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?
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
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
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.
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.
Have a look at ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-5NRiLOvmXdA.html it includes some relevant information. I do not recall any specific documentation on subject.
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.
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
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.
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).
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.
