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Accurate measurement of pulse fall time with low side MOSFET?
Grumpy:
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)?
Grumpy:
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.
F6FLT:
--- 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.
...
--- End quote ---
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.
Hakasays:
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.
Grumpy:
--- 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.
--- End quote ---
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.
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