Linear opto-couplers and the loop gain 'booby trap'

Vishay TCET1104

A power supply that I was examining used an opto-coupler in its feedback path for control of its output voltage. This is a commonplace and well established design approach, but the feedback loop’s unconditional loop stability was not altogether a certainty.

I decided to measure the opto-coupler’s transfer function in two ways as seen below. I knew that the device’s datasheet presented this characteristic, but I wanted a first hand look for myself (Figure 1).

Opto-coupler transfer function test set-ups.
Figure 1. Opto-coupler transfer function test set-ups.

The 1.33k and 150 ohm resistors just happened to be conveniently on hand so I used them. A 1k and a 100 ohm pair would have sufficed just as well, but I just didn’t feel like rummaging at that moment.

The two test results did seem to confirm the datasheet presentation (Figure 2).

Opto-coupler transfer function test results.
Figure 2. Opto-coupler transfer function test results.

The upper trace of Figure 2 goes with the upper sketch of Figure 1, the lower trace with the lower sketch. It was reassuring to note the symmetry of the two test results. However, upon some mental reflection, I realized that there was a loop gain booby-trap of sorts of which to be aware.

Using the lower trace of Figure 2 and measuring the transfer function slopes, we see the following (Figure 3):

Close-up examination of the transfer function.
Figure 3. Close-up examination of the transfer function.

The “linear” nature of the opto-coupler is not to be taken too literally. The device’s transfer function doesn’t switch or anything like that, but the first derivative of the output versus the input, which is to say the slope of the output versus the input, varies versus where you place the device’s operating Q-point. The variability in the device I examined was nearly 19 dB.

That much gain variation could have a rompin’-stompin’ effect on a feedback loop’s overall transfer function, possibly pushing a marginally stable feedback loop into conditional instability.

The cautionary note of all this is to be sure to check the true slope of whichever opto-coupler you’ve chosen for yourself and where you choose to set its Q-point.

Materials on the topic

  1. Datasheet Vishay TCET1104

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