ISL32740E/41E Isolated PROFIBUS/High-Speed RS-485 Transceivers
- Explains the purpose and benefits of galvanic isolation
- Gives a short overview on applications requiring isolated data transmission
- Lists the major differences between PROFIBUS and RS-485
- Discusses principle operation of GMR isolators
Interface Products from Intersil
Intersil was an original innovator during the early evolution of serial interface standards, including the introduction of the charge pump eliminating the negative supply voltage requirement.
Our extensive portfolio includes transceiver families that provide the industry's
- Highest output voltage (Vod = 3.1V @ 54Ω)
- Highest data rate (DR = 100Mbps)
- Highest withstand voltage (VAB = ±80V)
- Lowest supply current (Icc = 70µA)
- Lowest logic supply (VL = 1.35V)
thus, giving our customers all the choices they need to meet their serial communication requirements.
Applications That Use Isolation
Isolation in general is used in many markets, ranging from medical, industrial and consumer electronics, to telecom, computer, and office equipment. PROFIBUS and high-speed RS-485 related isolators are used in the following applications.
- Industrial networks of factory automation
- Industrial sensor modules connecting to a network
- Industrial robotics
- Point-to-point data links in medical and motor control systems
Industrial Applications that Use Isolated PROFIBUS or High-Speed RS-485
Industrial networks connect programmable logic controllers (PLCs) to instruments, motor drives, data acquisition (DACQ) and digital I/O modules. Often, the communication protocol is PROFIBUS-DP, which uses a slightly modified version of RS-485 as physical layer.
At the equipment/bus interface, galvanic isolation is implemented to keep the communication bus free from common-mode noise, which is of particular importance in the electrical noisy environment of motors and generators.
A second, emerging field of applications is industrial robotics. Here, the communication between a controller and the actual robot happens via high-speed RS-485.
Next, I’ll cover some technical highlights.
Defining Galvanic Isolation
- Is a means of preventing current from flowing between two communicating points while allowing the transmission of energy or information between these points.
- Is used to eliminate ground loops while withstanding large ground potential differences (GPDs).
The information transfer within the isolator can occur via light, electric fields, or magnetic fields.
Preventing Ground Loops
The network nodes of a communication network draw their supply from different locations in the electrical installation system and use their local grounds as reference potential.
However, remotely located power sources can experience large ground potential differences (GPDs) due to non-standardized earthing techniques. GPDs are the main contributor to common-mode noise in a data link.
The left diagram shows that by simply connecting the bus node circuits to their local grounds, a ground loop is created. Here, large GPDs can cause large common-mode noise on the bus exceeding the input common-mode voltage range of a transceiver. This can cause data errors and even lead to device damage.
The right diagram shows that by inserting an isolator into the signal path, the ground loop is broken, and the common-mode voltage removed from the bus.
Thus, the use of galvanic isolators in a data link not only prevents the design of unintentional ground loops, but also ensures reliable transceiver operation in the presence of high GPDs (that can range up to the isolator's breakdown voltage).
Proper Isolation of Bus Nodes....
High-speed interfaces, such as Ethernet, use transformers to isolate the differential data bus, which limits the transmission bandwidth to high-speed signals only.
However, PROFIBUS and high-speed RS-485 interfaces can operate at data rates of up to 40Mbps. Here, the most economical solution is the use of single-ended, CMOS/TTL logic isolators, implemented between the transceiver logic I/O and the adjacent UART or controller.
Of course, complete bus node isolation also requires the isolation of the local power supplies from the supplies of the bus-connected circuits using isolated DC-DC converters. The schematic on this slide shows both isolated grounds (GND1ISO and GND2ISO) are floating, having no relation whatsoever to the local Earth-grounds GND1 and GND2. With the impedance of each isolation barrier being about 1014Ω, the entire common-mode voltage, previously established across the receiver inputs, is now distributed across both isolation barriers.
Tougher Application Requirements for Network Isolators
Although it's possible to design an isolated bus node using separate transceiver and isolator components, the ISL32740 and ISL32741 transceivers combine both functions within a single package.
Legacy industrial network transceivers are mainly available in 16-lead standard wide-body SO packages. They only support EIA-485 transmission and operate at temperatures of up to 85°C.
The last decade however shows a rising trend towards PROFIBUS installations requiring higher data rates and thus, more precise switching characteristics. End equipment exposure to higher ambient temperatures required an increase in transceiver operating temperature, while small form factor designs of sensor modules connecting to a network called for smaller transceiver packages.
In order to expand the field of applications into medical and motor control systems, higher isolation robustness in the form of reinforced insulation was necessary. This in turn required the design of new wide-body SO packages, whose minimum creepage distance of 8mm exceeds the standard wide-body package specified by JEDEC.
Intersil's response to these increasing demands, the ISL32740 and ISL32741 transceiver portfolio shown next.
Industry's Strongest PROFIBUS Isolator Portfolio
The ISL32740 and ISL32741 have the same electrical performance. The main difference between them is the isolation rating, basic versus reinforced. The ISL32740EIBZ was our first PROFIBUS isolator in 16-SB-SOIC package, operating up to 85°C.
The ISL32740EFBZ is the same device but performing up to 125°C without parametric variations. The ISL32740EIAZ is the ISL32740 in a small 16Ld QSOP package. Due to the package's higher thermal resistance though, the upper operating temperature limit remains 85°C. The ISL32741EIBZ and ISL32741EFBZ are rated VDE-reinforced due to their thicker isolation dielectric. The EIBZ (85°C) version is the a low-cost variant of the EFBZ (125°C) version.
Note: Reinforced isolation describes a single isolation barrier with twice the dielectric strength of a basic isolation barrier. There is however a difference between an UL-reinforced and a VDE-reinforced rating. UL does not base its reinforced isolation rating on package geometry. It simply drops the isolation rating of basic insulation by half and calls it reinforced. By this definition, all ISL32740 transceivers are UL-reinforced for 1.25kV.
The German VDE0884-10 standard on the contrary, does specify a minimum creepage distance for the package. Standard compliance is established under so-called "partial-discharge tests" thus resembling real rather than ideal isolation barriers. The standard also includes tests for surge immunity, and therefore represents a much vigorous testing in comparison to UL1577. Hence, when promoting the ISL32741, always emphasize its VDE-reinforced rating.
GMR-Isolator Functional Principle
The ISL32740/41 utilizes a GMR isolator whose operating principle is shown in the left diagram. Here, a buffered input signal drives a primary coil, which creates a magnetic field that changes the resistance of the GMR resistors 1 to 4. GMR1 to GMR4 form a Wheatstone bridge in order to create a bridge output voltage that only reacts to magnetic field changes from the primary coil.
Large external magnetic fields, however, are treated as common-mode fields. Since they affect all four GMRs equally, the bridge output is zero. Thus, external fields are suppressed by the bridge configuration. The right diagram depicts the function of a single GMR resistor.
This resistor consists of ferromagnetic alloy layers, B1 and B2, sandwiched around an ultra-thin, nonmagnetic, conducting middle layer, A, typically copper. The GMR structure is designed so that, in the absence of a magnetic field, the magnetic moments in B1 and B2 face opposite directions, thus causing heavy electron scattering across layer A, which increases its resistance for current I drastically.
When a magnetic field H is applied, the magnetic moments in B1 and B2 are aligned and electron scattering is reduced. This lowers the resistance of layer A and increases current flow.
Internal Structure of a GMR Isolator
The crosscut above shows the internal structure of a GMR isolator. The current (I) flowing through the planar coil windings generates a magnetic field (H) that penetrates a proprietary polymer dielectric barrier and modifies the resistance of the magnetic sensors (GMR resistors) in bridge configuration.
The bridge output is conditioned and made available via output buffers located within the silicon substrate. Above the planar coil windings is a passivation layer that allows for the application of a magnetic shield. This shield has two functions:
- Shielding the isolator structure against external magnetic fields
- Strengthening the internal field and focusing it onto the GMR bridge
Emissions: Advantages of GMR vs. Transformer Isolation
A major advantage of GMR isolation over other isolation technologies is its low radiated emission and low EMI susceptibility. Unlike capacitive and magnetic isolators that utilize RF carriers or pulse-width modulation (PWM) to transfer DC and low-frequency signals across the barrier, GMR isolators do not require fancy encoding schemes. Neither do they include current hungry power transfer coils or transformers, as their signal transfer is virtually energy-less.
The absence of the above factors not only results in a significantly lower current consumption but also causes the radiated emission spectrum to be virtually undetectable. Furthermore, because GMR isolators have no pulse train of RF carriers to interfere with, they also have very low EMI susceptibility.
Typical Isolated PROFIBUS Application
In a typical isolated RS-485 bus, each transceiver requires two power supplies, one for the non-isolated control side and one for the isolated bus side. On the control side, the ISL32740/41 allows for operation down to VDD1 = 3V, thus enabling the direct connection to 3V microcontrollers. The bus side however, requires a 5V supply to generate strong bus signals, that are able to compensate for the frequency dependent cable losses at high data rates.
This PROFIBUS-DP application assumes a bus length of less than 100m. It uses dual failsafe biased terminations suggested in the PROFIBUS standard, to ensure very high noise immunity during bus idling. Because of the use of isolated transceivers, the GND2 terminals of both transceivers are floating and any common-mode voltage is removed from the bus.
The entire common-mode voltage, mainly due to the large GPD between bus nodes, now drops across both isolation barriers. This means that any GPD between the GND1 terminals of both transceivers can be as high as the working voltage of the isolation barrier. For the ISL32740, this voltage can be as high as 600V, and for the ISL32741 even 1kV.
Differences Between PROFIBUS and RS-485
The table on this slide speaks for itself with the exception of the last parameter VOD-PP. Here, PROFIBUS International places limits on the transceiver differential output voltage that prohibit the use of many RS-485 transceiver chips on the market. Under no-load condition, the maximum differential bus voltage is limited to 7VPP, while for maximum load, the minimum limit is 4VPP.
Many RS-485 transceivers with high output drive comply with the minimum value, but exceed the 7V maximum, because it is easy to produce drivers with differential voltages of up to 8.5V. In this case, more is definitely not better. Therefore beware, failure to use a transceiver complying with the 4VPP to 7VPP limits will cause an end equipment's PROFIBUS Certification to be rejected by the approval body.
Another hurdle is the requirement of measuring the differential voltage at the maximum specified supply voltage. For a 5V transceiver this usually is 5.5V. PROFIBUS test laboratories use that worst-case voltage in their compliance tests. PROFIBUS designers don't need to worry, because all end equipment designs using the ISL32740/41 transceivers are PROFIBUS compliant.
To ease the evaluation of the ISL3274x transceivers, simple evaluation boards are available. Supply and control signals are applied on the left side of the board, while the bus signals are taken from the right side of the board.
An isolated 3.3V to 5V DC-DC converter provides the power supply across the isolation barrier. Engineers new to isolation are referred to application notes AN1971 and AN1972, which explain the operating principle of GMR and its implementation into signal isolators.
Engineers concerned with safety requirements for electrical systems will find important information in application note AN1973, which explains why GMR isolators, certified to VDE V 0884-10, can be used in equipment requiring compliance with the latest edition of IEC 61010-1, Edition 3.
Intersil's ISL3274x family of isolated PROFIBUS transceivers offers the industry's
- smallest package
- widest operating temperature range
- highest isolation rating
They therefore should be easy to design-in into new PROFIBUS and high-speed RS-485 designs.