Measuring Dead Time in 48 V Power Converters, Part 2: Dynamic Measurements

Figure 1. P1 and P2 measure dt@lvl over the entire acquisition,
while P3 and P4 measure dt@lvl for only a single operational
cycle of zoom traces Z1 and Z3.

A primary engineering task for 48 Volt power conversion systems using bridge topologies is to ensure adequate dead time to prevent catastrophic shoot through occurring when both HI and LO FETs conduct at the same time. Being able to accurately measure dead time is therefore of critical importance. Part 1 of this series dealt with the basic dead time measurement. Part 2 will deal with studying the dynamic changes in the dead time measurement using statistical tools like tracks and histogram functions. 

As we learned in part 1, the dead time delay is measured using two instances of the measurement parameter Delta Time at Level (dt@lvl) as shown in Figure 1. 

In the Figure 1, the dt@lvl parameters P1 and P2 show the value of the last measurement in the acquisition which contains 10,000 switching transitions. The parameters P2 and P4 measure only the single timing cycle shown in the zoom traces. (Click any  image to enlarge it and see the detail.)

The values of both parameters are different, and you should ask the question: how does dt@lvl vary with time? To find the answer, turn on the measurement parameter statistics, as shown in Figure 2.

Testing for Near Field Radiated Emissions in the Time Domain

Figure 1. Triggering on a time-domain signal
likely to be coincident with radiated emissions
helps to locate where in the channel di/dt occurs.

There are three, principal root causes of the common currents that lead to radiated emissions in electronic devices:

1. Return path discontinuities 

2. Physical structures that are not tightly coupled to the return plane

3. Ground loops causing common currents in cables 

We’ll briefly demonstrate a bench top test for finding sources of near field radiated emissions caused by return path discontinuities using a real-time oscilloscope in the time domain.

Why the time domain? Although EMC compliance testing is done in the frequency domain, in the time domain we can see the signatures of near field emissions in a way that yields information about the root causes of those emissions. It is a type of pre-compliance EMC testing that can be easily done in your lab, without the expense of an anechoic chamber.

Near Field vs. Far Field Radiated Emissions

Figure 1. The near-field emissions we measure
in the lab may not be an accurate measure of
the far-field emissions on which EMC is certified. 

When we are testing a product on our bench top for EMC, we are in close vicinity to that product in a typically noisy environment. All we can measure is the near field, the electric or magnetic field strength in close proximity to our product. It’s important to keep in mind that near field measurements are not the same as the far field (3 m or more) measurements in the FCC Part 15 Radiated Emissions test described earlier, and here's why. 

Unintentional Antennas in Electric Circuits

Figure 1. Certain design features can introduce
unintentional antennas into electric circuits.

In our last post, we discussed how little radiated emissions it takes for an electronic product to fail an FCC certification test for EMC.

Where do these radiated emissions come from? No one designing an electric circuit board is designing them into their product on purpose. These sneaky antennas do not appear in the schematic. However, we can unwittingly introduce them into our product through certain styles of board and interconnect design features. It is sometimes jokingly said there are two kinds of designers: those who are designing antennas on purpose, and those who aren't doing it on purpose. We’re going to introduce two, basic models of antenna—magnetic dipole and electric dipole (Figure 1)—to reveal a secret source of radiated emissions.