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Benefits of integrated signal, power isolation

Posted: 13 Aug 2013 ?? ?Print Version ?Bookmark and Share

Keywords:industrial sensors? RTD? thermocouples? isolation? opto-couplers?

Robust and accurate measurements and controls are significant for industrial instrumentation and process control. Various industrial sensors such as RTD or thermocouples usually require input isolation, not only to prevent ground loops that can compromise measurement accuracy, but also to prevent voltage transients that can cause permanent damage to the instrument. Besides small differential signal of interest, these sensors can carry significant common mode potential that will reduce data acquisition accuracy or large voltage transients that can pose as a safety hazard. Similarly isolated analogue outputs are required to operate actuators such as hydraulic actuators, solenoids or motor starters safely and accurately.

Besides isolation for analogue inputs and outputs from system ground, isolation is also needed between inputs and between outputs to handle different common mode potentials for various sensors and actuators and prevent interference among them. Digital isolators have been successful in improving analogue system performance and reducing system size by replacing traditional analogue isolators or opto-couplers which are bulky and inaccurate. High resolution ADC placed right at the sense node converts the precise analogue sensing signals into digital streams before they are transmitted across isolator barrier to the system microcontrollers using highly integrated multi-channel digital isolators. Similarly, the digital commands from the system micro-processor are transmitted through isolation barrier using digital isolators first and then are converted to accurate analogue current or voltage controls using high resolution DACs.

Figure 1: (a) Size Comparison for Various Isolated Power Implementations (b) Inside Look of ADuM5010, a 150mW Fully Integrated DC/DC Converter (c) Inside look of ADuM521x, a Dual Channel Isolated with integrated DC/DC converter.

The digital isolators eliminate the signal distortion and degradation from analogue isolators or traditional opto-couplers, however, the DACs to interface with controls or the ADCs to interface with these sensors would require some isolated power to operate. While discrete isolated DC/DC converters can be used to provide the isolated power from system side, they are bulky and difficult to design as they require many discrete components. The outputs of these discrete DC/DC converters are not well regulated either and they become very inefficient at low loads, typical for low power high resolution ADCs or DACs. To enable robust and compact analogue measurement and control systems, small, integrated isolated power together with digital isolation is needed. isoPower, an extension to iCoupler technology, industrial leading digital isolators, provides isolated signal and power in a shrink small outline package (SSOP).

Signal, power isolation using micro-transformers
Let us take a close look at some isoPower devices, and see how they compare to traditional DC/DC implementations. Figure 1a shows size comparison for an isoPower device ADuM5010, an integrated DC/DC converter providing up to 150mW isolated power, in 20L SSOP, a DC/DC module, and an Optocoupler-based discrete design. ADuM5010 occupies merely 90mm2 board space including 4 surface mount ceramic capacitors while the discrete design consisting of a total of 16 components requires a board space of 470mm2, more than 5 times that of the isoPower device. The sizes for DC/DC modules vary, but its thickness is usually prohibitive for surface mounting. An isolated DC/DC in a SSOP is made possible with two chip integration as shown in figure 1b.

The left die has transformer switching circuit while the right die has rectification diodes and the feedback controller. Three transformers are integrated on the left die with the top smaller transformer providing feedback signal isolation while the bottom two transformers are used for isolated power transfer. Two more dies can be fit into SSOP to provide additional digital isolation channels as shown in figure 1c. This isoPower device ADuM521x is a dual channel isolator with 150mW integrated DC/DC converter. The two micro-transformers on the top right die provide the isolation for the two digital channels. Both the top left die and top right die have encoding and decoding circuits for transmitting digital signals across micro-transformers.

For transmitting power across the isolation barrier, these micro-transformers are switched resonantly at high frequency, around 120MHz, to achieve efficient energy transfer while the energy regulation is realised through a low frequency PWM feedback signal, around 500kHz, which controls the duty cycle that the high frequency resonant action is left on, as shown in figure 2.

Figure 2: Schematic for Isolated Power Transfer Using Micro-transformers.

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