sensata-Crydom’s LND44xx SSRs offer up to 75 A and 528 volts from a compact solution weighing just 75 g. (Image source: Sensata-Crydom)
The LN Series have been designed for lowest EMI operation. They use an optocoupler with an optotriac on the input, and back-to-back SCRs for output control to overcome potential EMI that can occur as a result of a slow dv/dt characteristic. The back-to-back SCRs feature a dv/dt of 500 volts/µs. The products also feature a patented trigger circuit that enables resistive load switching with minimal EMI. A schematic for the LN Series SSRs is shown in Figure 5.
Figure 5: Sensata-Crydom’s LN Series SSRs are designed to minimize EMI with features such as a patented trigger circuit and back-to-back SCRs. (Image source: Sensata-Crydom)
The result of these EMI mitigation features is conformance with IEC60947-4-3 Environment B for low-voltage domestic, commercial and light industrial locations (Figure 6).
Figure 6: Conducted RF emission test for the Sensata-Crydom LND4450 SSR. The threshold for compliance with IEC60947-4-3 Environment B is shown as a solid orange line. (Image source: Sensata-Crydom)
The LN Series is particularly suited to applications such as heaters in commercial ovens as shown in Figure 7.
Figure 7: Relays used in commercial ovens should comply with IEC60947-4-3 Environment B regulations. In this graphic, relay locations are marked by numbers, with “1” indicating where LND44xx SSRs would be a good choice. (Image source: Sensata-Crydom)
Relays are a simple and proven solution for switching a high-power circuit using a low-power activation circuit. EMRs are a good option where a low-cost solution is needed but are less suited for use in high-frequency switching applications and EMI-sensitive areas. SSRs are more expensive but offer robust and wear-free operation and are particularly compatible to control by digital electronics. However, designers choosing SSRs should be aware of the thermal challenges they bring due to higher heat dissipation in like-for-like applications compared with EMRs.
While all types of SSRs exhibit lower EMI than EMRs, some designs struggle to meet EMC regulatory requirements such as those specified in IEC60947-4-3 Environment B. As shown, the solution is to use SSRs with back-to-back SCR output stages. These offer zero-crossing switching which results in ultra-low RF emissions, making it easier to meet compliance.