In this whitepaper, we will discuss the challenges associated with designing industrial systems and how isolation products can be used to eliminate some of these issues. The paper will also dive into some specific applications and the products available to address the unique problems they present.
The Applications and Design Challenges of Industrial Automation
In the world of industrial design, there are a few critical parameters that cannot be compromised. First and foremost is safety; the goal is to achieve high levels of intrinsic safety without sacrificing system performance. Second, industrial environments are inherently noisy. Products introduced to these types of industrial settings should be highly immune to external noise fields and high voltage noise transients.
Most industrial systems are large infrastructure-type projects, which usually translates to high cost. The large investment means they need to be long-lasting to justify the expense. This financial commitment makes it necessary for the products to be resilient and durable under high stress conditions.
Finally, these systems tend to be high performance – whether that’s measured in terms of throughput or efficiency. Therefore, the products should have excellent performance characteristics, such as low propagation delays and low skews, and a high level of part-to-part compatibility to ensure that the product performance is stable and repeatable. Using isolation products when designing industrial systems can provide all of these benefits, in particular the safety and noise immunity characteristics, which are the most desired in this sector.
The Importance of Isolation
Isolation is required when circuits within a system that are not referenced to a common ground have to transfer data or control signals to each other. In some cases, the difference in common mode between the systems may vary greatly – even thousands of volts. This is fairly common with high-power systems such as motor control or high wattage power supplies or UPS systems. Here, safety is a primary concern - the high voltage can be a safety hazard for human operators as well as low-voltage electronic circuits.
Another important use of isolation products is for noise elimination. Since isolation products effectively cut the ground loop between two circuits while transferring data, this helps to eliminate ground loop currents and therefore noise issues. Level shifting is also a common use for isolation products, which are intrinsic level shifters.In the figure below the following can be observed:Safety is improved by isolating 400V from a 3.3V MCULevel Shifting is acheived so VDD1 and VDD2 can be different levelsNoise is reduced by preventing ground currents from disturbing sensitive circuitry
Traditional Isolation: Optical Coupling
Traditionally, isolators were constructed using optical coupling. Photons generated by LEDs transmit the signal from the input to the output side and photosensitive transistors convert it back to electrical signals. However, opto-coupled isolators have some intrinsic disadvantages when it comes to industrial applications including:
- Poor reliability where parameters drift over temperature, VDD and time
- Poor timing characterisits such as large proagation delays, skew, and varation from part to part
- Marginal common mode transient immunity (CMTI) due to large parasitic coupling between input and output
- Energy inefficiceny with a higher current requirements to drive even moderate performance requirements
Isolation Using Non-Optical (CMOS) Capacitive Coupling
For a few years now, isolators using non-optical coupling techniques have been available. Silicon Labs, for example, uses CMOS platforms and On-Off Keying (OOK) modulation to couple the signal level information across the capacitive isolation boundary, using SiO2 (silicon dioxide) as the high-voltage capacitor dielectric.
This method has a few distinct advantages:
- The construction of the product ensures it’s highly reliable over a long lifetime. With low propagation delays and low skews, it has excellent part-to-part matching due to the all-CMOS process platform used.
- The technique also offers high noise immunity to voltage transients – as high as 200 kV/µs, which is at least 2x better than any product available today.
- These types of isolators are much lower power compared to optos and even other digital isolators.
Application Specific Challenges
Programmable logic controllers are used extensively in industrial automation applications, such as machine control or assembly line control, amusement rides, and industrial lighting applications.
These controllers primarily consist of a set of inputs and outputs (commonly called I/O modules) and a central processing unit. The custom control program for any particular application is stored in the non volatile memory (NVM) so that for a given set and sequence of input events, the controller enables certain outputs. Thus, this is a real-time system which must respond within a certain time period to ensure correct operation and to maintain safety.
PLC systems may have 128 channels while the field inputs can be sourcing or sinking, depending on the geographic market sector. Commonly used optos are not fast enough for certain applications like servo motor control. Also, opto performance is poor based on temperature fluctuations and age.
The Si838x from Silicon Labs solves these problems:
- Our isolators can lead to huge board area savings because it packs 8x channels in a QSOP-20 package and up to 16x Si838x devices can be daisy-chained for 128 total channels.
- One SPI port interfaces to the processor while accommodating these 128 channels, reducing GPIO count for controller.
- Bipolar input capability allows it to be used with either sourcing or sinking inputs.
- High speed capability of 2 Mbps data rates makes it an ideal candidate for reliable servo control.
- Digital CMOS construction means it provides stable and reliable performance over a long lifetime.
Si838x: Application Diagram
The Si838x is typically connected to the 24V based digital input switches located on the field side through a simple resistor network. The values of the resistor can be chosen by the customer to satisfy compliance to IEC 61131-2 Type 1, 2, or 3 switches.
The stepped down voltage from the R network feeds into the Si838x input pins, which are LED emulators. The emulators utilize the power supplied by the switches directly and do not need any power supply on the field side. The CMOS outputs can be directly connected to a controller with 2.25V – 5.5V capability. Status LEDs (D2 in diagram below) can be connected to the outputs through a current limit resistor as shown below.
These status LEDs are required for compliance to IEC 61131-2. They indicate whether the field input is in ON or OFF state. No special connection is needed for the high speed channels (AHx) vs the low speed channels (Ax), except that capacitor C1 is recommended for noise immunity on the AHx channels. The Ax channels have built in de-glitchers that provide very high immunity without the need for external capacitors.
The diagram above illustrates how 16x Si838x can be daisy chained for a highly efficient use of controller GPIOs.
Target Application: Isolating Interfaces (PHY)
The block diagram below is for a card making measurements in the automation system. The measurements are captured by an embedded controller and then sent through an RS-485 (or CAN) bus that enables multiple ports to communicate effectively. The processor is isolated from the transceiver for all the reasons we discussed before – mainly for noise and safety considerations. A two or three-channel isolator is needed as shown. The RS-485 or CAN transceiver and the secondary side of the isolator needs a little less than 1W of isolated power. This makes it a target application for the Si88xx family. In fact, the Si88xx is a great fit for isolating devices like RS-485, CAN, and RS-232 transceivers in a range of applications such as motor control, PLCs, e-meters, solar inverters, and medical electronics.
Providing isolated power at a small level like 1 Watt can be tricky and can require many components and tuning of the control system. In many cases, the engineer for automation systems doesn’t have experience with creating power systems. However, isolated power is necessary for having an isolated communications bus, therefore a more straightforward method is preferred. The Si88xx solves this problem by providing an integrated dc/dc controller. With just a few external components, isolated communications and isolated power can be realized. This family can provide up to 5 Watts of power, more than enough for the transceiver and other circuits. It’s an ideal and efficient solution for isolating RS-485 and CAN buses.
Target Application: Power Supplies
Shown below is an AC to DC power converter. There are switching stages on both side of the transformer that require a gate drive. On the line (AC) side is the main switching stage. On the output side, there are sync FETs in high efficiency power supplies.
The most critical parameter for power supply design is improved power density, or watts per centimeter cubed. The other issue driving decisions for power supply customers is safety. Improving product lifetimes is also important to ensure safety for the end user. The newest IsoDrivers have features to address both power density and safety.
Switching Power Safely
The challenge with these types of systems is finding the best way to switch power both efficiently and safely. Due to the amount of power supply contained within these systems, safety issues can be costly, or even fatal.
The Si8239x has an output power status pin for reporting back to the controller. The device also has an auto mode where the IC automatically shuts down both driver outputs in case of an UVLO fault on either driver. This prevents flux imbalance in the transformer if one driver is off while the other is still switching, thus preventing the transformer from getting saturated or possibly damaged.
In the quest to increase power density, one of the best tools available is to increase the switching frequency of the modulation scheme. This has the advantages of reducing the size of the magnetic components such as inductors and transformers, increasing efficiency, and improving transient response.
However, there is a risk associated with higher frequency: the faster switching causes higher slew rates and thus high noise transients. These transients can cause the gate driver to glitch and therefore cause loss of modulation temporarily, which can affect efficiency, or it can cause permanent damage by inducing spurious turn-on of the MOSFET or by latching up the driver.
With a high performance noise immunity of 200 kV/µs, the Si827x eliminates the risk posed by faster switching speeds. The high noise transients generated by the faster switching will not affect signal integrity through the driver, thereby eliminating the risk of loss of modulation. The high latch-up spec of 400 kV/µs makes the Si827x extremely robust and prevents permanent latch-up damage.
Target Application: Motor Control
Below is a block diagram of a multi-phase inverter system for motor control. In this case it’s a 3-phase AC induction motor control. Usually, the switches here are IGBTs. Motor drives such as this type typically have three current measurements on the individual windings of the motor. Since each one of these current sensors will change voltage during the switching cycle, they must be isolated.
The Si8920 provides an interface to the shunt resistor that acts as the current sensor. Also shown in the diagram is an overall current measurement on the DC LINK supply. This measurement is used for monitoring the total system. The fast response offered by the Si8920 allows the system to respond to error and transient conditions quickly.
Si8281: Isolated IGBT Driver with dc/dc Converter
Shown below is the Si828x IGBT driver, which has an integrated dc/dc converter. The driver has built-in desaturation detection and logic to shut down the driver in a controlled manner for safety. The device also feeds back the fault state to the controller, which allows the controller to take action when a fault has occurred in the system. The controller is also fed back a power ready signal when the UVLO condition is met on the isolated side.
The Si8281 also has an integrated Miller clamp to ensure a strong shut off of the gate. With high common mode transient immunity, the device =offers the best timing specifications available with low propagation delay. All specifications are for the extended industrial temperature range from -40 °C to 125 °C.
The input and output sides of the amplifier are isolated by Silicon Labs industry leading isolation technology. This galvanic isolation is robust and allows the controller to stay immune while measuring the current on a high-power bus. The Silicon Labs underlying architecture for the device has little delay, ensuring the controller is instantly informed about changes in the system load and can respond faster. The amplifier has very low drift specifications for offset and gain.
The accuracy is maintained over the entire operating profile, which helps development by removing the need for calibration over temperature. The Si8920 also has excellent common mode transient immunity, which eliminates disruption to the current measurement during the switching process.
Si828x Simplifies PCB Layout
With the isolated DC-to-DC integrated into the device, the designer can place the power domain directly at the driver. The below figures show how the power domains are simplified using a driver with an integrated dc-dc converter, which can generate its own isolated driver power supply. The power domains are centered on the driver and can remain small. This simplified power architechture separates the different power domains for the drivers and reduces the distance that the power has to be routed across the board.
Noise and inductances are also reduced, creating amore compact and smaller PCB. The resulting PCB lay-out is efficient and provides numerous design benefits.
Target Application: Current Measurement in HV Systems
In a situation where the controller is managing a converter stage with 600 Volts on the rail or DC Link, the system controller can't measure the current directly because of the high voltage. The controller also isn’t able to connect directly to a sense resistor; therefore, isolation is necessary to facilitate the connection. This inverter switching process can be excessively loud and noisy and many transients must be rejected in order for an accurate measurement to be made. The Si8920 isolated amplifier provides a good solution to this type of current measurement on high-voltage rails problem.
Si8920: Isolated Analog Amplifier for Current Sense
The Si8920 is an analog amplifier with a low voltage and differential input, which connects directly across the shunt resistor. Either ±100mV (with a 16x gain) or ± 200mV (with an 8x gain), is specified for a ±1% max gain error. The device’s low group delay of 0.75 us allows fast response. The Si8920 also has hgh bandwidth and low noise, which allows wide usage with high accuracy. Noise measured is in the range of 0.03mVrms over 400kHz with 0.2% non-linearity. The high working voltage of 1200V allows connection to the high voltage bus with a 20-year lifetime guarantee.
Silicon Labs Isolation Portfolio for Industrial Systems
The Silicon Labs family of mixed-signal IC solutions offer superior isolation and high voltage expertise to enhance power system performance, flexibility and reliability while reducing system size and cost. The graphic below provides information on the various isolation families for the industrial market. As previously discussed, the Si86xx offers multiple digital isolation channels with up to 5kVrms isolation rating, while the Si86xxT provides an additional 10kV surge withstand capability. The Si823x provides half bridge or dual driver configurations with up to 5kVrms isolation rating. The Si826x is a pin to pin replacement for many popular opto-coupled gate drivers with superior performance and reliability.
|Family||Description||Unidirectional Channels||Bidirectional Channels||Isolation Rating (kV)||Input Type||Package Type||Max Data Rate||Max Propagation Delay||Input Supply||Output Supply||Forward Channels||Reverse Channels|
||High Data Rate, Low Power Digital Isolators||0, 1, 2, 3, 4, 5, 6||0, 1, 2||1, 2.5, 3.75, 5||Digital||NB SOIC16, NB SOIC8, QSOP16, WB SOIC16||1.7, 150||13, 55||2.5 5.5||2.5 5.5||0, 1, 2, 3, 4, 5, 6||0, 1, 2, 3|
||High Data Rate, Low Power, 10kV Surge Isolators||2, 3, 4||0||5||Digital||WB SOIC16||150||13||2.5 5.5||2.5 5.5||1, 2, 3, 4||0, 1, 2|
||High Data-rate, Isolators with Integrated dc-dc||2, 4||0||3.75, 5||Digital, Low||WB SOIC16, WB SOIC20, WB SOIC24||100||23||3 5.5||3 5.5||0, 1, 2, 3, 4||0, 1, 2, 3, 4|
||24V Digital Input Isolator for PLCs||8||0||2.5||LED Emulator||QSOP20||200, 2000||4 µs, 4 µs/100 ns||2.25 5.5||8||0|
Si8920 Analog Isolators
|Part Number||Data Sheet||Evaluation Kit||Package Type||Temperature Range (°C)||Output Mode||Isolation Rating||Initial Accuracy|
|Si8920ISO-KIT||GW DIP8||-40 125||Analog||3.75||1.50%|
|Si8920ISO-KIT||GW DIP8||-40 125||Analog||3.75||0.75%|
|Si8920ISO-KIT||WB SOIC16||-40 125||Analog||5||0.75%|
Isolated Gate Drivers
|Family||Isolation Rating (Output-Output)||Package Type||Output Configuration||Input Type|
||0, 3500, 900||LGA14, NB SOIC16, NB SOIC8, QFN14, WB SOIC14, WB SOIC16||Dual Driver, High Side / Low Side, Single||LED emulator, PWM, VIA, VIB|
||0||GW DIP8, NB SOIC8, WB SO6||Single Driver||LED emulator|
||3500||WB SOIC16||Dual Driver, High Side / Low Side||PWM, VIA, VIB|
||2100||NB SOIC16, NB SOIC8||Dual Driver, High Side / Low Side, Single Driver, Separate Pull Up/Down||PWM, Single Input, VIA, VIB|
||3750, 5000||WB SOIC16, WB SOIC20, WB SOIC24||Single Driver||PWM|