In the development of motor system, technicians often need to focus on the consideration and design of isolation. One is to protect users from harmful voltage, 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, prevent data from being disturbed, and consider the impact of long-term high-voltage environment on the life of the isolator. Therefore, in the design of motor control system, isolation poses a huge challenge to technicians.
The motor control system may include a variety of isolation devices, such as: isolated grid driver in the drive circuit; Detect isolated ADC, amplifier and sensor in the circuit; And isolated SPI, RS-485 and standard digital isolator in communication circuit. These devices need to be carefully selected for both safety reasons and performance optimization.
For a long time, technicians have been used to using optocoupler devices to solve the isolation problem. However, with the development of technology, the electromagnetic coupling isolator based on micro-transformer provides a feasible and, in many cases, superior alternative; This paper will discuss these two isolation solutions and provide suggestions for technical personnel in the isolation design of the motor system.
The optocoupler uses light as the main transmission method, as shown in Figure 1. The sending side includes an LED, the high-level signal turns on the LED, and the low-level signal turns off the LED. The receiving side converts the received optical signal back to an electrical signal using a photodetector. The isolation is provided by the plastic packaging material between the LED and the photodetector, but can also be enhanced by an additional isolation layer (usually based on polymer).
One of the biggest disadvantages of optocoupler is that the aging of LED will make the transmission characteristics drift; The designer must consider this additional issue. LED aging causes the timing performance to drift with time and temperature. Therefore, signal transmission and rise/fall time will be affected, which will complicate the design. In addition, the performance expansion of optocoupler is also limited. In order to improve the data rate, we must overcome the inherent parasitic capacitance problem of the optocoupler, which will lead to higher power consumption. Parasitic capacitance also provides a coupling mechanism, resulting in the degradation of CMTI (Common Mode Transient Immunity) performance of optocoupler-based isolation devices.
The magnetic isolator based on the micro transformer is based on the standard CMOS technology and adopts the 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 pulse manner to generate a magnetic field. The magnetic field passes through the barrier and induces a current in the second coil on the other side of the barrier. Due to the standard CMOS structure, it has obvious advantages in power consumption and speed, and there is no life deviation related to the optocoupler. In addition, the CMTI performance of the isolators based on transformers is better than that of isolators based on optical couplers.
The transformer-based magnetic isolator also allows the use of conventional signal processing modules (to prevent the transmission of spurious inputs) and advanced transmission encoding and decoding mechanisms. In this way, bidirectional data transmission can be realized, different coding schemes can be used to optimize the relationship between power consumption and transmission rate, and important signals can be transmitted to the other end of the barrier faster and more consistently.
According to the performance and power level of the application, as well as the specific control and isolation scheme, the motor drive has a variety of system designs. Figure 3 shows the common isolation communication block diagram of inverter or low-end motor driver. In this system, the controller potential is the same as the power level, and the communication interface is isolated because it is usually a lower speed and simpler interface. In such systems, the power inverter may have low-end grid drivers, which do not need to be isolated because they share the same ground with the motor control module. High-end drivers can be isolated, but technologies such as level conversion can also be used, especially when the power inverter voltage is not too high. In this block diagram, the motor controller is directly connected to the inverter feedback without isolation. When the power level is high, using this architecture will have limitations. The additional noise generated by the switch signal on the motor may submerge the feedback signal used to monitor the motor current, which may cause the motor to lose control.
Figure 3 Control block diagram of isolated communication motor
For high performance drives, such as large polyphase drives used in industrial motors and train traction motors, isolation control and communication will be required, as shown in Figure 4. In this system block diagram, for the reasons of anti-noise and improving communication speed, control and communication are located at the safety side of the isolation barrier. Because the motor control module is located on the safe side of the barrier, all grid drivers need to be isolated. The specific isolation voltage and safety requirements are determined by the specific structure and the position of the isolation barrier. In the block diagram, inverter feedback is used to help control motor drive, which is one of the most important aspects of motor control. As shown in the figure, the inverter feedback is connected to the current measurement nodes iV and iW in the two phases of the three-phase AC motor. In the isolation 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 three-phase motors to prevent users from being exposed to high voltage.
Figure 4 Block diagram of isolation control and communication motor control
In addition, in large motor applications, when the motor control switch circuit changes step by step in the bridge voltage, the common-mode voltage change on the isolation barrier may cause noise. Common mode transient immunity (CMTI) is the ability of the isolator to withstand this high voltage swing voltage transient and the output of the isolator is not disturbed. The CMTI of the optical coupler may not be very high, because its receiving element is very sensitive and vulnerable to the capacitive coupling effect. The capacitive coupling of optical coupler is a single-ended structure, and there is only one path for signal and noise to cross the barrier. This requires that the signal frequency must be much higher than the expected noise frequency, so that the barrier capacitance provides low impedance for the signal and high impedance for the noise. When the frequency of the motor control signal is low (usually lower than 16 kHz), the high-frequency component of the common-mode transient will be higher than the signal frequency, and its amplitude may be enough to disturb the output of the optocoupler.
Figure 5 Transformer coupling digital isolator
Figure 5 shows a digital isolator based on a miniature transformer. The transformer has a differential input structure, which provides different transmission paths for input signals and noise, so it must have greater common-mode noise immunity, and there is no restriction that the signal frequency is higher than the noise frequency required by the optical coupler. The improved electrical noise immunity enables the device to work reliably in high noise environment.
Figure 6 shows the high bridge voltage of common-mode transient and switching noise of fast dV/dt during motor control switching. The digital isolator must be able to resist this interference. The oscilloscope waveform shows that the fast common-mode transient (CMT) from GND2 to GND1 must be higher than 150 kV for the transformer-coupled digital isolator with switch keying structure to disturb the data/ μ s. Moreover, the time for the isolator output to be disturbed is very short, only 3 ns. The key to realize ultra-high CMTI is that the transmitter must continuously generate differential carrier signals, and the receiver must have high input common-mode variation immunity.
As people pay more attention to system performance, efficiency and safety, motor control architects face increasingly complex challenges when designing robust systems. The gate driver based on optical coupler is the traditional choice, but the transformer-based solution not only has more advantages in power consumption, speed and time stability, but also has obvious advantages in system performance and security. This allows the designers to reduce the dead time and improve the system performance while preventing the switch of the upper and lower bridges from being switched on at the same time. In addition, it also supports faster 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 transformer should be the ideal choice for technicians.