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Motor control

Categories:Industry application
Time of issue:2020-05-28 00:00:00
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In the development of motor systems, technicians often need to focus on the consideration and design of isolation. One is to protect users from harmful voltages, and the other is to protect sensitive devices and equipment in the system. In addition, in a bad environment, the system is also required to resist high-voltage transients to prevent data from being disturbed, and at the same time consider the impact of a long-term high-voltage environment on the life of the isolator. Therefore, in the design of motor control systems, the challenge of isolation to technicians is huge.

The motor control system may contain a variety of isolation devices, such as: isolated gate drivers in the drive circuit; isolated ADCs, amplifiers and sensors in the detection circuit; and isolated SPI, RS-485, Standard digital isolator. Whether for safety reasons or to optimize performance, these devices require careful selection.

　　 For a long time, technicians have been accustomed to using optocoupler devices to solve isolation problems. However, with the development of technology, electromagnetic coupling isolators based on micro-transformers provide a viable and often superior alternative; this article will discuss these two isolation solutions for technicians in the design of isolation of motor systems Provide suggestions.

　　Optocouplers use light as the main transmission method, as shown in Figure 1. The sending side includes an LED, a high-level signal turns on the LED, and a low-level signal turns off the LED. The receiving side uses a photodetector to convert the received optical signal back to an electrical signal. Isolation is provided by the plastic encapsulation material between the LED and the photodetector, but it can also be enhanced with additional isolation layers (typically based on polymers).

figure 1

One of the biggest disadvantages of optocouplers is: LED aging will cause the transmission characteristics to drift; designers must consider this additional issue. LED aging causes timing performance to drift with time and temperature. Therefore, signal transmission and rise/fall times are affected, which complicates the design. In addition, the performance expansion of optocouplers is also limited. In order to increase the data rate, it is necessary to overcome the inherent parasitic capacitance problem of the optocoupler, which will lead to increased power consumption. Parasitic capacitance also provides a coupling mechanism, resulting in degradation of the CMTI (common-mode transient immunity) of optocoupler-based isolation devices.

　　Micro-transformer-based magnetic isolator is based on standard CMOS technology, using electromagnetic coupling transmission principle, the isolation layer is composed of polyimide or silicon dioxide, as shown in Figure 2. The low-level current is transmitted through the coil in a pulsed manner, generating a magnetic field that passes through the isolation barrier and induces a current in the second coil on the other side of the isolation barrier. Due to the use of standard CMOS structure, it has obvious advantages in power consumption and speed, and there is no problem of life deviation related to the optical coupler. In addition, the CMTI performance of transformer-based isolators is superior to optocoupler-based isolators.

figure 2

Transformer-based magnetic isolators also allow the use of conventional signal processing modules (to prevent transmission spurious inputs) and advanced transmission codec mechanisms. In this way, two-way data transmission can be achieved, using different encoding schemes to optimize the relationship between power consumption and transmission rate, and to transmit important signals to the other end of the isolation barrier more quickly and consistently.

　　According to the performance and power level of the application, as well as the specific control and isolation schemes, motor drive has a variety of system designs. Figure 3 shows a block diagram of isolated communication commonly used in inverters or low-end motor drivers. In this system, the controller potential is the same as the power level, and the communication interface is isolated because this is usually a lower speed and simpler interface. In such systems, the power inverter may have low-side gate drivers that do not need to be isolated because they share the same ground as the motor control module. High-end drivers can be isolated, but techniques such as level shifting can also be used, especially when the power inverter voltage is not too high. In this block diagram, the motor controller does not use isolation and is directly connected to the inverter feedback. When the power level is high, the use of this architecture has limitations. The additional noise generated by the switching signal on the motor may overwhelm the feedback signal used to monitor the motor current, which may cause the motor to lose control.

Figure 3 Block diagram of isolated communication motor control

For higher performance drives, such as large multiphase drives used in industrial motors and train traction motors, isolated control and communication will be required, as shown in Figure 4. In this system block diagram, control and communication are located on the safety side of the isolation barrier for reasons of noise immunity and increased communication speed. Because the motor control module is located on the safe side of the isolation barrier, all gate drivers need to be isolated. The specific isolation voltage and safety requirements are determined by the specific architecture and the location of the isolation barrier. In the block diagram, inverter feedback is used to help control the motor drive and is one of the most important aspects of motor control. As shown in the figure, the inverter is feedback connected to the current measurement nodes iV and iW in the two phases of the three-phase AC motor. In the isolated control and communication system diagram, the inverter feedback must be connected across the isolation barrier, so isolation is also required here. In many high-power motor applications, the architecture will require enhanced isolation of the high voltage of the three-phase motor to prevent users from being exposed to the high voltage.

Figure 4 Block diagram of isolated control and communication motor control

In addition, in large motor applications, when the motor control switching circuit produces a step change in the bridge voltage, the common mode voltage change on the isolation barrier may generate noise. The ability of the isolator to withstand this high-voltage slew-rate voltage transient and the output of the isolator is not disturbed is the common-mode transient immunity (CMTI). The CMTI of the optocoupler may not be very high because its receiving element is very sensitive and susceptible to capacitive coupling effects. The capacitive coupling of the optocoupler is a single-ended structure, and there is only one path for signal and noise across the isolation barrier. This requires that the signal frequency must be much higher than the expected noise frequency, so that the isolation barrier capacitance provides low impedance to the signal and high impedance to noise. When the frequency of the motor control signal is low (usually below 16 kHz), the high frequency component of the common mode transient will be higher than the signal frequency, and its amplitude may be sufficient to disturb the output of the optocoupler.

Figure 5 Transformer-coupled digital isolator

The digital isolator based on a micro-transformer shown in Figure 5, the transformer has a differential input structure, which provides different transmission paths for the input signal and noise, so it must have greater common-mode noise immunity, and there is no light The coupler requires the signal frequency to be higher than the noise frequency limit. The improved electrical noise immunity allows the device to work reliably in high noise environments.

Figure 6 shows the high-bridge voltage and common dV/dt switching noise during common-mode transients during motor control switching. The digital isolator must be able to resist this interference. The oscilloscope waveform shows that for a transformer-coupled digital isolator with a key control architecture, to disturb the data, the fast common mode transient (CMT) from GND2 to GND1 must be higher than 150 kV/μs, and the time the isolator output is disturbed is very high Short, only 3 ns. The key to achieving ultra-high CMTI is that the transmitter must continuously generate differential carrier signals, and the receiver must have a high input common-mode change immunity.

　　As people pay more attention to system performance, efficiency and safety, motor control architects are facing increasingly complex challenges when designing robust systems. The optocoupler-based gate driver is the traditional choice, but the transformer-based solution not only has advantages in power consumption, speed, and time stability, but also has obvious advantages in system performance and safety. This allows designers to reduce the dead time and improve system performance while preventing the upper and lower bridge switches from turning on at the same time. In addition, it also supports quicker response to system commands and errors, which can also enhance system reliability and improve security. In view of these advantages, the electromagnetic isolation scheme based on micro-transformers should become an ideal choice for technicians.

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