Oscilloscope measurements in power electronics: I. Single ended probe

[@r_be]

Sam, I sincerely hope you never stop this power electronics series. Best of luck and thank you.

[@sambenyaakov]

😊👍

[@UpcycleElectronics]

Thank you very much. This is the most clear explanation I have seen on this subject, especially the last parts about the voltage ratings of parts, and the capacitive coupling of EMI through an isolation transformer. I have often wondered why modern oscilloscopes are not designed for isolation in the first place. Oscilloscopes have been built without any exterior metal for well over a decade. It seems to me that the standard practice of an earth reference is just an attempt at selling me overpriced differential probes. Now I must go watch your latest upload on differential probes to further isolate my ignorance. Thanks for the upload. -Jake

[@sambenyaakov]

😊Thanks

[@gillywild]

Hi, firstly thank you very much for publishing these videos. I am enjoying them very much . You addressed the issue of floating the scope but not the practice of floating the DUT (the convertor being tested) in my experience this is the most common solution and I wondered what issues this could pose other than the isolation transformer must sometimes be very large and expensive?

[@BrijeshKumar_electrical]

Thanks for the nice video on Oscilloscope... If there is any practical related to above discussion like effect due to ground voltage/current, effect due to flux during high frequency. Can we create artificial EMI and see the effect through oscilloscope. Regards

[@sambenyaakov]

In any PCB that includes both power and signals you can clearly see these effects.

[@kokotmkokot4926]

Thank you very much for these videos!

[@sambenyaakov]

Thanks for comment

[@MrMropi]

thank you so much for these videos you are the best

[@sambenyaakov]

👍😊

[@BogdanSerban]

Digital multi channel scopes can compare two signals, so channel 1 would be the gate and channel 2 the source (of the high side switch), and the ground clips are disconnected. The resolution and speed are poor, but at least you can do the measurement. I also thought about isolating the converter itself and not connecting the scope to the mains earth.

[@sambenyaakov]

Ground clips need to be connected.

[@user-mj3ef]

Please more basic courses in English. Great teaching thank you sir!

[@sambenyaakov]

Thanks. Doing both.

[@catalin3407]

Since you broight up the common mode problem, I have a question related to it. Isolated converters have a Y cap across the transformer to mittigate common mode problems. However, I could not found any relevant documentation on the subject on how to select such cap. It seems to me that it is trial and error ? Could you give your view on this ? Thanks.

[@sambenyaakov]

Have a look at kzbin.info/www/bejne/a3-1mn-Fq9KLmqM it includes some relevant information. I do not recall any specific documentation on subject.

[@catalin3407]

@@sambenyaakov thank you. Good information.

[@grigorioss3936]

Probably this has been solved by now, it's a question I was trying to answer myself recently. My thoughts is that you'd want to use the largest Y cap value possible (less impedance for CM noise to filter better), while adhering to leakage current regulations. Let me know if there's some more thorough guidance I can check for this.

[@jubul1988]

Hi sir, can you please address my queries 1.power ground and analog ground should be common or different. 2. In a 3 phase inverter fed motor drive when using more than 48 volt battery why it is required to use isolated gate driver. 3. I have seen spike in current wave form and the mosfet failed when i used to probe vgs and vds .i am using isolated scope yokogawa dl850ev.what may be the possible reason . Thanks in advanve

[@sambenyaakov]

1. Separated but them connected in one point. 2. The reason is high power 3. There are capacitive paths that can still nject noise.

[@petergriffin760]

Does isolated channels oscillos (like FLUKE-190-104) do the trick with floating measurements?

[@sambenyaakov]

Yes indeed. But notice they do not provide CMRR data.

[@NiHaoMike64]

You show wrapping the ground wire around the probe which is not desirable since that makes an inductor! I was taught to just hold the wire against the probe. I wonder why there aren't more scopes that are double insulated and therefore do not need a ground. As well as battery operated scopes that are basically ideal for floating measurements, especially for signals too small for a differential probe to resolve.

[@sambenyaakov]

Thanks for good comments. Wrapping a bare wire on metal does not make a coil if tied. Differential probed are not that expensive anymore (save the high end) and they provide a very good solution for power electronics. For very low signal, true differential scope are around (e.g. Picoscope 4000 , if I remember right the number. I do own one).

[@sambenyaakov]

Just realized that when referring to wrapping you meant the first mode I showed in video, wrapping of the isolated ground wire. You are right, but holding the wire against he probe is rather cumbersome. Thanks again for comment.