RU-vid's algorithm recommended your videos (esp. the Miller Plateau video) right when I was working on a related problem on a design. Very serendipitous...and your videos are great!
Thanks a lot for the great video. Is it possible to use a whiteboard instead please? I understand that this is a weird request. But the marker on paper makes a sound that is hard for some people (not everyone) to tolerate. I am sorry for this strange comment. Thank you again. you rchannel is amazing.
Hey, I thought I was the only one to "design by tweak" I tried the "math" way and the values always turned out wrong so ended up tweaking the values anyway!
awesome video!, I wish you could of talk about adding diode in parallel to resistor and see it's effect on the scope. You can also combination of diode and resistor to control the turn-on and turn-off speed.
Does the oscilloscope's probe capacitance (e.g. 1pF) not change the gate signal too much? Or is the gate capacitance order of magnitude bigger than the 1pF of the probe?
Gate capacitance is hundred or hundreds picofarad at least typically and oscilloscope probe is 12-25 picofarad (fine one) so it doesn't change much. If you work with more powerfull transistors it is neglisible. It is only consern at rare cases.
As real examples, a power mosfet like the IRL530 has a typical input capacitance around 1nF and the IRFZ44 around 2nF, while for the small-signal 2N7000, it's just 60pF max.
as a 10:1 passive probe is typically around 11 pF, it won't load the drive path very much, relative to the gate capacitance of a big mosfet, but really loads a smaller device, where an active probe becomes indispensable! Also, a typical passive probe, if you use a "ground lead" often adds a resonance at 100-125 MHz. probing is always raising its head as a tricky factor in switching analysis...
But what if you're just switching on/off a load (no fast switching requirements) ? Wouldn't a high resistor value be preferable, in order to get lower emi emissions?
Then a higher then optimum will be prefered I guess. Here another problem may arise: your load may be placed at some relatively large distance and you'll have a high transients with ringing on a wire inductance. Then yes - it is more reasonable to to make it as slow as possible.
The problem then becomes the amount of time the mosfet spends in its linear region. If you are sinking a high current with a high voltage supply, you will need to make sure that the power dissipated during the switching is within limits for the device and its heatsink.
I must admit I was hoping for a bit more than "try different resistor values, and see what looks best on a scope". But that's fair enough advice in most cases, and you did supply the link to a more detailed analysis.
@@ferrumignis Yes, indeed, as I remarked. Most of the time, of course, you'll have an estimate of the maximum parasitic inductance when you know the length and size of the leads/tracks between the driver and the mosfet. That's sufficient to determine the minimum value of resistor needed to damp the LCR circuit.
Wow thanks so this should be done under load...if I'm trying to rive a dc traction motor should I do it under load or off the ground freewheeling with the scope
