Thank you, Sam, this gave me a better understanding or the relation between snubbers and gate current. For example, in my case current prototype, I use a C3M0015065K, which has a Coss of 300 pF and a Crss of around 31 pF. Those are each dependent on the Vds, but the ration Coss/Crss is around 10 and constant over a very wide voltage range. VT is around 2 - 3 V, and with that, the calculations around 20:00 into the video suggest that then, if we choose Rg 1 ohm or less, the this internal Coss of the MOSFET serves as a snubber for the Turn-Off transition for up to a IL of about 20 A, and we don't need to add any additional capacitance at all ! That's a wonderful relation that I did not know existed. Thank you for explaining the parameters involved in this process in a delightfully clear way.
Professor, this is really great! I love how you used a datasheet of an actual MOSFET and did the calculations based on that. I didn't watch this just to pass an exam, in fact I'll be designing a commercial product based on this theory. Thank you very much for giving this valuable content for free!
Great lecture, as usual, Prof.!!! @10:33 , shouldn't the gate current be an exponentially decaying current with an initial peak of Vg/Rg since the gate current ig = Vgs/Rg ?
Hi Arun. You have a keen eye. You are the first one to notice my goof. You are correct of course. I must have had the gate voltage i mind. Anyway, this does not harm the explanation since the relevant part is the flat portion. Thank for the comment.
Hey sam sir, It is really worth it watching great content and explanation of snubber by you. could you tell me what type of mosfet can we choose from matlab simulation library to design RCD snubber ckt ? I would be thankful if anyone revert on it asap.
This was a very interesting and educational video. Running SPICE simulations on some previous applications indicated that while it was possible to increase efficiency (power provided to the load divided by input power) with this snubber design, the optimal capacitance for minimizing dissipation in the MOSFET generally left the peak voltage levels above safe limits that the snubber was being used to enforce. I guess there is no free lunch...
Sir, I am using a 3 phase contactor to switch a 25kvar capacitor at 440V(33A at 440V), can u suggest the right values of capacitor and resistance that can be used as a snubber to supress arc and protect the contacts of the contactor.
The values depend on stray inductance that you would expect. Energy from inductance moves to cap and builds up its voltage whle the R is chosen to reduce Q to about 1.
This was a great explanation, I do have a few questions? Doesn't this just move the switching losses from turn-off to turn-on? Is the benefit is that the turn on losses are less significant, maybe there isn't a similar constant Vgs area at turn on that generates this problem? Couldn't a similar effect be accomplished with a forward-biased turn off diode in parallel with the turn on gate resistor or a diode and series resistor in parallel with the turn on gate resistor. I've seen this in other circuits and application notes, but am not knowledgeable of all the fundamentals yet and how it compares with this switch technology and application. It would appear this diode only or diode and smaller turn-off resistor would speed up the process of turning off the gate with the inefficiency of pumping energy in and out of another passive device. Could the independent turn-off diode and resistor be better for higher voltage mosfets where the Vt is much greater than the diode forward voltage and for really low voltage mosfets the diode forward voltage is too close to Vt, this parasitic capacitor would be a better choice unless a loss less type is used. Is there a brief explanation of the Vo/Vi limits of the loss-less system for boost circuits? Thanks again for your time :-)
In RCD snubber the losses are moved to resistor, in lossless snubber they are recycled. Pleas break down the ret of comments into sections so I can follow :-)
Professor, Excessive capacitance will cause excessive current spikes when the MOSFET is turned on. I usually connect a resistor in series, which can improve the spike problem. Generally, the resistance of this resistor is around 100 ohms.
@@sambenyaakov In your analysis, you use a continuous input current with low ripple. Would the analysis change if the input current IL were pulsed? 15:16 Also, why are the capacitors Cgd and Co constant?
@@sambenyaakov The switching transitions in the datasheet are measured with a resistive load and do not fully represent a practical circuit. However, the calculations are made for an inductive load. Therefore, the calculated and measured values will be very different from each other. Am I wrong?
@@sambenyaakov Dear Sam Ben-Yaakov Thank you very much for your excellent videos and lecturers. The are very useful as material resources for students in my work place, Brunel University London.
Excuse me ,Sir, I have some questions, when the switch turn off the Vgs will drop but in your video you said assume the vgs is constant I think it is weird, can you explain more detail? thanks
@@sambenyaakov So, as i understood this, there is a current flowing through transistor, snubber and gate resistor until Vds reaches the clamp voltage. In other words - transistor won't completely turn-off until Vds reaches it's clamp, but we can, sort of, moving losses out of the transistor to the snubber resistor, by increasing part of snubber current? Does this mean that turn-off losses will be more than the turn-on losses? Thank you!
