The Use of Robust Digital Isolators in the Harsh Environments of Electric Motor Drives

Introduction

Robust digital isolators are required in the harsh environments of electric motor applications. These very difficult environments have requirements for immunity to high voltage transients causing data upsets and the effect of high voltage electrical stress to the isolation lifetime of the isolator. The typical solution for isolation in these applications has been optocouplers, which are made to withstand high voltages due to their thick layers of internal insulation. The drawback to optocouplers is their use of light emitting diodes (LEDs) that lose their light intensity over time and vary with temperature, which creates design and reliability issues. There are new, more robust digital isolators that eliminate the use of LEDs and their reliability issues, and have improved insulation capability to compete with the optocoupler. These digital isolators have advantages of increased immunity to high voltage transients,a requirement in motor control applications. This article will describe in detail how these new digital isolators operate and how their advanced capabilities will outperform optocouplers in these applications.

Applications

There is a range of system designs for electric motor drives, depending on the performance and power level of the application, as well as the particular control and isolation schemes. Figure 1 shows the isolated communications block diagram often used for inverters or low end motor drives. In this system, the controller is located at the same potential as thepower stage, with the communications interface being isolated, as this is typically a lower speed and simpler interface. In these systems, the power inverter may have low-side gate drivers that do not need to be isolated since they share the same ground as the motor control block. The high-side drivers can be isolated, but techniques such as level shifting can also be used, particularly if the power inverter voltage levels are not too high. In this block diagram, the motor controller has a direct connection to the inverter feedback without the use of isolation. This architecture has limitations when used at higher power levels. The additional noise generated by the switching signals to the motor could overwhelm the feedback signal used to monitor the motor current and, potentially, could lead to loss of control of the motor.

For higher performance drives—for example, large multiphase drives used in industrial motors and in traction motors used in trains—isolated control and communications would be needed, as shown in Figure 2. In this system block diagram, the control and communications are together on the safe side of the isolation barrier for reasons of noise immunity and higher communication speed. Now that the motor control block is on the safe side of the isolation barrier, isolation is required for all the gate drivers. The specific isolation voltage and safety requirements are determined by the detailed architecture and isolation barrier locations. In the block diagram, the inverter feedback is used to help control the motor drive and is one of the most critical areas of motor control. The inverter feedback is shown connected to current measurement nodes iV and iW in two phases of the three-phase ac motor. In the isolated control and communications system diagram, the inverter feedback must connect across the isolation barrier; therefore, isolation is required here as well. In many of the high power electric motor applications, the architecture will require reinforced isolation from the high voltages of the three-phase motor, to protect the user from exposure to high voltages. These reinforced applications havethe largest isolation voltage requirements, which may require the isolator to have larger internal insulation thickness depending on the materials.

Insulation

The insulation capability of an isolator is its ability to withstand high voltages over its operating lifetime. Various isolation material types will have different capabilities under environmental conditions, voltage transients, and voltage waveforms. The optocoupler has been the traditional high voltage isolator due to its thick insulation, high withstand voltage capability, and the decades of field experience. Optocouplers use plastic molding as the insulation, and the process can include air voids in the insulation, which cause partial discharge, and leads to insulation failure. For this reason, agency certification requirements for high voltage testing of the insulation will include partial discharge testing. Unlike the optocoupler, the digital isolator uses internal layers of insulation produced in a well-defined and highly controlled semiconductor manufacturing process for its primary isolation barrier. This eliminates voids in the insulation and makes the insulation structure much simpler and more robust. Digital isolators eliminate the use of LEDs and their reliability issues, and they have been made more robust with process improvements to increase the insulation layer thickness and composition. Some digital isolators use silicon dioxide in thin layers to produce an insulation that has a high dielectric strength, which has been widely used as an insulator on semiconductor die. The disadvantage of silicon dioxide insulation is that it is integral to the IC and damage to the IC can damage the isolation. This limitation of silicon dioxide can be addressed by the use of polyimide insulation, which is a semiconductor process that has been used for decades to help provide strength and stability in integrated circuits. Polyimide internal insulation is a post process and has independent integrity. If the IC is damaged, the independent polyimide insulation will remain intact. When manufactured in multiple layers, polyimide can be used as a reinforced insulation, which may be required in motor drive applications. Engineers using digital isolators will need lifetime data from the manufacturer to show how the device will perform over time, temperature, humidity, and voltage to meet the challenge of replacing the optocoupler.

Figure 1. Isolated communications motor control block diagram.

Figure 2. Isolated control and communications motor control block diagram.

Environment

Environmental conditions for motor control applications can have large extremes of temperature and moisture. The example of the traction motors of a train can illustrate some of these extremes, in this case where the locomotive engine may be pulling a train of loaded cars up a mountainous track on a cold winter day. The ambient temperature could be below −40°C and the electric motor could be exposed to this outside air, but then the train could enter a long tunnel, and the temperature around the electric motor and engine could rapidly rise due to the heat from the engine. The motor and its insulators need to be able to operate within these temperature extremes and not have adverse effects over time and temperature. The optocoupler is known for its degraded performance change over temperature, with the internal LED producing less light and the detector obtaining less output signal over time and temperature. When used as a multichannel isolator, the optocoupler has an increasing mismatch of the channels over time. In contrast, the digital isolator does not rely on detecting a signal from an internal LED, and it uses semiconductor IC processes for dependable circuits to transmit and receive digital signals across the isolation barrier.

Digital Isolator

The technology of the digital isolator structure is illustrated in an example with the block diagram in Figure 3. The digital isolator responds to eitherinput logic levels or logic pulses, depending on the architecture. There are different methods available to encode and decode the signals to send and receive logic data across the isolation barrier. The pulse encoding technique, shown in Figure 4, has the advantage of consuming low supply current at low data rates when the time between encoding and decoding pulses is long. The carrier technique known as on-off keying (OOK), shownin Figure 5, consumes more current at low data rates than the pulseencoding method. At higher data rates(above 10 Mbps) the OOK method consumes less supply current than the pulse encoding technique. The advantage of the OOK technique over the pulse encoding technique, is that the OOK technique has a simpler logic, which yields a lower propagation delay and a higher maximum data rate. The pulse encoding technique has the disadvantage that if external noise causes the output data to be upset, this can last for a microsecond or more until the internal error correction or a new data edge occurs. For the motor control application this could mean the gate driver switches or feedback control signals could be out of control for a long enough time that the switch circuits or the motor drive could be damaged. Using the OOK technique, if a data upset is caused by a voltage transient, the upset may disturb the data output only for the short time that the noise occurs since the signal is constantly driven. In addition, due to the simpler architecture, the OOK digital isolator can be designed to be very robust to electrical noise generated in a motor control application.

Figure 3. Digital isolator block diagram.

Figure 4. Digital isolator: pulse encoding data architecture.

Figure 5. Digital isolator: on-off keying data architecture

Noise Immunity

The noise in a large electrical motor application can be generated from a common-mode voltage change across the isolation barrier when the motor control switching circuits create a step change in the bridge voltage. The ability of the isolator to withstand this high slew rate voltage transient without an upset at the isolator output is defined as thecommon-mode transient immunity (CMTI). The CMTI of an optocoupler may not be very high since it has very sensitive receiver elements prone to capacitive coupling effects. The capacitive coupling of an optocoupler is a single-ended structure, with only one path for the signal and the noise across the isolation barrier. This requires that the signal frequencies be well above the expected frequency of the noise so that the barrier capacitance presents a low impedance to the signal and a high impedance to the noise. At the low signal frequencies of motor control signals, which are typically less than 16 kHz, the high frequency components of the common-mode transient will be above signal frequency and may be of sufficient amplitude to upset the output of the optocoupler. Looking at the case of a transformer-based digital isolator in Figure 6, the transformerhas a differential input structure that gives the input signal and the noisea different transmission path, which inherently has a greater immunity to common-mode noise, without the limitation that the optocoupler has for the signal frequencies to be higher than the noise frequencies. Improved immunity to electrical noise allows reliable operation in high noise environments. Figure 7 illustrates the switching noise of high bridge voltages and fast dV/dt of a common-mode transient that the digital isolator needs to be immune to during motor control switching. The scope waveform shows that for the on-off keying architecture with transformer coupled digital isolator, it would take a very fast common-mode transient (CMT) of more than 150 kV/μs from GNDto GNDto cause a data upset, and the isolator output would only be upset for a very short duration of only 3 ns. The key to achieve very high CMTI is that the transmitter has to keep generating a differential carrier signal and the receiver has to have a high immunity from input common-mode variation.

Figure 6. Diagram of a transformer coupling digital isolator

Figure 7. Common-mode transient dV/dt in motor control applications.

Surge Capability

High voltage transients or surges can occur in motor control applications, and these surges can have peaks of over 10,000 V with a rise time of only 1.2μs. The requirement to withstand these surges has been met by optocouplers due to their thick layers of internal insulation. The digital isolators using silicon dioxide have limits on how thick the insulation can be made without the internal stress causing cracking. Insulation in digital isolators that use polyimide can improve their surge capability, and this has been shown to be very effective when polyimide insulation is made in multiple layers with a total thickness of 30μm. In Figure 8, the surge testresults of 30μm polyimide shows that it is very robust and can withstand ±20 kV peak.

Figure 8. Polyimide insulation surge test results.

Summary

A comparison of isolators in Table 1 shows how digital isolators have improved performance over optocouplers in the harsh environments of electric motor applications. The noise immunity (CMTI) of optocouplers is a minimum of only 10 kV/μs, but the digital isolator has many times more immunity to the voltage transients causing motor control upsets than the optocoupler. While optocouplers and their LED aging issues are generallylimited to 85°C operation, digital isolators will perform in high temperatures to 125ºC. This article has shown how these digital isolators operate and how their advanced capabilities will outperform optocouplers in motor control applications.

Table 1. Comparison of Isolators for Motor Control Application

Optocoupler 
Isolator
Capacitive 
Coupled Digital
Isolator
Transformer 
Coupled Digital
Isolator ADuM225N
Insulation Material Moldingcompound Silicondioxide Polyimide
Minimum Internal InsulationThickness(μm) 80 14 25.4
Data Architecture LED and PINdiode On-off keying On-off keying
Minimum Common-Mode Transient Immunity(kV/μs) 10 60 75
Reinforced Surge Isolation
Voltage VIOSM(V peak)
8000 6250 10,000
Operating Temperature(°C) –40 to +85
 
–40 to +125 –40 to +125

Об авторах

Brian Kennedy

Brian Kennedy

Brian Kennedy is an applications engineer with the Digital Isolator Group at Analog Devices, Inc. He has been with ADI since April 2008 and is responsible for Gate Driver and Power Supply Digital Isolation Products. He has a Bachelor of Science in Electrical Engineering (BSEE) from State University of New York (Buffalo.)