Oscilloscope Common Ground

[Glenn0010]

Hi all,

As part of a project I'm running, I want to measure the Vds voltage on a high side mosfet in an inverter driving a BLDC motor. In the mean time I'd also like to look at the phase current of the motor on the second channel of the scope. I plan on using a battery operated current clamp for the Oscilloscope

Therefore since the oscilloscope has common ground between channels will this prove to be an issue since I do not want to blow anything up. I don't think it should be an issue but I want to cross check with you guys. I will then repeat the same test however this time for the gate voltage.

Thanks

[Paul Rose]

Battery operated clamp for current should be OK for channel 2.

Vds measurement depends.  You want the voltage across drain and source.  IF the circuit under test is powered by a grounded supply, and neither drain nor the source are grounded, then you don't have a safe place for the ground clip of your first channel.

You could measure drain to ground OR source to ground, but not drain to source.

If either source or drain are grounded, then you are good ( clip the ground lead to the grounded terminal ).  Or if the circuit itself is powered from a floating supply ( i.e. lab supply where negative is not dedicated to ground, or a battery ).

[Simon]

You would in theory require a isolated/differential probe, or be certain that your scopes ground is not shared with the DUT, this however can be dangerous if your BLDC drive is powered at anything over 50-60V. If the supply voltage is stable you could as suggested measure the voltage of interest with respect to ground and work it out. If you have a scope with more than 2 channels and enough shared ADC speed you could use the third channel and take a non isolated differential measurement and use math functions to get a trace of the particular voltage you are interested in.

[Glenn0010]

Hi thanks for the feedback. Yes my motor is powered from a 48V nominal battery(53V max) and I've  taken the high side measurments before (Vds, Vgs)

So having a battery operated motor and a battery operated clamp (different batteries for both) should I be good to go to take my measurements?

[Glenn0010]

My bldc drive is powered using a 53V max battery. hence I shouldn't need a different probe correct? Also why would the max voltage make a difference?

[Paul Rose]

With a battery powered device (and no other ground connections on your inverter), you will be fine to measure Vds with one probe.  You will also be fine to measure current via a clamp on your second channel at the same time.

[Glenn0010]

Thanks, yes my device is purely battery powered.

[Simon]

My bldc drive is powered using a 53V max battery. hence I shouldn't need a different probe correct? Also why would the max voltage make a difference?

It makes all of the difference. The scopes ground is also mains earth for your safety especially as the whole scope chassis is connected to the ground clips. the last thing you want is even on a battery (non earthed) devico is to hook your scope ground to some potential and then risk touching something else near the other side of the supply voltage.

[Glenn0010]

It makes all of the difference. The scopes ground is also mains earth for your safety especially as the whole scope chassis is connected to the ground clips. the last thing you want is even on a battery (non earthed) devico is to hook your scope ground to some potential and then risk touching something else near the other side of the supply voltage.

Is what you mean by this, the risk of shorting out the supply with the ground clip due to the proximity of the pins, devices etc?

Do you recommend I proceed with the test of the current probe and the Vds measurement at the same time?

[Simon]

If your DUT is not mains earth referenced/connected you won't tave a problem and being under the HV limit means there is little chance of injury.

However it is a good idea to be aware of you scope and ho it's connections work. For example you cannot take a mains powered scope and measure the mains because even if you connect the ground clip to what you believe is neutral you will still be in danger and would cause an RCD to trip. If you connect your ground clip to live you create a direct short circuit and render the chassis live.
If you are measuring a high voltage device that is isolated from the mains then you still have to be careful because as soon as you connect the ground clip to something you have earthed that potential. Lets pretend your BLDC is 200V with negative used as earth and perhaps some of the chassis connected to it. If you were to connect your earthed scope probe to +200V you now have a situation where there is 200V (or -200V) between your chassis and mains earth (the whole building including your bench. Ands that's pretty much the end of the life of anyone touching the chassis of that device.

In your case given the low voltage you should be ok if you current probe is isolated. If in doubt use the two probe method and math function but note this is not a substitute for for an isolated probe and you cannot measure voltages that exceed you scope+probe input range.

[Glenn0010]

What you said is definitely good and thanks for the well written explanation. I'm just going to use one of those multi meter current clamps things and they have current reading on them so that is the simplest

[SiliconWizard]

If you can have access to a differential probe, I would definitely use that for Vds measurement to avoid any potential issue. Connect the oscilloscope ground to your circuit ground, and measure Vds differentially.