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OverUnity Research > Electrical / Electronic Theory and Learning Center > Learning Center Miscellaneous > Accurate measurement of pulse fall time with low side MOSFET?
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Author Topic: Accurate measurement of pulse fall time with low side MOSFET?  (Read 1187 times)
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img  Grumpy
Accurate measurement of pulse fall time with low side MOSFET? « on: 2022-09-01, 22:41:21 »

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I have a typical circuit with a load switched by a low side MOSFET, driven by a gate driver.

How can I measure the pulse fall time with an oscilloscope that has a 1x-10x passive probe?

When the MOSFET turns off, the probe takes a very long time to discharge, so I can't see the real pulse fall time.

Or is something else slowing down the fall time?

(I'm very new to MOSFETs and low side switching...as you can tell.)

EDIT:
Can I connect the output through a capacitor and run the other side to ground through a 50 ohms resistor and scope across the resistor (with scope inverted)?
   
img  Grumpy
Re: Accurate measurement of pulse fall time with low side MOSFET? « Reply #1 on: 2022-09-02, 00:09:56 »

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Using a capacitor and resistor like I mentioned above works, but the voltage level is greatly reduced.
So, I doubt what I see is accurate at all, but it does resemble the gate pulse for the most part.

EDIT:
Figuring the probe and capacitor are making a divider, I'll try a resistor from probe to ground.
   
img  F6FLT
Re: Accurate measurement of pulse fall time with low side MOSFET? « Reply #2 on: 2022-09-02, 08:12:48 »
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Quote from: Grumpy on 2022-09-01, 22:41:21
I have a typical circuit with a load switched by a low side MOSFET, driven by a gate driver.

How can I measure the pulse fall time with an oscilloscope that has a 1x-10x passive probe?

When the MOSFET turns off, the probe takes a very long time to discharge, so I can't see the real pulse fall time.
...

We can put a simple fast transistor in emitter follower with a low emitter load resistance (for example 50 ohm), as a buffer in output of the MOSFET.
It will have a high impedance for the MOSFET so it will not disturb its operation, and a low impedance for the scope so it will not be disturbed by it, and will isolate one from the other.
In all cases, the scope must of course have a maximum working frequency compatible with the MOSFET fall time (F > 1/toff). The probe in 1x will be the best option.

« Last Edit: 2022-09-02, 10:39:33 by F6FLT »


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img  Hakasays
Re: Accurate measurement of pulse fall time with low side MOSFET? « Reply #3 on: 2022-09-02, 15:19:10 »

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Depending on voltage level, you might get results by putting a small piece of tape over the target wire and clipping over that, letting the probe couple dielectrically.  You'd likely lose the DC component though :(

Lazy me would also try putting a series resistance between probe and device-under-test just to see if it helped.



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img  Grumpy
Re: Accurate measurement of pulse fall time with low side MOSFET? « Reply #4 on: 2022-09-02, 17:25:37 »

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Quote from: F6FLT on 2022-09-02, 08:12:48
We can put a simple fast transistor in emitter follower with a low emitter load resistance (for example 50 ohm), as a buffer in output of the MOSFET.
It will have a high impedance for the MOSFET so it will not disturb its operation, and a low impedance for the scope so it will not be disturbed by it, and will isolate one from the other.
In all cases, the scope must of course have a maximum working frequency compatible with the MOSFET fall time (F > 1/toff). The probe in 1x will be the best option.

How do I connect the transistor to the MOSFET output? A cap and resistive divider to the base?

I should mention that this MOSFET is going to operate from with a Vs of 50v to 4kv, DC.
   
img  Vasik041
Re: Accurate measurement of pulse fall time with low side MOSFET? « Reply #5 on: 2022-09-02, 20:29:53 »
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With such high voltage you can try using 1 : 100 probe
Probe has known (quite small) capacitance, you can take it into account in you measurements if it has significant influence.
   
img  F6FLT
Re: Accurate measurement of pulse fall time with low side MOSFET? « Reply #6 on: 2022-09-04, 13:18:16 »
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Quote from: Grumpy on 2022-09-02, 17:25:37
How do I connect the transistor to the MOSFET output? A cap and resistive divider to the base?

I should mention that this MOSFET is going to operate from with a Vs of 50v to 4kv, DC.

The problem is no longer the same if the voltage must exceed the KV. There I have no real solution. To measure the voltage accurately, a very expensive high voltage probe is needed..


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