Power Electronics

Single Component Isolates Data And Power

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Analog Devices has extended its line of iCoupler digital isolators by developing three devices that provide both data and power isolation in a single eight-pin SOIC package. For applications that require isolation of both data and power, and where the isolated power requirement is 50 mW or less, the new iCouplers can save pc board space and lower design costs. The iCouplers are designed to replace the combinations of optocouplers, transformers and supporting components that are currently used to isolate data and power lines.

For example, in power supply applications that employ secondary-side control, one iCoupler device can eliminate the need for the additional transformer windings and discrete components that typically provide the isolated power for startup. The iCoupler devices employ chipscale microtransformers to obtain galvanic isolation through inductive coupling. These devices take the place of optocouplers that are normally used for isolation in signal paths. According to the vendor, the microtransformers offer several advantages over optocouplers including the ability to integrate up to four channels on a single die, better performance versus price for certain parameters and lower power consumption.

In each of these new devices, three microtransformers are fabricated on a single semiconductor die. Two of these transformers are used for signal isolation, but the third transformer is the basis for an isolated dc-dc converter that is implemented on chip. (The company refers to this combination of an isolated dc-dc converter with independent signal channels as its isoPower technology.)

The dc-dc converter delivers up to 50 mW of isolated, regulated power at 5 V. Although the device as a whole specifies operation from a 2.7-V to 5.5-V supply, the dc-dc converter is enabled at supply voltages above 4.5 V. This converter switches at 300 MHz, which makes it possible to use the tiny microtransformer. Of course, small size is achieved at the expense of efficiency, which is only about 15%. However, the device targets applications where small size and low cost outweigh the need for efficiency.

Analog Devices cites a design example that originally used an optocoupler for signal isolation and a 1-W, isolated dc-dc converter module for powering an interface logic chip. This design required 12 components, 510 mm2 of board area, and had a bill-of-materials cost of $6.75. When the design was changed to use a new iCoupler, component count dropped to three parts, board area to 60 mm2 and estimated cost to $2.15.

The three devices built with isoPower are the ADuM5240, ADuM5241 and ADuM5242. The products vary by the directionality of each channel. The ADuM5240 has two signal outputs on the same side as the power output. The ADuM5241, which is intended for bi-directional communications, offers signal channels in each direction. The ADuM5242 has the two signal outputs on the same side as the power input, making it ideal for the power supply applications with secondary-side control (see the figure).

In terms of the data paths, the ADuM524x isolators specify operation up to 10 Mbits/s with propagation delays of < 55 ns and channel-to-channel matching better than 3 ns. All three ADuM524x devices carry a 2.5-kV isolation rating and will carry regulatory safety approvals from the North American and European agencies upon product release.

The ADuM524x are sampling to lead customers now, with general sampling scheduled for July 2006. Each device is priced at $2.95 per unit in 1000-piece quantities. For more information, see www.analog.com/icoupler.

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