Two or three things about isolation devices and motor control

　　In harsh industrial scenarios, motor drive control and isolation are inseparable. A motor system often contains a variety of isolated devices, isolated ADCs, isolated gate drivers, isolated SPI's, etc. Although the introduction of isolation inevitably brings power, latency, cost and size limitations, industrial motors are becoming more and more reliable and require more and more protection features as isolation technology is revolutionized and motor drive control systems move toward higher switching frequencies and smaller dead times. Nowadays, motor systems require isolation devices that can withstand high-voltage transients, prevent data from being disturbed, and also eliminate the impact of high-voltage transients on isolator life.

　　Optocouplers fall behind?

　　Optocoupler-based isolation is the most traditional approach used for motor drive control and was once the most typical isolation solution for motor systems. Optocoupling uses physical means to isolate the high-voltage circuit system from the adjacent low-voltage system, thereby isolating unwanted signals. The internal insulation of the optocoupler is thick enough to withstand high voltages, a performance that has always been prominent in optocouplers. However, the disadvantage of optocouplers is equally obvious, that is, the need to use light-emitting diodes.

　　Its light intensity, inevitably, decreases over time and with temperature, resulting in timing drift over time and temperature. This not only affects performance, but also complicates the design of the device to an extreme degree. On the other hand, optocoupler-based isolation is often caught in a dilemma in maintaining good CMTI and expansion. If you want to overcome the data rate limitation, you have to work on the parasitic capacitance of the optocoupler, but then the power consumption will definitely rise and the common mode transient immunity of the optocoupler will be greatly reduced.

　　Even Broadcom, a manufacturer that is far ahead in optocoupler technology, will choose magnetic couplers for digital isolators. Broadcom's leading common mode transient immunity industry index for optocoupler-based isolators is 50 kV/µs, which is already a very high CMTI under optocoupler technology. with CMTI inferior to magnetic coupler and capacitive coupler, optocoupler isolation must find another way to stop the decline. Some optocouplers are manufactured with unique diffusion junctions that provide fast leading and landing times with low drive currents for improved common mode rejection on circuit loop isolation.

　　Although isolation based on this technology has gradually fallen out of favor due to the voltage withstanding advantage of thick insulation layer on high voltage industrial control environment, there are still stable applications in high voltage scenarios with high motor control signal frequencies (motor control signal frequencies below 16 kHz can cause significant interference to the optocoupler).

　　What the fully developed capacitive coupler lacks

　　As an alternative to optocouplers, capacitive couplers have many similarities with magnetic couplers, but are not as prominent as magnetic couplers, and are in a lukewarm position. As the technology with the smallest internal insulation thickness of the three, silica-based capacitive couplers are nearly 10µm thicker than polyimide based magnetic couplers.

　　In harsh motor application environments, for voltage transients that disrupt motor control, we should not only look at the typical CMTI of isolated devices, but also at their minimum CMTI. the minimum CMTI of optocouplers is around 10 kV/µs, that of capacitive couplers is around 60 kV/µs, and that of magnetic couplers is around 75 kV/µs. The capacitive coupler does not lag much behind the magnetic coupler in terms of immunity and transmission rate, but it is not as strong as the magnetic coupler in terms of surge protection.

　　High-voltage transients or surges can occur in motor control applications, and such surges can peak at over 10.000 V with a rise time of only 1.2 μs. Optocouplers are protected against high-voltage surges by an extremely thick internal insulation layer, and there are limits to the thickness of insulation that can be made for capacitive coupler isolation devices using silicon dioxide so that internal stresses do not cause cracks. It is not as robust as polyimide for magnetic coupling with limited thickness, so this means that the surge protection capability of capacitive coupling will be limited.

　　Improved Delay Timing for Magnetocoupler Applications

　　The CMTI performance of magnetocoupler-based isolation is clearly superior to that of optocouplers, and this class of isolation is based on standard CMOS technology, which also provides a significant improvement in power consumption and speed. Power consumption and CMTI are the first indicators that people focus on when choosing an isolation device, which is of course very important, but there is another performance that is often overlooked, and that is the transmission delay of the isolation device.

　　As a measure of the drive signal across the isolation barrier, the transmission delay can vary greatly depending on the isolation technology. Smaller transmission delays reduce the limitations of gate drive designs, giving more flexibility in timing margins in particular.

　　A comparison of gate drive delays based on optocouplers and magnetocouplers is shown below, with data taken from the maximum delays of first-class isolated gate drives.

　　Transfer Delay MAX Delay Deviation

　　Optocoupler 700ns200ns

　　Magnetocoupler 60ns12ns

　　It is easy to see that the magnetic coupler is able to transfer to the other end of the isolated gate much faster. And another extremely important setting of the motor, the dead time, is also affected by the transfer delay. the switching delay of the MOSFET/IGBT is the factor that affects the length of the motor dead time that is not related to the isolated transfer delay, the rest is affected by the transfer delay mismatch.

　　In increasingly high power motor applications where motor impedance becomes lower, increased motor current distortion torque ripple can be extremely detrimental to the motor if transmission delays and mismatches cannot be controlled to a minimum. Beyond high CMTI, isolated devices give motor applications more flexibility and reliability in design by improving transmission delays.

　　Summary

　　The development of industrial motor drive control is moving toward higher switching frequency, smaller dead time, and faster switching speed, while demanding higher reliability and requiring more and more protection features. While focusing on the immunity and power consumption of isolation devices, more attention should be paid to the performance of isolation devices in terms of delay reduction and dead time reduction, which is equally important for enhancing system reliability and safety.