Optimizing power conversion for isolated sensor interfaces

-December 04, 2012

In the world of industrial controls only a few things are certain; the next product will have a smaller form factor, more channels and have a lower target cost per channel. The expectation is that technology has improved since the last design and all of these things are possible. To a large extent that is the way things have worked out in the past, and your luck may be holding.

The data interfaces have been improving steadily from the era of optocouplers to the latest high speed low power highly compact digital isolators.  In this article we will examine one aspect of isolated sensor interfaces that gets less attention than it deserves. How do we get isolated power to the ADC and conditioning circuits while shrinking the size of the interface and improving performance? 

In the past, the analog interface boards did not have high channel counts, so there was enough room on the board for a modest dc-to-dc converter to be designed to provide power to the sensor interface.  Power dissipation was not a great concern since there were only one or two interfaces to a module. Currently analog PLC modules, as illustrated in Figure 1 can have four, eight or even 16 independent isolated channels.  Multiple copies of a modest dc-dc converter take up a lot of space and create a lot of heat.

Figure 1 Typical Multichannel sensor interface

A good place to start a discussion of power is with a generic analog interface as shown in Figure 1.  The active circuits consist of a signal conditioning element like an op amp or instrumentation amp, and an ADC with a serial interface which can be interfaced with the FPGA through digital isolator channels.  This circuitry typically needs significantly less than 150mW.

The basic challenge of providing power to the sensor interface is optimizing the supply to work well within the required power range.  Operation at 0-150mW means that the fixed quiescent power of the controller and feedback elements that make up the power supply will be a large portion of the total power used so the efficiency will be lower.  This can be seen in the quiescent current values in Table 1 for various supply configurations. 

Alternately many simple power supply designs require a minimum load to operate properly so power must be wasted in resistive dead loads to ensure that the supply functions properly. While it is very simple to drop a 555 Timer and transistor on the board and get some power, it is difficult to make an efficient and reliable supply that works a low power levels.

There are three basic categories of dc/dc converter used for this power range

1)  Unregulated switching supplies or modules
2)  Regulated switching supplies or modules
3)  Chip scale power converters

Each of these supply architectures needs increasing complexity of control circuitry, and in the case of the first two options increasing component count and solution size.

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