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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?

The 3rd phase current can be derived mathematically from the other two to save on power dissipation and component costs. The AD7401A contains a differential track and hold stage, a Σ- modulator, and digital isolation all in a single package. The high voltage side analogue signal is converted to a digital serial data stream which is then transmitted across the isolation barrier to the low voltage side. The AD7401A also contains a clock input pin which allows for the measurements on each device to be made simultaneously with a single clock source. As seen in table 2, optocouplers with a similar level of integration and cost exist, however, the digital isolator technology still provides better power consumption, speed performance, and reliability based on the fundamental construction of the devices in addition to the superior modulator performance of the AD7401A.

Table 2: Phase current digital isolator vs. optocoupler.

Digital communication: I2C is a two-wire, multi-drop communication interface frequently used to provide digital or analogue IO expansion capability to a controller. This approach is typically reserved for 'housekeeping' type of functions that are monitored or updated periodically. The FlexMC high voltage board uses an I2C interface to communicate with a PFC controller as well as an ADC monitoring the bus voltage, bus current, and IGBT temperature. The ADuM1250 allows for all this functionality on the high voltage side to be monitored by the controller utilising just a two wire peripheral interface through a single isolator. In contrast, there is no single optocoupler device that can provide I2C isolation. As a result, as can be seen in table 3, the ADum1250 is a clear choice for I2C isolation in cost, size, component count, and performance over optocoupler options.

Table 3: I2C digital isolator vs. optocoupler.

Isolated power: Another area where the use of digital isolator technology has benefited this design is with the generation of very low level isolated power supplies. Two ADuM5000 devices were used to generate isolated 5V supplies with up to 500mW of power output capability. These were used to power the analogue side of the Σ- converters which float with the rapidly changing motor voltage. These isolated supplies use the same technology as the data isolators so they have an internal transformer switching at 180MHz. This frequency is 3 orders of magnitude higher than standard DC-DC converters allowing for ta much smaller size. The ADUM5000 devices are a simple way to provide low power isolated voltages all in an SOIC-16 package.

About the author
Gaetano (Guy) Fichera is director of electronics at Boston Engineering Corporation. Boston Engineering is a contract engineering services firm that offers turn-key project development services to meet clients' immediate and long-term needs for software, electrical, mechanical, and controls engineering.

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