For my next project at my workplace I am looking for an oscilloscope which gives me good support in measuring the losses in one of the half-bridge switches over one commutation period. The last time I did this the measurement results were far off from reality as the scope just added switching and conduction losses where the letter ones were totally wrong due to offset errors of the voltage probe. Furthermore it was not possible to select a certain interval over which the losses should be averaged. In short, I am looking for something that can:

  1. Distinguish between switching and conduction losses
  2. Calculate conduction losses based on an Rdson I enter (or even reliably calculate it)
  3. Average losses in a specially defined interval
  4. Make calculations based on all acquired samples within the interval, not just of those printed on the screen

I addition I hope to be able to calculate the junction temperature of the switches my measuring the forward voltage drop of the body diode at certain current values.

The commutation frequency of my signal is between 50 - 150 Hz, PWM frequency is 10kHz, voltage rise and fall times < 100ns. Off voltage on the switch is 30V, on voltage <100mV.

At the moment I am considering either a LeCroy HDO4000 or a Keysight 4000 S-Series oscilloscope with the additional power measurement software. If the software works the way I hope, this will save me a lot of time during development. The manuals of these packages however only show measurements performed on DC-DC converters, nothing on DC-AC converters.

Does anyone of you have experiences with these tools or other recommendations?

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    \$\begingroup\$ There's not a lot of difference between DC-DC and DC-AC converters, as far as the exciting fast edge switching stuff is concerned. I would be very reluctant to buy a 'scope on the basis of any specific measurement software. Far more flexible and controllable, possibly much cheaper as well, is to use a general purpose scope, dump traces to files, then post-process in MATLAB or Python. But then I like writing analysis stuff, and hate spending money. \$\endgroup\$ – Neil_UK Mar 27 '16 at 13:51
  • \$\begingroup\$ @Neil_UK Thank you very much, this is really helpful for me! I was hoping to save me the post processing with the 12bit scope, but this seems not to be the case. Seems I can't get around learning the Python stuff and that a good measurement still is a time consuming work... \$\endgroup\$ – christoph Mar 31 '16 at 8:22

Why consider an oscilloscope?

Why not a power analyser. Newtons4th or Voltech/Tektronic

There a range specially tailored for inverters & "PWM Mode".

The PPA5530 from Newton4th can operate in 3 phase 2 wattmeter + phase 3

In the 3 phase 2 wattmeter configuration, the voltages are measured relative to phase 3. The phase 1 voltage input is connected across phase 1 and phase 3, and phase 2 voltage input is connected across phase 2 and phase 3, thus measuring phase to phase voltage directly. Phase 1 and 2 current inputs are connected normally. There is no need to measure the current in phase 3 as phase 3 has no voltage relative to itself so the power contribution is zero. In this mode, the neutral channel displays the synthesised phase 3 current. PPA55xx “KinetiQ” user manual 8-2 The advantage of this connection method is that 3 phase power can be measured with only 2 wattmeters. This frees up phase 3 of a 3 phase instrument to simultaneously measure the power of a single phase input (3 phase 2 wattmeter + phase 3 configuration). This allows direct measurement of efficiency in a 3 phase motor drive or 3 phase inverter application. The frequency reference for the independent phase 3 may be selected to be voltage, current, the mains line frequency, or the same as phase 1 & 2. In this mode, frequencies up to 1kHz can be measured with phase 3.

The PM6000 from Voltech/Tek is a 6 channel power analyser so you can 3 a 3watt meter method on input and output.

This allows you to measure the utility 3phase power draw and the load 3phase power sink.

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  • \$\begingroup\$ Thank you for your reply! This is a completely different approach, I haven't even considered power analyzers yet. Need to make some further research as it seems... \$\endgroup\$ – christoph Mar 31 '16 at 8:19

In our lab at the university we use a lot of LeCroy oscilloscopes ( waverunner, wavesurfer, hdo4000 and hdo8000) and they are really good. We use them for measurements on a.c. machines and for experiments on high-voltage (6,5 kV) IGBTs. But even with 12 bit it's hard to measure the forward voltage of a semiconductor, when it can take blocking voltage. With a 10:1 divider and 30V dc link voltage it may be also critical to measure less then 100 mV exactly. But measuring the switching transients is no problem. But you can test the additional software before buying them. I think you have to export the trace files, which is very easy with LeCroy, and use a MATLAB script.

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