Volume 40 December 2006
Digital Isolation Offers Compact, Low-Cost Solutions to Challenging Design Problems
By David Krakauer [email@example.com]
This article discusses two kinds of devices that embody these advances. In the first example, isolated power, chip-scale microtransformers are complemented by switches, rectifiers, and regulators to produce an isolated, regulated dc-to-dc converter; when integrated with isolated data channels it provides a complete isolation solution. In the second example, bidirectional isolation, integrating the requisite buffers and drivers creates an isolator that has truly bidirectional isolation channels without the need for external signal conditioning.
Figure 1 shows a 4-channel digital isolator, which houses three dice in a single package. Two CMOS interface circuits (left and right) integrate drive and receive electronics. The middle die contains four chip-scale microtransformers, each comprising metal (AlCu and Au) coils on either side of a 20‑μm polyimide insulation layer. The polyimide is capable of withstanding more than 5 kV rms for one minute.
Figure 1. Construction of iCoupler digital isolator.
Unfortunately, in most applications that require isolated data transmission, isolated power must be available on both sides of the isolation barrier, or it must be provided separately. System designers typically introduce isolated power by designing an isolated power supply using discrete components—including a transformer with the appropriate isolation rating—or by purchasing a commercial off-the-shelf isolated dc-to-dc converter.
Each approach has its advantages and disadvantages. In the first instance, isolated power supplies may be custom tailored to an application, allowing system designers to optimize their cost, isolation rating, power output, or other important specifications depending on the application requirements. The downside, however, is that custom solutions tend to be bulky, require safety certification, and can lengthen development times.
Commercially available isolated power supplies, on the other hand, can reduce time to market, but they carry a price penalty and may not be optimized to fit a particular application. While smaller in size than their custom counterparts, they are still fairly bulky, with only limited availability of surface-mount package options.
A third way is isoPower, which combines the benefits of both options. iCoupler digital isolators condition and drive data across the transformers as described in the article, “.” isoPower uses the same chip-scale microtransformer technology, but instead of transmitting only data, isoPower employs switches, rectifiers, and regulators to generate power that is isolated to the same degree as the data channels.
Figure 2 shows the isolated power section of the Coupler products with isoPower. Four cross-coupled CMOS switches generate an ac waveform that drives the transformer. On the isolated side, Schottky diodes rectify the ac signal. The rectified signal is passed to a linear regulator, which maintains the output voltage at a nominal 5-V setpoint. Efficiency can be significantly improved by giving up one of the isolation channels to provide feedback across the isolation barrier to the transformer switches., , and , the first i
Figure 2. isoPower digital isolator implements isolated power.
Figure 3 depicts the transformers used in the ADuM524x family. The chip-scale microtransformers are made from 6-µm thick gold, separated by a 20-µm polyimide insulation layer, which is capable of providing greater than 5-kV rms isolation. Because the transformer coils, only 600 μm in diameter, have a low L/R ratio compared with conventional transformers, high-efficiency power generation requires high-frequency switching—on the order of 300 MHz.
Figure 3. Chip-scale microtransformers.
As noted earlier, the transformers used to generate power employ the same process as those used to isolate data. The only significant difference between data and power channels is the conditioning circuitry on either side of the isolation barrier.
Figure 4. Isolated SPI interface using iCoupler technology (a) and optocouplers (b).
The small size and low cost of an isoPower solution opens up new possibilities for the placement and distribution of isolated sensors and reduces the cost of existing solutions, thereby enabling wider adoption of isolated sensors.
A case in point is turbidity sensors: they measure the amount of particulates in a liquid solution and can be used to determine the cleanliness of a volume of water. They are increasingly being used in home appliances, such as dishwashers and washing machines, both to conserve water and to improve cleaning performance. Conventional appliances wash or rinse for a set time, overestimating the required level of cleaning to ensure that the load is fully clean at the end of the cycle. A turbidity sensor, however, can let the system know when to stop cleaning. The machine will use the optimal amount of water for the optimal time, thus minimizing waste while maximizing useful cleaning performance.
Because turbidity sensors must be immersed in the water, they present two challenges to an appliance designer. First, the sensor must be small enough to fit unobtrusively anywhere within the space where clothes or dishes are to be placed. The size of the sensor is, therefore, critical. Second, the powered circuit is immersed in water, so the sensor must be safely isolated from the rest of the system. If the physical insulation should fail, the user and the system electronics must not be harmed, and there must be no possibility of fire. Both the power and the data must therefore be isolated.
The block diagram shown in Figure 5 demonstrates a cost-effective solution. Thelow-power ADC uses a 3-wire interface to convert the analog output of a turbidity sensor. The digitized turbidity data is transmitted across the galvanic isolation barrier of the and ADuM5242. The 50 mW of isolated power from the ADuM5242 is sufficient to supply the ADuM1200, the AD7823, and the turbidity sensor. The combined area of the isolators and converter is less than 100 mm2, excluding external components.
Figure 5. Isolated turbidity sensor.
Figure 6. Bidirectional isolation vs. unidirectional isolation.
The inter-integrated-circuit (I2C) bus is a popular 2-wire, bidirectional communication protocol that was developed to provide simple, low-cost, short-distance communication between an on-board controller and its peripherals. I2C buses limit the cost of applications in which multiple devices share a single bus with a host controller, as shown in Figure 7. Two bidirectional wires—one for the data and one for the clock—are used to achieve low cost at the expense of data rate, so I2C is typically used in systems with many peripherals running at data rates less than 1 Mbps. Systems that use a limited number of peripherals running at higher data rates will often employ protocols such as SPI.
Figure 7. The I2C bus provides communications between host and peripherals.
isolation challenge has been that optocouplers are based
on diodes that can transmit in only one direction, and
are therefore inherently unidirectional. A bidirectional
I2C bus could be isolated using optocouplers,
but the implementation isn’t pretty (Figure 8a). A special
buffer is used to separate each bidirectional channel
into two distinct channels: transmit
Once separated, the four unidirectional channels can be
individually isolated and then recombined. This solution
requires four isolators and expands the bus from two wires
to four wires. Additional circuitry is also required,
making this solution costly and large, and defeating the
original purpose of the
Figure 8. iCoupler simplifies bidirectional isolation.
The good news is that by adopting the new digital isolation techniques the circuitry that is used to separate, isolate, and recombine the data channels can be integrated into a single package. This approach can be implemented with the new Coupler solution is.and hot-swappable dual I2C isolators. Figure 8b illustrates how much more compact the i
Figure 9 shows how bidirectional isolation is achieved within the package. Just as the discrete solution employs a buffer to separate the two bidirectional channels into four unidirectional channels and four isolators, so, too, does the ADuM125x. The difference is that all the electronics are integrated onto a single IC. A designer sees only the 2-wire interface, and the entire device is less than 40 mm2, a 90% reduction compared with the optocoupler/buffer solution, which takes up about 350 mm2.
Figure 9. Bidirectional isolation using the ADuM1250.
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