![]() Optocoupler Comparisonįor example, controllers introduce dead time between switching signals to prevent any high and low side transistor pairs from conducting simultaneously (i.e. When selecting the appropriate isolation device for PWM control signals, digital isolators provide significant advantages in performance and cost over comparable optocoupler options (see comparison table shown in Table 1). ![]() Precise control of pulse widths, dead time, and channel-to-channel delay are critical when optimizing control performance. Switching frequencies are typically in the 10 kHz – 20 kHz range. Pulse-width modulation of the power stage is at the heart of all motor drives. In addition, there are several other functions that benefited from the use of isolators, including digital communication and low voltage, low power, isolated dc-to-dc conversion. These signals, as shown in the block diagram, pass through the isolation barrier. Two critical hardware elements to a closed loop motor control design are the pulse-width modulated (PWM) controller outputs and the motor phase current feedback. & amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp lt img src=' ' alt='Figure 1'& amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp gt This paper will explore how the selection of digital isolators over optocouplers improves this design. The motor power electronics are floating at high voltage potentials while the ADSP-CM40x processor is referenced to earth ground, thus the need for isolation. At the center is an isolation barrier between the high voltage power electronics and the controller. The drive receives a universal ac input, provides a power factor corrected (PFC) front end, and drives a permanent magnet synchronous motor (PMSM) while providing the necessary feedback conditioning for a sensored or sensorless control running on the ADSP-CM40x, an ARM ® Cortex ™-M4 mixed-signal control Processor with a 16-bit high precision analog front end. Isolation in a Motor Drive Designįigure 1 provides a block diagram of the high voltage FlexMC motor control drive developed by Boston Engineering Corporation ( /), which interfaces to an ADSP-CM40x mixed-signal control processor. Also, since digital isolators are transformer based and optocouplers are LED based, digital isolators provide significantly better reliability/MTTF over optocouplers. Transformers are also differential and provide excellent common-mode transient immunity. The transmission speed in transformers is inherently much faster than optocouplers. Transformers pulse current through a coil to create a small, localized magnetic field that induces current in another coil. ![]() In contrast, transformer based digital isolators use transformers to magnetically couple data across an isolation barrier. One way to improve speed is to increase the LED current, but this comes at the cost of increased power consumption. The speed of an optocoupler is directly related to the speed of the detector photodiode and the time that it takes to charge its diode capacitance. ![]() As the LED is driven on and off, logic high and low signals are generated on the electrically isolated photodiode side. Optocouplers use light from LEDs to transmit data across an isolation barrier to a photodiode. This article will compare methods of isolation in a traditional motor controller design to highlight the benefits of digital isolators. The use of digital isolators provides several benefits as compared to traditional optocouplers-some of which include reduction in cost, component count, and improved reliability. There are several methods of providing electrical isolation-primarily, optocouplers and digital isolators are used. Isolation is an integral part of ac voltage motor drives. Digital Isolation for AC Voltage Motor Drives
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