WHITE PAPER Working With Optically-Isolated Relays Learn how these solid-state relays can improve the performance of data acquisition systems and industrial machines it comes to these performance advantages. Optically-isolated solid state relays, in particular, can outshine other solid-state devices that use electrical or magnetic operating principles. In this paper, you will learn more about the operating principles of optically-isolated relays, how to apply them in different applications and how to maximize their already-long lifecycles. PhotoMOS relay Product lineup of PhotoMOS devices LED MOSFET Not too long ago, all relays performed their switching duties MOSFET through electromechanical means. Today, however, engineers can also opt for solid-state relays that use semiconductors to switch their output circuits. The choice between traditional Photo-Cell electromechanical relays and the solid-state varieties often comes down to reliability and performance. PRINCIPLES OF OPERATION With no moving parts, solid-state relays avoid all the obvious Optically-isolated relays are characterized by the use of a light mechanical failure modes associated with traditional relays. They emitting diode (LED) on their input side, MOSFETs on the output also tend to offer desirable electrical characteristics and design side and an array of photo sensors in between. advantages including: In operation, current flows through the LED, which then emits light. The photo sensor array detects the emitted light, triggering * Low power consumption. * Low leakage current. * Stable on-resistance over lifetime. * High reliability with extremely long life. * Small size. * Fast switching speeds. resin that allows light to pass through while providing a dielectric * High vibration and shock resistance. barrier between the input and output. * No contact bounce or switching noise. The most basic method to drive an optically-isolated relay is switch the load circuit. The design and packaging of the optical and electronic components are crucial aspects of the relay's performance. The LED and photo array, for example, are molded in a translucent Keep in mind that solid-state devices are not created equal when Panasonic a voltage drop that drives the MOSFETs. The MOSFETs finally Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 to apply a switchable voltage directly to the input pin of the www.panasonic.com/industrial PhotoMOS through a resistor to limit the current through the these relays also switch and protect small motors, power LED. Choosing the correct RF value for the resistor will ensure supplies and control devices with load currents up to 10 amps. that the LED reaches full intensity while preventing it from being overdriven by the input voltage (see Design Tip, "Calculating Input Resistance (RF) Correctly"). TEST AND MEASUREMENT USES These industrial uses represent the next wave of applications for optically-isolated relay technology, which has been widely accepted as a way to switch high-precision data acquisition and measurement systems. Most optically-isolated relays today will ultimately become part of Like test and measurement systems, industrial equipment can sophisticated test and measurement systems. To keep pace with benefit from high switching speeds, low on-resistance, low advances in the electronics industry, these systems increasingly capacitance and small package size.Yet motors, power supplies require solid-state relays that combine low capacitance, low on- and controls can reap additional benefits by moving from resistance, physical isolation and high linearity. traditional electromechanical relays to optically-isolated relays: All these characteristics play an important role as data Low Power Consumption. A typical optically-isolated acquisition devices become faster and more precise: relay requires 10 to 20 times less power than an equivalent electromechanical relay. For example, a 5 mA PhotoMOS Low capacitance improves switching times and isolation can often do the same job as an electromechanical relay characteristics for high frequency load signals. that requires anywhere from 50 to 100 mA, depending on the Low on-resistance reduces power dissipation when switching high currents and increases switching speeds to improve the precision of measurement. When considering on-resistance electromagnetic force needed to close the coil. A few milliamps here or there may not sound like a big deal, but in a plant with many small devices the savings add up quickly. values, pay close attention to the temperature range the relay Protection. Thanks to a built-in protective circuit in our latching- must withstand. Rising temperatures decrease the mobility of type models, PhotoMOS can safeguard motors, power supplies electrons, driving up the on-resistance. Starting with a relay that and other industrial devices from possible disturbances on has low on-resistance will minimize the effects of temperature the output side. These disturbances-such as voltage peaks drift. or overcurrent conditions-can arise due to short circuits or Physical isolation. Sometimes referred to as galvanic separation, physical isolation between the relay's input and output or between different output channels enhances precision by minimizing noise. Optically-isolated relays offer a true physical separation of the input and output, and the best of these products exhibit isolation voltages as high as 5,000 volts AC. High linearity ensures accurate measurements. improper use. The protective circuit is located on the output side of the component and recognizes high currents. This arrangement protects both the DMOSFET on the output side and the load circuit against overcurrent conditions. As soon as a dangerous load current arises, the load circuit switches off completely. It can be switched on again only after the input signal has been reset. Elevated Temperature Tolerance. The PhotoMOS protective With a variety of signals at work in a typical test system, circuit can play a particularly important role when the relay it's particularly important to find relays that offer the right must perform at elevated operating temperatures. Because the combination of electrical characteristics. For example, many voltage drop across the shunt increases as rising temperatures systems have both DC and AC switching needs and will require drive up resistance in the component, the protective circuit relays that combine low-on-resistance and low capacitance: responds to lower and lower current levels as temperatures rise. The low on-resistance minimizes signal loss when switching In essence, it exhibits a negative temperature coefficient, which DC signals, while low capacitance improves isolation when allows it to offset the increased power dissipation associated switching AC signals. with elevated temperatures. INDUSTRIAL APPLICATIONS TOO Reliability. Solid-state relays such as PhotoMOS shine when it comes to reliability. Without the moving parts of an Not all optically-isolated relays end up in test electromechanical relay, solid-state relays typically have and measurement applications. Increasingly, an excellent mean time to failure (MTTF). In general, solid- Panasonic Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 www.panasonic.com/industrial state relays tolerate shock and vibration loads that threaten motor protection and the value proposition becomes even more electromechanical relays. Solid-state relays also eliminate the compelling. Keep in mind, too, that the savings can be greater in buzzing that can affect electromechanical relays driven by PWM applications that require the relay to remain in its closed state for and other methods intended to conserve input power. long periods of time. Solid-state relays can be operated closed Low operating cost. Solid-state relays may have a higher price tag than electromechanical relays. The total cost over without the elevated temperatures and extra current draw of their electromechanical counterparts. the relay's life-cycle, however, tips the scales back in favor of Saves space, speeds development. Integrating the protective solid-state technology. Most of the operating cost advantages mechanism in the relay, rather than relying on a separate come from reductions in power consumption and a longer life- component, saves space. And it speeds development time cycle for fewer relay replacements. Factor in the cost benefit of because there's one less component to work into your design. DESIGN TIP Account For LED Power Losses To Maximize Relay Life Optically-isolated relays inherently have a long lifespan, diode array in the IC. So it takes longer to bias the MOSFET thanks to their lack of moving parts and the robustness of gates. their solid-state electronics. You can, however, make them last even longer by accounting for LED power losses. Elevated Temperature Effects. At elevated ambient temperatures, more LED current is needed to generate the same amount of lamination. This lamination will then be time. Instead, all LEDs experience a power loss in proportion converted to produce the necessary electrical voltage and to the time that current is applied to them. With optically- current to charge the gates of MOSFETs and maintain ON isolated relays, including PhotoMOS, this loss of LED power state. affects the device's operating characteristics and lifecycle. Careful design is required Rising Currents. As LED power falls, the relay's operating to set up the series currents will rise accordingly. On a typical PhotoMOS relay, limiting resistance of for example, LED power might drop by roughly 3% after a 5 the input LED to ensure mA input current has been applied for 100,000 hours. As a proper operation of the result, the relay's operating (IFon) and turn off (IFoff) currents relay across the operating would rise from their initial value by 3%. range of the relay. This change in the electrical characteristics of the PhotoMOS In many applications, the LED turn turn of offf current mA Keep in mind that LED power does not remain constant over 1.0 0.8 0.6 0.4 0.2 0 -40 -4 0 -20 0 20 20 40 40 60 60 80 80 Ambient temperature temperature C Load voltage: 400 V (DC) Continuous load current: current: 120 mA (DC) has lifecycle implications. As LED sensitivity degrades with electrical change related continued usage, more current is needed to generate the to optically-isolated same amount of light. This light is used to charge the gates relays may not make a practical difference. Adding 3% to of internal MOSFETs and ultimately turn the relay on. an already fast on-time, for instance, won't matter in every Slower Turn-On Time. The turn-on time of optically-isolated 85 85 application. relays slows as LED power falls. Going back to our example Yet even incremental changes in performance or lifecycle can of a 3% degradation of LED power after 100,000 hours at 5 be significant in cutting edge applications. Examples include mA, the turn-on time would likewise slow down by 3%. Put high-speed test and measurement systems, differently, a PhotoMOS with a turn-on time of 0.03 mS out of the box will have a turn-on time of 0.0309 mS after 100,000 hours of use at 5 mA. In these cases, the datasheet alone won't tell you whether you have picked the right relay for the job. You will have to evaluate the relay based on the electrical characteristics This slowdown occurs because light intensity diminishes, that will emerge after an extended period of operation that which reduces the voltage and current output of the photo corresponds to your application. Panasonic Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 www.panasonic.com/industrial DESIGN TIP Calculating Input Resistance (RF) Correctly When calculating the correct RF value for the resistors used . This margin will ensure with optically-isolated relays, make sure you take the forward safe operation over the entire voltage (VF) into account. temperature range. If the Since the LED operating current increases as the temperature rises, we must use the typical recommended IF value of 5 mA at the maximum operating temperature of 85C to ensure safe operation. The LED forward voltage (VF) depends on the forward current (IF) and the temperature. Figure 1 shows the LED forward voltage versus ambient temperature graph for the AQV210 PhotoMOS. The LED VF with IF of 5 mA at 85C is 1.03 V. 5V - 1.03V coefficient of 250 ppm (parts per million) per C, the appropriate RF a path to ground for the power supply Vcc thus turning on 1.4 the collector and the emitter of the transistor. 1.3 Using the same example of the AQV210 PhotoMOS, RF 1.2 50m 50mA 30mA 30m 20mA 20m 10mA 10m 5mA 1.1 value will be the next lower value 0 resistors: RF=680 power supply is one method that is typically used by circuit the LED. When calculating the RF in this circuit, we must 1.0 from the standard a transistor as a control mechanism to switch an external account for the voltage drop, typically 0.4 to 0.7 V, between LED dropout voltage. V a temperature directly and require some additional components. Using the logic circuit. When the transistor is turned on, it will create = 794 Assuming a 5% tolerance and relays, some logic circuits can not drive the PhotoMOS In this scenario, the transistor is controlled by the output of 5mA IFon calculations. designers. The maximum RF value can be calculated as follows: = value should be used for the isolated relays are significantly lower than electromechanical Panasonic optically-isolated relay, the AQV210 PhotoMOS. RF = ripple, the lowest possible Vcc Although power consumption and drive current for optically- Let's for example calculate the RF value for a popular Vcc - VF supply voltage (Vcc) contains a can be calculated as follows: Assuming a 5% tolerance and a temperature coefficient of 250 ppm per C, RF of 680 W can no longer guarantee safe operation over the entire temperature range. In this case, use the next lower standard -40 -20 0 20 40 60 80 85 Ambient temperature C resistor to ensure that RF is lower than the maximum allowed value of 714 : RF=560 . Figure 1: LED forward voltage vs. ambient temperature DESIGN TIP Calculating Mean Time To Failure Mean Time To Failure (MTTF) equals 1/the failure rate . is expressed in terms of failures per unit of time (FIT), where 1 FIT=1 failure per billion device hours. The failure rate of Panasonic's PhotoMOS optically-isolated relays is 20 FIT which means that MTTF is 1/(20*10*-9) based on the THB test per MIL HDBK-217F. Based on the MTTF, expected time to first failure exceeds 50 million hours of operation. Panasonic Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 www.panasonic.com/industrial PRODUCT GUIDE Many Types of PhotoMOS Relays More than 300 different types of PhotoMOS optically-isolated * Linearity. Optical MOSFET-based relays like PhotoMOS relays are available to meet a wide variety of electrical and have highly linear input and output characteristics package size requirements. The PhotoMOS products most that outshine those of alternatives such as Triacs or suited to motor protection and other industrial uses include: OptoCouplers. PhotoMOS relays can also control small analog signals without distortion, unlike Triacs and AQZ207 SIL package with 1 A load current. Bipolar transistors whose offset voltages distort and clip signals. AQY277A DIP4 SMD with 0.65 A load current. * AQV252G with 2.5 A load current. Minimal Signal Propagation Delay. Measurement applications benefit from a reduced length of internal For test and measurement applications, consider our Low bonding and flat lead terminals, which results in reduced CxR PhotoMOS Model AQY221N2M. It offers: signal propagation delay. * * Low capacitance of 1.1 pF. A laterally diffused metal- Besides using Low CxR PhotoMOS relays for switching oxide-semiconductor (MOSFET LDMOS) lowers the signals and I/O lines to devices being tested, these relays relay's capacitance. may also be employed in data acquisition circuits. For Low on-resistance of 9.5 ohm. A vertical-type doublediffused metal-oxide-semiconductor (DMOS) limits the relay's on-resistance. * instance, they can be used to select the gain of operational amplifiers. With the help of an optically-isolated relay, the device's digital control unit and the analog signal system can be physically isolated, enhancing the precision of the device Fast Switching and Physical Isolation. Thanks to the by minimizing noise. low capacitance and on-resistance values, this relay supports switching times as fast as 20 s and provides the isolation required to switch high-frequency load signals. Part Number Builder AQ # Term Output Type V-load Function Isolation PKG. T/R-Tube A B C D E F G H A Y: 4 pin V: 6 pin W: 8 pin S: 16 pin Z: SIL 4 E nil: Std D: V Drive E: Std G: High Current L: Current Limit N: Low Cout K: ShortCkt Prtctn B 1: 1FA (DC) 2: 1FA (AC/DC) 4: 1FB (AC/DC) 6: 1FA/B (AC/DC) F nil: 1,500 V H: 5,000 V C 0: 1: 2: 3: 5: 8: G nil: DIP (Through Hole) A: DIP (Surface Mount) M: SON S: SOP T: VSSOP V: SSOP D 0: 350V 1: 30~40V 2: 60~80V 3: 250V 4: 400V H nil: Plastic Tube W: T/R 3.5k X: T/R 1k Y: T/R 3.5k Z: T/R 1k HF (Low Ron) GU (General Use) RF (Low Ron Low Co) HS (High Sensitivity) HE (Low Ron) GU (General Use) Panasonic 5: 100~150V 6: 600V 7: 200V 8: 1,500V 9: 1,000V KL: SC Self Reset M: Soft On/Off P: DAA R: Low Ron T: Optocoupler T2: 2-Optocoupler V: Varistor Prtctn Two Riverfront Plaza, 7th Floor, Newark, NJ 07102-5490 www.panasonic.com/industrial