Optocouplers have been the unchallenged signal isolation solution for more than four decades, but digital isolators fabricated in complementary metallic oxide semiconductor (CMOS) process technology are gaining favor in the design community because of their superior performance, reliability and integration. This white paper explains the underlying technologies of digital optocouplers and digital CMOS isolators with side-by-side performance and reliability comparisons.
2.0. Digital Optocouplers and CMOS Digital Isolator Basics
Digital optocouplers are available in several versions, from 4-pin devices costing pennies to expensive higher-speed devices. The most popular, moderately priced optocouplers are typically 8-pin LED-based devices that require an output side bias voltage and use input-side current to control the output state. Referring to Figure 1A, most optocouplers are composed of a light-emitting diode (LED), photo detector and output driver (usually a transistor or buffer). Current flowing though the LED causes emitted light to strike the photo detector, creating a current flow from VDD to the base of the output device, driving the output low. Conversely, the absence of LED current inhibits current flow to the output device, resulting in output logic high.
CMOS Digital Isolator
Figure 2A shows an x-ray view of a single-channel optocoupler where the LED and photo coupler die are attached to a split lead frame separated by a physical gap (distance through insulation [DTI] 80 µM to 1,000 µM gap between the LED and optical receiver die) and a transparent insulating shield. More recent optocoupler packaging also includes the addition of silicone filler (see Figure 3). Optocouplers are fabricated in gallium-arsenide (GaAs) process technologies that are notorious for wide parametric variation over temperature, high input current, relatively short time-dependent device breakdown (TDDB) and inherent wear-out mechanisms.
Figure 3. Cross Sectional View of an Optocoupler
The basic operating principle of the CMOS digital isolator is somewhat analogous to that of an optocoupler, with the exception that output logic state control is determined by the presence or absence of a high-frequency (HF) carrier instead of light. Figure 2B shows the die configuration and bonding of a 150 Mbps, six-channel CMOS digital isolator. The base isolator die is designed and fabricated such that wire-bonding two identical die together (Figure 2B) forms a transmitter and receiver separated by a differential capacitive isolation barrier. The isolator output state is determined by either an input logic level or an input current (similar to the optocoupler), depending on the part number.
The heart of any digital isolator is the isolation barrier that safely withstands the applied high-voltage stress. The optocoupler isolation barrier relies on a combination of a physical gap (i.e., distance through insulator or DTI); polyimide tape, silicon filler and plastic mold compound for insulation (see Figure 3). This hybrid methodology not only makes increased optocoupler integration difficult, but increases fabrication complexity and thus cost and reliability. That said, many international standards still specify isolation barrier requirements based on optocoupler DTI. Fortunately, the standards agencies tests are based on barrier withstand voltage regardless of device implementation. Please see Section 4.0 in this document for a discussion regarding insulation material and its effect on isolation performance.
CMOS construction and careful attention to design enable CMOS digital isolators to achieve higher degrees of performance, reliability and ease-of-use, primarily by virtue of these key enabling technologies:
- Mainstream, low-power CMOS process technology instead of GaAs: CMOS is the highest performance, most reliable and cost-effective process technology available. Devices fabricated in CMOS exhibit superior timing, integration, operating stability and reliability characteristics compared to gallium arsenide processes used in optocouplers.
- High-frequency carrier modulation instead of light: The combination of the precision, high-frequency carrier and narrow receiver pass band provides tight frequency discrimination for outstanding noise rejection and thus higher data integrity.
- Fully differential isolation path instead of single-ended: Optocouplers are single-ended devices and are subject to CMT perturbations, whereas the differential signal path and high receiver selectivity of CMOS digital isolators provides high rejection of common-mode transients up to 50 kV/µS, external RF field immunity as high as 300 V/m and magnetic field immunity beyond 1000 A/m.
- Proprietary EMI design techniques: CMOS digital isolators meet the FCC Part B emission standards for conducted and radiated EMI. For more information on CMOS isolator emissions, susceptibility and reliability vs. optocouplers, see the Silicon Labs white paper, “CMOS Isolators Supersede Optocouplers in Industrial Applications”.
2.1. Isolator Requirements:
Isolator parametric requirements are dictated by end applications, and designers tend to stress the importance of certain isolator parameters over others. For example, isolators operating in noisy industrial environments might cause a designer to prioritize high common-mode transient immunity (CMTI) over low-power operation, whereas a high-precision ADC application might cause the designer to prioritize low jitter over pulse width distortion, and so on. In short, many applications will care most about just a few key isolator parameters, and designers will often verify these parameters independently to ensure they meet the system requirements. A list of the more common isolator parameters and their definitions is shown in Table 1 for reference.
|Channel-to-Channel Skew||tPSK||The propagation delay time difference between channels in a multi-channel isolator|
|Common-Mode Transient Immunity||CMTI||The ability for the isolator to reject fast common-mode transients|
|Device Startup Time||tSU||The time delay from power on to proper isolator operation|
|Input High, Low Thresholds||VIH, VIL||Voltage threshold value that must be crossed to achieve input high or low levels|
|Input Leakage Current||IL||Parasitic leakage current flowing into an input-side pin|
|Input-Side Bias Current||IDDI||Current that provides bias current to the isolator input-side circuits|
|Jitter||tJIT(pk)||Variation of the pulse edge position of the data stream|
|Undervoltage Lockout ||UVLO||The VDD bias level required to enable or disable isolator operation|
|Maximum Data Rate||Dmax||Highest isolator data rate (usually specified in Mbps)|
|Minimum Pulse Width||t(min)||The minimum width pulse that can be recognized by the isolator|
|Output Impedance||Zout||The characteristic output impedance (typically measured in Ω)|
|Output Rise, Fall Times||tr, tf||The time period for an output edge to rise from 10% to 90% of VDD or fall from 90% to 10% of VDD)|
|Output Voltage High, Low||VOH, VOL||The output threshold levels that define a logic 1 and logic 0|
|Output-Side Bias Current||IDD||Bias current flowing into the isolator output-side|
|Propagation Delay||tPHL, tPLH||The time period from an isolator input event required to propagate to the isolator output|
|Propagation Delay Skew||tPSK(p-p)||The difference between the min and max propagation delays (tPLH or tPHL) for any given group of isolators operating under the same conditions|
|Pulse Width Distortion||PWD||The difference between high-to-low and low-to-high propagation delay (|tPLH – tPHL|)|
3.0. Device Performance
This section compares the performance of CMOS digital isolators (most notably the Si86xx series) against competing optocouplers, such as the HCPL-4506, HCPL-2400 and others. Note that all Silicon Labs isolators (i.e., the Si84xx, Si86xx and Si87xx families) are based on Si86xx technology and, therefore, have substantially the same performance and reliability. While Table 1 lists many key isolator parameters, designers appreciate isolators that have attributes like those listed below:
- Well-defined, key operating parameters that exhibit little change over voltage, temperature and device age
- Isolators vulnerable to significant changes due to temperature, current and/or device age force the designer to give up design margin, reducing system performance. This is especially true in isolated high-speed data systems that rely on tight timing relationships.
- High reliability
- Isolator lifetimes of 20+ years are now required by applications, such as HEV/EV, solar and wind energy systems, and typically operate in elevated temperature environments over 120 °C.
- High CMTI
- To ensure data integrity in noisy industrial environments, robust isolation devices with high immunity to common-mode transient events are required.
- Minimal electromagnetic interference (EMI) and high external electrical and magnetic field immunity
- EMI is a source of potential data corruption, especially in medical applications that acquire low-amplitude signals, such as electrocardiographs (ECG). External magnetic and electric fields can degrade system performance, so the ability of an isolation device to reject external field interference is critical.
- Ease of Use
- The best isolation devices are well documented, predictable, reliable and intuitively easy to apply; these points are covered in detail in the paragraphs below.
3.1. Timing Variation with Temperature:
Figure 4 compares propagation delay time and pulse width distortion stability over temperature for the HCPL-2400 20 Mbps digital optocoupler and the Si8660Bx 10 Mbps CMOS digital isolator. Referring to Figure 4A, the HCPL-2400 data sheet curves show propagation delays tPHL and tPLH are not matched, so additional system design margin may be required to ensure the slower of the two propagation times is met (depending on the user’s timing requirements). The propagation delays remain uniform at a worst-case value of 33 ns between –40 °C and +35 °C but slow to 40 ns at 70 °C (and potentially increase with elevated temperature as indicated by the dashed lines). The blue curves at the bottom of Figure 4A show the propagation delay time of the Si8660Bx over the temperature range from –40 °C to +125 °C. Note that tPHL and tPLH are symmetrical and faster than the propagation times of the HCPL-2400. The HCPL-2400 PWD data sheet curve (Figure 4B) is similar to that of Figure 4A, suggesting the same underlying thermal sensitivity. PWD remains flat at 3 ns over the temperature range of –40 °C to +38 °C but worsens to 6 ns at 69 °C. The Si8660Bx curve at the bottom of Figure 4B shows a considerably lower PWD over the temperature range of –40 °C to +125 °C with a deviation of 3 ns at the graph’s limit (69 °C) and 4.5 ns at +125 °C.
Figure 4. HCPL-2400 and Si8660Bx Isolators Propagation Delay and PWD over Temperature
Figure 5 shows effect of the HCPL-2400 optocoupler input current on propagation delay time as documented in the manufacturer’s application note. The optocoupler was tested with and without a “peaking” capacitor (Cp), which momentarily increases LED current during turn-on and turn-off for improved response. Again, tPHL and tPLH are not symmetrical, and, therefore, must be examined individually. The optocoupler of curve A is subjected to an input current of 0.5 to 1.0 mA with peaking capacitor Cp = 20 pF. The combination of elevated input current and faster turn-on and turn-off due to Cp results in a flatter response and shorter propagation time than the other curves. The Si86xx curve at the bottom of the chart shows the propagation time of the CMOS digital isolator, which exhibits a flatter, symmetrical curve over the temperature range from –40 to +125 °C.
Figure 5. Optocoupler vs. CMOS Isolator Propagation Delay
While these examples demonstrate the changes in propagation time and pulse width distortion, other optocoupler parameters can wander with temperature, device age and other influences, such as optocoupler current transfer ratio, threshold settings and output impedance mostly caused by behaviors related to the LED emission and the underlying GaAs process technology. Section Summary: Optocouplers suffer from asymmetric propagation delays and large parametric deviation with temperature, both of which are related to the underlying GaAs process technology. These issues cause designers to compensate by sacrificing design margin, potentially reducing system performance. By comparison, CMOS digital isolators exhibit symmetrical delays and little parametric deviation over an operating temperature range much wider than that of optocouplers.
4.0. Device Reliability
This section summarizes the isolation-related specifications in the data sheet, distinguishes between long-term and short-term ratings, provides a simple tool to compare products from different manufacturers and explains why the SiO2-based insulation barrier from Silicon Labs delivers the best performance from an isolation perspective.
Isolation devices have voltage withstand specifications in their respective data sheets. For example, the following table is from the data sheet for the HCPL-4502, an open-collector Avago optocoupler.
|Maximum Working Insulation Voltage||VIORM||630||560||Vpeak|
|Input to Output Test Voltage, Method b*
VIORM X 1.875 = VPR, 100%
Production Test with tm = 1 sec,
Partial Discharge < 5 pC
|Input to Output Test Voltage, Method a*
VIORM X 1.5 = VPR, Type and sample test,
tm = 60 sec, Partial Discharge < 5 pC
|Highest Allowable Overvoltage*
(Transient Overvoltage, tini = 10 sec)
Of these ratings, only the Maximum Working Insulation Voltage (VIORM) implies a guarantee over time. All other specifications are only valid for time zero. In other words, these specifications are not guaranteed over the lifetime of the product, but only for a single instance of short-term (1 second or 1 minute, typically) voltage stress. Isolation devices are composed of a dielectric that is the insulation barrier, which provides the voltage withstand capability. All dielectrics exhibit aging; this implies a lifetime for which the voltage-withstand specification is valid.
Aging is due primarily to dielectric degradation from thermal and/or electric energy fields. For a particular dielectric, this is determined by the inherent material and manufacturing properties (impurities, defects etc.). Aging is a non-reversible process, and at the point of dielectric fatigue or failure, an avalanche current flows through the dielectric, causing permanent damage to the dielectric. High-voltage isolation lifetime implies that, at the end of this time period, its dielectric properties can no longer be guaranteed.
The amount and type of dielectric used in the isolator construction determines isolator performance and lifetime. Silicon Labs uses silicon dioxide (SiO2) for insulation because it is the most commonly-used dielectric in the semiconductor industry. Its defect levels are among the lowest of all dielectrics, thereby guaranteeing long product service lifetimes. SiO2is also one of the highest quality dielectrics available and has a breakdown threshold of 1000 V/µm compared to ~50 V/µm for the mold compound used in optocouplers for insulation. In addition, SiO2 is one of the most stable dielectrics across variations in temperature and moisture and is an inorganic material as opposed to mold compound, which is organic. With SiO2 as the insulation material, Silicon Labs isolation products offer Industry-leading withstand properties compared to optocouplers.
All isolation product manufacturers provide this lifetime information; when this information expires, the devices are no longer guaranteed to function as isolators. In all cases, the failure mode is a resistive short between the two (once isolated) grounds of the isolation device – a short from the input to the output side. This failure mechanism is independent of the coupling technique and applies to all isolators; it is a fundamental dielectric property. For an example, refer to high-voltage lifetime reports from the manufacturers below: Avago: App Note AN1074, Texas Instruments App Note SLLA 197.
For measuring high-voltage lifetimes of dielectrics, it is important to understand how dielectric aging is physically explained and characterized by various models like the E-model, 1/E model, a combination of the two or other lesser-known models. The E-model is the most conservative, and its mathematical statement is given below. It states that the time to fail (TF) has an inverse exponential relationship to the applied electric (or E) field, according to Equation 1:
ln(TF) = (ΔHo/Kb*T) – Ψ*E (Equation 1)
- TF is the device time to failure
- ΔHo is the enthalpy for dielectric breakdown (also referred to as activation energy, which depends on the dielectric materials used and the process technology)
- Kb is Boltzmann’s constant
- T is the temperature
- E is the electric field across the isolation barrier, given by the ratio of applied stress voltage (Vrms) to thickness of the insulation barrier
- Ψ is the field acceleration parameter. This depends on dielectric material properties as well.
Time Dependent Dielectric Breakdown (TDDB) methodology is used to estimate the continuous operating voltage of an isolator. It starts by measuring the time-to-fail for given devices under highly-accelerated voltages. This data is then extrapolated to the working voltage or continuous rated voltage for the product to determine its lifetime. In the methodology used by Silicon Labs to characterize the dielectric using an E-model, the temperature is a constant (worst case Tj of 150 °C was used), and the insulation barrier thickness is a constant for a given product. Thus, the equation reduces to the form of Equation 2:
TF = exp(-B*V) (Equation 2)
- B is a constant for a given product/device under test
- V is the applied voltage.
When plotted on a log-normal graph, as shown below, the exponential relationship expected in the E-model is mathematically represented by a straight line. Note: Not all isolator manufacturers use the E-model. Silicon Labs’ decision to use the E-model is based on the fact that it is the most widely-accepted model and the most conservative.
Standard TDDB methodology was used to calculate lifetime expectancy. Breakdown tests were run at various voltages. Results were then plotted on a Weibull distribution to extrapolate the raw data to 10 ppm fail rates. This implies that, at the stated time-to-fail values, only 10 ppm of the devices under test would have failed. This is an extremely low failure rate, and thus the lifetimes stated are very conservative. These 10 ppm values were used to plot the TDDB graph of time to fail (TTF) vs. voltage as shown below. The mean time to fail (MTTF) is also plotted in the TDDB graph below.
Figure 6. TDDB Graph of Time to Fail vs. Voltage (10 ppm and Mean)
Based on the data in Figure 6, the results for various working voltages are summarized for convenience in Table 3 below. At a working voltage of 800 Vrms, the 10 ppm time-to-fail is 265 years, while at 1200 Vrms working voltage, life expectancy is 53 years. This is the best high-voltage lifetime rating of any isolator available today.
One concern that customers have is whether the product has been tested at voltages much closer to the working voltages as a litmus test for TDDB extrapolation. To address that concern, Silicon Labs has started a test at 1600 Vrms. At the time of this writing, there have been no fails with more than 1000 hours logged on the test.
|Si86xx Applied Voltage||10 ppm Time To Fail, years
||Mean Time To Fail, years
|Vrms||Vpeak or V|
Optocoupler manufacturers have also conducted similar studies to rate their products (see Figure 7). The chart below is from App Note AN1074 on www.avagotech.com. No TDDB model is mentioned, only a safe operating area is defined as shown in Figure 7 below. According to the chart, only voltages less than 800 V continuous even have a lifetime rating. At 1600 V, safe lifetime is only 200 hours. The Silicon Labs product lifetime for 1600 V is 10 years.
Figure 7. Avago High Voltage Lifetime Rating for Optocouplers
5.0. Common Mode Transient Immunity (CMTI)
Common-mode transient immunity (CMTI) refers to the ability of the isolator to reject fast common-mode transients, typically measured in kV/µS. Common-mode transients are one of the leading causes of data corruption in isolation applications. CMTI is important because high slew rate (high‐frequency) transients can couple through parasitic capacitance across the isolation barrier, corrupting data. The schematic diagram for the CMTI test set-up and test board photo used in this paper appear in Figure 8, while Figure 9 shows the dominant parasitic couplings within the HCPL-4506 optocoupler.
Figure 8. CMTI Test Schematic and Test Board Photo
Referring to Figure 9, the single largest parasitic coupling is CLEDN, which is effectively short-circuited across the impedance of VCM when the optocoupler is configured in grounded cathode mode as shown. That said, the remaining parasitic couplings (albeit weaker) remain capable of creating CMT-induced data errors. The Si8712A is an optocoupler upgrade that directly replaces commodity optocouplers without changes to the PCB and provides substantially better performance, reliability and power efficiency (note the Si8712A is the low-power, low CMTI Si87xx product version).
Device testing was performed with an external pull-up resistor instead of the internal pull-up because the parasitic coupling of CLED02 is weaker than that of CLED01. The stronger parasitic coupling of CLED01 combined with the high 20 kΩ internal pull-up resistor would increase the probability of data errors due to CMT events for the optocoupler and, to a much smaller extent, for the Si8712A. The test results shown in Figures 9 through 13 compare side-by-side the CMTI performance of the Si8712A CMOS digital isolator to that of the HCPL-4506 optocoupler. While both isolation devices are configured identically per Figure 8, the internal parasitic couplings of the Si8712A are 2 to 3 times lower than those of the optocoupler, enabling higher CMTI performance.
Figure 9. HCPL-4506 Optocoupler Internal Parasitic Couplings
Figure 10A. Positive CMTI Response for CMOS Isolator
Figure 10B. Positive CMTI Response for HCPL-4506
Figure 10 applies logic high to the anode pins of both devices, causing their output to remain low. The top trace of Figure 10A shows a positive-going 1.5 kV CMT pulse, while the bottom trace shows the effects of the internal coupling to the collector pull-up resistor. As shown, the amplitude of this disturbance is on the order of 100 mV for the Si8712A, compared 500 mV for the optocoupler (Figure 10B).
The test of Figure 11 applies logic zero to the input of both devices causing their output to remain logic high while a negative-going CMT event perturbs the devices under test. Figure 11A shows the output of the Si8712A dipping negative to -250 mV(pk) compared to -1.1 V(pk) for the optocoupler. Note that the Si8712A exhibits both lower amplitude and faster recovery time. The CMT event of Figure 12 increases fall time to 200 ns, which has the effect of reducing CMT amplitude but increasing recovery time.
Figure 11A. Negative CMTI Response for CMOS Isolator
Figure 12A. Negative CMTI Response for CMOS Isolator
Figure 11B. Negative CMTI Response for HCPL-4506
Figure 12B. Negative CMTI Response for HCPL-4506
While the above tests show the effect of single parasitic events, multiple couplings can combine to create unwanted effects as shown in Figures 13A through 13D. Referring to Figure 13A, the optocoupler input is maintained logic high (output low), and this time the internal 20 kΩ pull-up resistor is used. The LED cathode is exposed to a 40 ns CMT rise time, which rapidly level-shifts the LED cathode and anode voltages, causing parasitic current to be injected through CLED02 to the Vo pin while CLED01 diverts current from IF into the RL pin. This “1-2” combination causes the current in both the LED and the internal pull-up resistor to be simultaneously reduced, further reinforcing the momentary output turn-off, as evidenced by the 1 V(pk) glitch of Figure 13A.
Figure 13. Internal Parasitic Coupling within the HCPL-4506 Optocoupler
Figure 13B shows the same test as Figure 13A, except that the CMT rise time is increased to 80 ns, which has the effect of reducing the internal pull-up coupling but increasing the amplitude of the LED parasitic turn-on. This trend continues in the 120 ns CMT pulse of Figure 13C and is especially pronounced in Figure 13D, where the internal pull-up coupling is virtually gone, but the parasitic turn-on pulse amplitude is several volts. Note the LED parasitic turn-on is delayed due to the long optocoupler propagation delay time of Figure 13D.
Section Summary: The optocoupler’s inherently high internal parasitic coupling and parasitic interdependencies can individually or collectively create false positives and negatives of various durations and amplitudes, corrupting data transmission and reducing overall system performance. This issue is compounded by the notoriously poor CMTI characteristics of optocouplers.
6.0. Electromagnetic Interference (EMI)
EMI refers to any electromagnetic disturbance that disrupts or otherwise interferes with the effective performance of authorized electrical equipment. As it pertains to isolators, radiated and conducted EMI is generally the product of high-frequency switching circuits (and their harmonics) that couple into other circuits and/or the environment. Radiated EMI is analogous to broadcast in that it travels from transmitter to receiver through free space, whereas conducted EMI travels along a conductor. The FCC sets limits for both conducted and radiated EMI, and all devices in a given class must comply.
Specifications published by various certification agencies place limits on conducted and radiated EMI; one of the more common specifications is FCC Part 15, which covers circuit assemblies used in or near the home. Testing to this specification is conducted in an open air environment using a 10 meter antenna positioned approximately 5 meters above the ground plane. Another specification, SAEJ1752-3, is more IC-centric in its test methodology and recommends mounting the IC to be tested on a small circuit board, then measuring the radiated emissions from the board while operating within the actual application environment.
EMI testing to FCC Class B Part 15 (Figure 14) was performed using two of the test boards, where each board contained a 6-channel CMOS digital isolator with all inputs tied high to maximize emissions from the internal transmitters. The EMI plots below show the total emissions from 12 channels switching at full speed on two boards (results summarized in Figures 14A-14D). This independently-conducted test shows that the CMOS digital isolator passed FCC Class B Part 15, achieving low EMI operation primarily through a combination of the field-canceling internal differential signal path, the sizing of the isolation capacitors, and the design of the low-power main oscillator. (Note that the low-frequency noise in Figure 14A was caused by tester noise and not the CMOS digital isolator.)
Section Summary: The optocoupler has perhaps the lowest EMI of all silicon-based isolators, with the exception of the Silicon Labs Si8xxx family. These CMOS digital isolators passed FCC Class B Part 15 and have industry-leading EMI performance.
6.1 RF Immunity:
RF immunity is the ability of the isolator to reject strong local electromagnetic fields, thereby maintaining error-free data transfer. Intuitively, one would assume that ambient RF fields would interfere with the CMOS digital isolator’s internal HF data transmission. However, CMOS digital isolators demonstrate an extremely high degree of external RF immunity by virtue of their design.
Local fields induce common-mode voltages within a CMOS digital isolator’s internal signal path that are rejected by a combination of the CMOS digital isolator’s differential isolation signal path and high receiver selectivity due to the tight matching of the signal levels on each side of the internal differential signal path. This architecture causes common-mode voltages at the receiver differential inputs to be rejected. The receiver then amplifies only the differential input voltage within a very narrow frequency band, and rejects all other input. Together, these mechanisms reject interference from a variety of external fields and enable very high CMTI and robust operation, even in electrically noisy environments.
Figure 15. CMOS Isolator E-Field Immunity
The CMOS digital isolator can be used in close proximity to large motors and other magnetic field-producing equipment. It is theoretically possible for data transmission errors to occur if the magnetic field is far too large and/or far too close to the isolator. In actual use, however, CMOS digital isolators provide extremely high immunity to external magnetic fields and have been independently evaluated to withstand magnetic fields of at least 1000 A/Meter (per IEC 61000-4-8 and IEC 61000-4-9). Such a field could be generated by 107 amperes through a 0.1 m conductor located 0.1 m away from the CMOS digital isolator. It is highly unlikely that such a condition would be found in any operating environment. Such a field would most likely destroy surrounding circuitry before damaging the CMOS digital isolator isolation barrier circuit.
Section Summary: While optocouplers provide the singular benefit of intrinsic external field immunity, the CMOS digital isolator exhibits virtually equivalent field rejection through its high receiver selectivity due to the tight matching of signal levels on each side of the internal differential signal path.
7.0. Ease of Use
Large and interdependent parasitic couplings, temperature-dependent operating parameters and operating changes with device age make optocouplers challenging devices to apply. Conversely, CMOS digital isolators have substantially lower first-order parasitics, fewer interdependent couplings and significantly better performance over operating temperature. Common optocoupler design issues are summarized in the following section.
7.1. Current Transfer Ratio:
Optocouplers have a temperature-dependent “gain term” called the current transfer ratio (CTR). Typical CTR values will be between 10% and 50% for optocouplers with an output phototransistor and up to 2000% for optocouplers with a Darlington transistor output pair. Other CTR dependencies include output transistor current gain hfe, VDD supply voltage, forward current through the LED and ambient operating temperature. The value of CTR varies with absolute current level and peaks at an LED current level of about 10 mA; then it falls away at both higher and lower current levels.
Figure 16. Optocoupler CTR
7.2. Digital Optocoupler Turn-on Thresholds:
Digital optocoupler turn-on and turn-off thresholds tend to be positioned at relatively low levels and are temperature-sensitive as well. The actual threshold values can be rather nebulous from one unit to the next. Lab measurements have shown that most optocouplers begin turn-on below the data sheet-specified threshold, sometimes requiring additional external components for correction.
Figure 17. Si87xx Input Current Turn-on/Turn-off Thresholds vs. Optocoupler Thresholds
Figure 17 compares the Si8712A CMOS digital isolator thresholds to that of a typical digital optocoupler. As shown, the CMOS isolator is either on or off, which eliminates the need for a CTR specification and its related temperature sensitivities. The digital optocoupler has the diode characteristic of the internal LED, making consistent and precise turn-on/turn-off thresholds very difficult to implement.
7.3. Added External BOM:
Optocouplers operating in electrically noisy environments commonly use additional components to increase CMTI performance, as shown in Figure 18. Input and output circuit modifications that help increase optocoupler performance include an external (instead of internal) pull-up resistor, reverse biasing or shorting the LED to ensure it remains in its correct state in the event of a CMT and adding an external RC to filter momentary output glitches, such as those shown in Figures 10 through 13 inclusive.
Figure 18. External Circuits for Optocoupler CMTI Improvement
These optocoupler fixes should be made with care as some of them can combine to create unwanted results, as outlined in Table 4.
7.4. Degradation with Device Age and Current:
Given the typical service life of industrial equipment, long-term reliability and consistent performance are becoming of paramount concern to system designers. Optocoupler timing parameters (e.g., propagation delay, pulse width distortion) can change with three different variables: LED wear-out, excessive LED input current and operating temperature.
One of the key optocoupler wear-out mechanisms is LED light output (LOP), a material-based phenomenon, which causes the LED to lose brightness over time. Lower emission decreases the signal seen by the photo detector, negatively impacting optocoupler timing and output impedance characteristics. Figures 19A and 19B show manufacturer’s LOP data based on normalized light output over a period of 10,000 hours for LEDs made from both Gallium Arsenide Phosphide (GaAsP) and Aluminum Gallium Arsenide (AlGaAs). Light output degradation typically worsens with increasing temperature and increasing LED current. In the worst case, the light output can fall below the minimum value needed for proper device operation.
Figure 19A and 19B. Optocoupler Manufacturer’s LOP Test Data
LOP can change nominal light output by as much as 20%, causing significant changes in operating characteristics. Since the degradation rate is partially related to forward current, the system designer must take into account the effects of LOP when choosing the LED forward current operating range, forcing a trade-off between longer device service life and lower CMTI. The LOP wear-out precludes the use of optocouplers above approximately 100 °C and 25-year lifetime applications, such as solar panels.
Section Summary: Many optocouplers have multiple, highly-temperature-dependent parameters, such as the current transfer ratio and input turn-on and turn-off thresholds that complicate design and negatively impact system performance. Optocouplers operating in electrically noisy environments commonly use additional external components to increase CMTI performance. Optocouplers also have both a narrower temperature operating range and inferior reliability (10x lower service life compared to CMOS digital isolators). By comparison, CMOS digital isolators are either on or off, eliminating the need for a CTR specification and its related temperature sensitivities. In addition, CMOS isolators have tight input thresholds, superior CMTI, and greater parametric stability over temperature and current, industry-leading reliability and no wear-out mechanisms.
8.0. Optocoupler Replacement Upgrades
Optocouplers have traditionally dominated signal isolation applications because of the absence of alternative pin- and package-compatible isolator solutions. The Si87xx CMOS digital isolator (Figure 20) directly replaces optocouplers without PCB modification while providing significant gains in performance and reliability. The Si87xx input-side “harvests” input bias voltage from the anode input while the output side is powered by a local voltage source of up to 30 Vdc. The Si87xx is also compatible with external optocoupler circuits, including those that implement the remedies shown in Figure 18.
Figure 20. Si87xx Optocoupler Replacement CMOS Digital Isolators
The Si87xx input-side die includes a diode emulator, high-frequency transmitter and galvanic isolator stage. The diode emulator performs two functions: it mimics the behavior of an LED to ensure compatibility with existing (external) optocoupler input circuits and it enables the transmitter when anode current is at or above its threshold value. When enabled, the transmitter propagates a high-frequency carrier across the isolation barrier to the receiver, which forces the output driver low when sufficient in-band energy is present. Conversely, input current below the threshold value disables the transmitter, causing the receiver to turn-off the open-drain MOSFET and allowing the external or internal pull-up resistor to pull the output pin high. This simple architecture provides a number of features and benefits over optocouplers:
- Package and pin-compatible, drop-in optocoupler upgrade with typically no PCB changes required
- Direct, package and pin-for-pin optocoupler replacement
- Silicon Labs patented CMOS isolation technology for higher performance and reliability
- Isolation ratings: 3.75 kV and 5.0 kV10 kV surge-tolerant per IEC 60065
- Faster, tighter timing: 1 Mbps or 15 Mbps versions, 60 ns prop delay, 20 ns PWD
- CMTI: 35 kV/µS (A-Grade), or 50 kV/µS (B-Grade) and 2x to 3x lower internal parasitic coupling vs. optocouplers
- Precise input current thresholds: 2.2 mA (A-Grade) and 3.5 mA (B-Grade)
- No CTR
- Very low parametric change with temperature
- Wide –40 °C to +125 °C operating temperature range
- High reliability: TDDB > 60 years at worst-case voltage and temperature
|Issue||Digital Optocoupler Solutions||Si86xx CMOS Digital Isolators||Si87xx CMOS Digital Opto Retrofit Isolators|
|Improve CMTI: Spurious Turn-On||
|Improve CMTI: Spurious Turn-Off||
|Address CTR Variation||
|Address LED Aging Effects||
If parasitic coupling through CLED01 and CLED02 causes Si87xxA false turn-on, replace Si87xxA with higher threshold Si87xxB.
Increasing CL requires more current for the same tPHL and tPLH.
Increasing RL requires more LED current for the optocoupler.
Table 4 compares common LED-based optocoupler issues and solutions against those of the Si87xx series of CMOS digital optocoupler replacements. Note the generous application of input current required for the optocouplers to sustain normal operation.
While optocouplers have been the dominant signal isolation solution for a long time, a new breed of higher performance CMOS isolator solutions will ultimately outmode the venerable optocoupler by virtue of the following benefits:
- Broad offering: Single, multichannel (6 channels max), logic input and optocoupler replacement versions
- Optocoupler upgrades: Advanced, drop-in optocoupler upgrades with higher performance and reliability
- Tight timing performance: tPHL/tPLH, PWD, channel-to-channel skew, unit-to-unit matching and more
- High CMTI: up to 50 kV/µs
- High reliability, longer service life: 60+ year TDDB
- High surge immunity: 10 kV surge tolerant (Si87xx)
- Stability: Very low parametric changes over operating temperature
- Low EMI: meets FCC Class B Part 15
- Efficiency: Low operating power per channel
- High e-field immunity: Electric field immunity up to 50V/M, magnetic field immunity above 1000A/M
- Industry-leading ESD tolerance: up to 6 kV HBM
- Ease of use: No CTR, 2x to 3x lower internal parasitics compared to optocouplers, intuitive device behavior
In conclusion, the benefits of CMOS digital isolators bring a high level of added value to power system designs. The increases in performance, reliability and ease-of-use make these advanced solutions the ideal choice for both system retrofit and new designs.