Frederik Dostal, Subject Matter Expert, Power Management at Analog Devices
Optocouplers are devices which have traditionally been heavily used in power systems. They allow a feedback signal to be fed from the output of the circuit to a controller that dynamically adjusts the circuit for better regulation. Optocouplers provide isolation by turning the electrical signal into a light signal using an LED. At the other side of the isolation barrier, the light is turned back into electrical signal by a photosensitive device. There are quite a few downsides to optocouplers – for example, they are inefficient because the LED needs biasing, they are slow, physically they are bulky, and they tend to degrade over time, so they have to supply extra current to the input to get the desired current at the output. The headroom required to account for that degradation is wasteful. Finally, they are unreliable at temperatures over 85oC, a definite drawback for today’s power dense solutions.
Fortunately, there are alternatives to optocouplers that can do the job even better with fewer drawbacks. The first of these options is just to leave out the optocoupler altogether. In some circuits, such as flyback converters, voltage can be sensed across the primary side windings during the transformer’s ‘off time’, allowing the output voltage to be tracked. This provides a much more compact design that outputs a regulated voltage. This type of design uses a controller, such as the LT830X, a small device with an integrated switch.
Frederik Dostal, Subject Matter Expert, Power Management at Analog Devices explains, “When there's no current through the primary side winding, the reflected voltage from the secondary side can be sensed. However, while the output voltage is regulated, it's not as well regulated as circuits with a real feedback path. That problem can often be mitigated using clever design. The second drawback is that a little output load is required, because if there's no current flowing in the output, then the sensing scheme through the primary side winding doesn't work too well. Some applications can live with these drawbacks, but others need a better solution”.
For those more demanding applications, there are other types of isolation available that have neither the drawbacks of optocouplers or those of primary side sensing technology. Capacitive and magnetic isolation are techniques that are growing in popularity for power designs and other applications. Both are faster and more efficient than optocouplers and they can work in higher temperature ranges. They also are better regulated than primary side sensing designs and don’t need a load. Analog Devices mainly uses magnetic isolation in its power products as that was the technology the company had originally developed, although the company has launched capacitive isolation designs more recently.
The company’s iCoupler magnetic isolation technology can isolate up to 5kV in a wide package, or 2.5kV in a reduced footprint package. It is capable of operating in temperatures of up to 125oC, and consumes around a third of the power of a traditional optocoupler. The technology is available in a discrete component, such as the company’s ADuM3190 that works with a separate controller, or as an integrated solution with the isolation packaged alongside the controller as found in the company’s ADP 107x range. That family of iCoupler integrated devices includes solutions for topologies that can handle different power levels, for example the ADP1074 Active Clamp Forward Controller and the ADP1071-x Flyback Controller.
Dostal concludes by saying, “Our integrated isolation technologies use Analog Devices’ iCoupler technology. The sensing path feeds the feedback pin, or compensation pin of the primary side controller and gives very good regulation of the output voltage. There’s also no requirement for a minimum output load. It is a perfect feedback path, like a piece of wire, but it offers the galvanic isolation that is needed for most transformer based technologies”.
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