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How Can You Determine the Turns Ratio of a Power Transformer in a Sealed DC-DC Module?

Updated: Mar 18, 2022

by Steve Sandler

I do not know if there was a legitimate reason for this request, but I was once asked to determine the turns ratio of a power transformer inside a sealed, DC-DC converter module. In any case, there is not much usable information since the module is sealed, but we can apply a simple trick. Every switch generates magnetic and electric fields and some of this will be able to be captured in the air surrounding the module.

For this measurement I selected a small H-field probe and sniffed around the outside of the DC-DC module until I located a signal that looked like the primary switch. A picture of the measurement setup is shown in Figure 1.

Figure 1 A small Beehive H-field probe was used to sniff around the outside of the module until the screen showed a measurement that looked like the primary switch waveform.

A lot of information can be obtained from this measurement. One thing we could determine is that the DC-DC converter is operating in discontinuous conduction mode. We can see this because the duty cycle changes with changes in load current. We can also tell that this is buck-boost derived, since the lower pedestal voltage is proportional to the input voltage while the step above it is fixed.

Using cursor voltage measurements, the two voltages are easily related. Here I used a ruler from the picture, just to show that the measurement can be accomplished even without the cursors, say from a picture of the screen.

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