MOSFET Turn-Off Snubber

An intuitive explanation on the operation of a turn-off snubber and a method to calculate the required capacitance.

[@CSparzo]

Sam, your videos are top notch and essential MOSFET material that is rare.

[@sambenyaakov]

😊Thanks.

[@anuradhapriyankara5226]

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!

[@sambenyaakov]

Thanks

[@fakduken9775]

As an electronics enthusiast, I really enjoy your videos. Very clear explanations, thanks.

[@sambenyaakov]

🙏😊

[@2meters2]

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.

[@sambenyaakov]

Thanks for comment

[@MicheleDondi]

thank you so much for your clear and useful explanation, everiday is a good day for learning!

[@sambenyaakov]

Thanks

[@jhonnyginting8985]

Thank you sir you make it's critical clear enough.. straight to the applications. Very nice

[@sambenyaakov]

😊

[@topphemlig1191]

Great explanation. I've always wanted to see an algebraic explanation for something I've been using while relying on select on test values

[@sambenyaakov]

Thanks for coment

[@premsuthar4285]

Interesting analysis. Enjoyed it and now got precise equation to size the cap.

[@sambenyaakov]

Nice to read.

[@xyilmaz3563]

very clean and constructive explanation. well done!

[@sambenyaakov]

Thank you!

[@SachinAttri27]

Thank You For your Lectures. Great material.

[@sambenyaakov]

Tanks for comment.

[@xarmagarri]

Really helpful. Thanks for sharing your knowledge!

[@sambenyaakov]

Thanks for comment.

[@Andreasonline3]

Absolutely great video.

[@sambenyaakov]

🙏👍🙂

[@knoblauch001]

So clear and so nice, thank you very much!!

[@sambenyaakov]

Thanks.

[@anoimo9013]

very sharp clear explanations

[@sambenyaakov]

Thanks.

[@hamidk4772]

Outstanding Job. 🙂

[@sambenyaakov]

Thanks Hamid.

[@ats89117]

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...

[@sambenyaakov]

Indeed

[@chuckjls]

Great video.

[@sambenyaakov]

Thanks😊

[@perrykleez]

very nice explanation, thanks!

[@sambenyaakov]

Thanks

[@monolito8556]

Thank you so much Sam !

[@sambenyaakov]

😊

[@Alex-cj7ro]

very informative and nicely presented. thank you professor👍👍

[@sambenyaakov]

🙏🙂

[@sahar37b]

useful video, thanks.

[@sambenyaakov]

Thanks

[@williamkopanchuk]

Thank you!

[@sambenyaakov]

👍😊

[@williamkopanchuk]

@@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.

[@ssackirann]

Thank you professor

[@sambenyaakov]

😊🙏

[@TheMailrouter]

Equation at 15:30 should read IL+GM*VT instead of IL+VT

[@TSulemanW]

very detail explaination nicely

[@sambenyaakov]

😊

[@fabriziocattaneo3603]

thank you so much

[@sambenyaakov]

Thanks

[@mohamedarrach3688]

Hi dr Sam. I would know if can we have a choppe, based on thyristors, with 3 levels at the output voltage ?

[@user-nu3yh4wn3z]

Thanks for this explanation. Perhaps talk about the capacitor technology because a lot of rms and peak current in your application (and 400V)

[@sambenyaakov]

Good point. Perhaps.

[@yanivnisinman7603]

הרצאה טובה. תודה.

[@sambenyaakov]

תודה

[@user-en7ci6uk3k]

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]

Good point. Oscillations will commence if Q is high

[@georgestanciu5466]

Thank you professor. Great presentation. Can you tell about lossless turn off: how to calculate the Ls inductance and what is best diodes to use?

[@sambenyaakov]

Thanks. What do you mean by Ls?

[@georgestanciu5466]

@@sambenyaakov At minute 6:12 the inductance of Ls can be calculated?

[@davecook2428]

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 :-)

[@sambenyaakov]

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 :-)

[@prathmesht42]

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.

[@sambenyaakov]

I am not that familiar with MATLAB MOSFET models. Sorry.

[@cobushoffmann4249]

Hi Professor, Great video once again. Also what books would you recommend for power electronics design ?

[@sambenyaakov]

I don't know about design, but a basic power electronics book is: Fundamentals of Power ElectronicsAuthors: Erickson, Robert W., Maksimovic, Dragan

[@user-hm6xc6ll2f]

sir please explain series connection of mosfet for dynamic voltage equlization

[@sambenyaakov]

Good subject will try/

[@arunkaruppaswamyb1907]

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 ?

[@sambenyaakov]

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.

[@boonedockjourneyman7979]

How many times have I flailed my way through an explanation of MOSFET heating? I always seem to start with a story about resistance. I am humbled.

[@sambenyaakov]

(-:

[@qqqqqq6686]

Dear Prof, Please explain once again and perform same analysis for a recycled snubber. Can we skip Ls and D2 ? What are they doing?

[@mikemarachov3230]

Professor, Can we use this idea in a 3phase inverter - For low and high side?

[@sambenyaakov]

Yes definitely..

[@idofabhay]

interesting 👌👌

[@sambenyaakov]

Thanks

[@deepalps]

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.

[@sambenyaakov]

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.

[@pallavishirsat]

Hi.. which snubber circuit is better to use RC snubber or RCD snubber. What's are advantage n disadvantage of both configurations

[@sambenyaakov]

RC and RCD are for different purposes. RC damps oscillation and RCD controlls dV/dt. See ny video on snubbers.

[@rj8528]

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]

Which minute in video are you referring to?

[@rj8528]

@@sambenyaakov 13:24

[@audiokees4045]

I have a supply who has a burned resistor and a 10 nF cap, a idea how to calculate the resistor? THanks./

[@sambenyaakov]

Is this a snubber resistor?

[@audiokees4045]

@@sambenyaakov Yes, but in the meantime I have repair it, just calculate a new one. Thanks for the answer.

[@sujoybha]

Can this be used for 3phase H bridge for motor control? Or is there a better circuit?

[@sambenyaakov]

There are more clever RCD snubbers for half bridge configurations.

[@sujoybha]

@@sambenyaakov if you come across any configuration names, please let me know. Also thanks a billion for these videos.

[@ajayrai7017]

great explanation....but how to know the gate resistor value to calculate snubber capacitance??? is Rg value available in MOSFET Datasheet??

[@sambenyaakov]

Rg is selected by user. See kzbin.info/www/bejne/d6KUeqpsd96Ld9k and other relevant video in my KZbin channel

[@ajayrai7017]

thanks professor@@sambenyaakov

[@ADGGMB]

Great Video, What was the Vpulse values for simulation?. Regards

[@sambenyaakov]

To which minute of video are you referring to?

[@ADGGMB]

@@sambenyaakov Hi, min 24:10

[@sambenyaakov]

15V

[@panditji8823]

sir, may u tell me pls, when and where to use CCM and DCM mode of operation in SMPS and which mode is beneficial with which topology.pls guide

[@sambenyaakov]

Practically any PWM converter will enter DCM at low power. To maintain CCM at low power one would need very large inductance.

[@panditji8823]

where to use DCM and CCM ,is there any trick or voltage by which we could elaborate the reasion.

[@ANoNIMkillir]

Why do we ignore Cgs? This capacity should have a contribution to Ig isn't it?

[@sambenyaakov]

Yes and no. As Vgs reaches the threshold the current through Cgs becomes zero. The total charge takes into count both capacitances.

[@ANoNIMkillir]

@@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!