Integrated signal and power isolation provides robust and compact measurement and control
I. Introduction
Robust and accurate measurements and controls are important 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 analog outputs are required to operate actuators such as hydraulic actuators, solenoids or motor starters safely and accurately.
Besides isolation for analog 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 analog system performance and reducing system size by replacing traditional analog isolators or opto-couplers which are bulky and inaccurate. High resolution ADC placed right at the sense node converts the precise analog 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 analog current or voltage controls using high resolution DACs.
The digital isolators eliminate the signal distortion and degradation from analog 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 analog 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).
II. isoPower: Signal and 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.
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 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 realized 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
Two cross-coupled HVCMOS switches together with a center-tap transformer provide the sustained oscillation, which is enabled or stopped by a tail switch controller by the feedback PWM signal. The Schottky diodes for efficient rectification at 120MHz and a modified type-II controller are implemented in die 2. For smooth power up, a soft-start circuit is implemented in die 1.
At power up, the soft start circuit provides PWM signals with slowly increasing duty to slowly charge the output voltage to a pre-set level slightly lower than the targeted output voltage, at which the feedback signal is enabled and it takes over the control of the tail switch from soft start output. This soft start sequence ensures minimum output overshoot at the power up, which is important for system reliability.
To transmit the data robustly across isolation barrier, differential encoding and decoding is used. The leading edge is encoded as a short pulse of positive polarity and the falling edge is encoded as a short pulse of the same duration but of negative polarity. The differential receiver would convert these pulses of different polarities into the output with the correct logic state.
To ensure the output is updated when the input has not changed state for a long time, periodic refresh pulses are also transmitted across the transformers, a positive pulse for logic high and a negative pulse for logic low. Differential receiver allows the common mode pickup during common mode transient events be rejected and large common mode transient immunity greater than 50kV/μS is achieved.
3.75kV, 1 minute, UL rated isolation is achieved through 32μm thick polyimide layers sandwiched in between the primary coils and the secondary coils. A cross-section for the isoPower transformers is shown in Figure 3. Both the top winding and the bottom winding have 6μm thick Au for efficient power transfer. The polyimide is very good against HV transients, providing over 10kV surge capability. Besides high surge capability, the polyimide has also well-defined aging behavior.
Figure 3: Cross-section of isoPower Transformers
The time to failure is collected at various voltages from 1.2kV to 3kV, and it is plotted in Figure 4.
Figure 4: High Voltage Lifetime for isoPower Products
Based on fit to a typical polyimide aging model, 
, the life time is over 100 years at 400Vrms, good enough for many industrial applications.
Besides high voltage behavior, EMC requirements such as radiated emissions are of concern for industrial applications. The isoPower transformers are closely coupled with only a few hundred μm in diameter, so radiated emissions from the transformers are very limited. The isoPower transformers are also implemented is s-type configuration as shown in Figure 5.
Figure 5: isoPower Transformer Configuration
In far field, the flux from one half of the transformer would cancel out the flux from the other half by design each half with opposite phase. While the radiation emissions from the transformers are negligible, we need to take precaution in designing PCB for using isoPower products. Besides good bypassing to contain noises within the part, isolated islands of PCB need to be carefully designed not to form efficient dipole antenna. CISPR Class A or B can be met for the isoPower parts with properly designed PCB.
III. Signal and Power Isolation Needs for Measurement and Control
To make reliable and safe measurements of current, voltage, temperature, pressure, flow rate or other parameters for a variety of industrial applications in the harsh environments, isolation is essential to maintain data integrity and equipment or user safety by eliminating the ground loops and the direct exposure of the user or equipment to hazardous voltages.
Besides signal isolation, power isolation is also needed to establish bias for the ADCs or DACs to interface with various sensors or transducers. There is significant challenge to pack isolation components such as those based on opto-couplers in a small area considering the fact that there are many analog inputs or outputs that not only require isolation from the back plane but also between those analog channels. Integrated signal and power isolation is the idea solution to deliver compact and robust isolation to each of the analog I/O port.
An example of implementation of isolated signal and power for use in industrial instrument or process control equipment is shown in Figure 6.
Figure 6: Fully Isolated Industrial Measurement and Control System Using isoPower
A single isoPower component such as ADuM5211, a two channel digital isolator with 150mW isolated power, would satisfy all the isolation needs for a given analog input channel. One isolated data channel is used to send the system clock from the micro-controller to the ADC, and another data channel in the opposite direction would send the acquired data from the sensor back to the micro-controller. Multiple ADuM5211 can be used for multiple sensors that require isolation between them.
Similarly, a single isoPower component such as ADuM5210, a two channel digital isolator with 150mW isolated power, can handle all the isolation needs for a given analog output channel. The two data channels are used to send both the clock and the digital commands from the micro-controller to the DAC to control the actuator. Multiple ADuM5210 can be used for multiple actuators or analog outputs that require isolation between them. The red horizontal dashed lines illustrate the isolation barriers between various sensors and actuators and the long red vertical dashed line to the right indicates the isolation between the system and analog I/O ports.
For communication between many pieces of process control equipment, additional isolation as shown by the short red vertical dashed line is needed between the system controller and communication bus such as RS485, RS232, CAN and others. Again, a single isoPower component such as ADuM5287, full integrated isolated RS-485 transceiver with 500mW isolated power, can be used for the complete bus interface isolation for a single piece of equipment. Here ADuM2587 provides integrated RS485 transceiver and three channels of digital isolation, of which two are used to send Drive and Drive Enable to the micro-controller to the bus and the other one is used to send received data Receive from the bus to the controller. As we can see, integrated signal and power isolation significantly simplifies the measurement, control and communication interface for industrial equipment.
There are similar signal and power isolation needs for battery measurement or test equipment. Voltages and Currents for each battery cell need to be monitored continuously to maintain battery life span but the whole battery stack in series can be hundreds or even thousands of volts in total. Battery monitor ICs to interface with various battery packs in different locations of the stack need to be isolated from system controllers and isolated power is needed to allow proper system shut down in the event
of battery failure.
IV. Conclusion
Integrated signal and power isolation using micro-transformers in a small package dramatically simplify the design for industrial measurement and process control system and it significantly reduces board space and cost compared to those based on opto-couplers and discrete transformers. It provides reliable and accurate measurement and control in harsh industrial environments.
About the author
Baoxing Chen is an ADI fellow. He joined Analog Devices in 1997 after he graduated from University of Michigan with a Ph. D in Physics and M.S. in Electrical Engineering. Baoxing also has a B.S. in Physics from Nanjing University in China. Baoxing pioneered the use of micro-transformers for transmitting signal and power through the isolation barrier. At ADI, he has been leading iCoupler® and isoPower® technology developments including circuit architectures and process developments. Baoxing has published over 25 papers in areas ranging from material, device, circuits and applications. He holds 20 US patents.
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