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Digital isolators for AC voltage motor drives

Posted: 02 Jul 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Isolation? AC voltage motor drives? optocouplers? digital isolators? controller?

Isolation is an essential part of AC voltage motor drives. There are several methods of providing electrical isolationprimarily optocouplers and digital isolators are used. The use of digital isolators provides several benefits as compared with traditional optocouplerssome of which include reduction in cost, component count, and improved reliability. This article will compare methods of isolation in a traditional motor controller design to highlight the benefits of digital isolators.

Opto-coupler vs. digital isolator background
Optocouplers use light from LEDs to transmit data across an isolation barrier to a photodiode. As the LED is driven on and off, logic HIGH and LOW signals are generated on the electrically isolated photodiode side. 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. One way to improve speed is to increase the LED current, but this comes at the cost of increased power consumption.

In contrast, transformer based digital isolators use transformers to magnetically couple data across an isolation barrier. Transformers pulse current through a coil to create a small, localized magnetic field that induces current in another coil. The transmission speed in transformers is inherently much faster than optocouplers. Transformers are also differential and provide excellent common-mode transient immunity. Also, since digital isolators are transformer based and optocouplers are LED based, digital isolators provide significantly better reliability/MTTF over optocouplers.

Figure: Motor-control block diagram.

Isolation in a motor-drive design
The figure provides a block diagram of the high voltage FlexMC motor control drive developed by Boston Engineering Corporation. It 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. At the centre is an isolation barrier between the high voltage power electronics and the controller. The motor power electronics are floating at high voltage potentials while the controller is referenced to earth ground, thus the need for isolation.

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. These signals, as shown in the block diagram, pass through the isolation barrier. In addition, there are several other functions that benefited from the use of isolators including digital communication and low voltage, low power, isolated dc-dc conversion.

PWM isolation: Pulse-width modulation of the power stage is at the heart of all motor drives. Switching frequencies are typically in the 10-20kHz range. Precise control of pulse-widths, dead-time, and channel-to-channel delay are critical when optimising control performance. 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 shown in table 1).

Table 1: PWM digital isolator vs. optocoupler.

For example, controllers introduce dead-time between switching signals to prevent any high and low side transistor pairs to conduct simultaneously (i.e. shoot-through). This dead-time is a function of the delays in the turn on/off of the power switches and the uncertainty in the delay introduced by the isolation circuits. The ADuM1310 iCoupler digital isolator provides a channel-to-channel matching of only 2ns, as compared to 500ns for optocouplers. Utilising digital isolators allows for the dead-time to be greatly reduced, thus improving power inverter performance. Furthermore, as can be seen in the comparison table, in addition to performance, the ADuM1310 provides a much more integrated solution reducing component count and BOM cost.

Phase current and I2C comparisons
Motor phase currents: Most advanced motor drives utilise motor phase current as a primary feedback. To provide continuous feedback, very low resistance shunt resistors are placed in series with the motor phases. However, this adds the complexity of trying to measure millivolt level signals and common-mode voltage swings of hundreds of volts switching at high frequency with rapid dv/dt. For this design, two isolated Σ- modulators, AD7401A, were used to measure the motor winding current and the digital bit stream is then processed by digital filtering circuitry on the motor control IC.

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