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Some doubts about RCD snubber

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I_think

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1) If snubber is to protect the switching transistor, then if the max voltage during turnoff of device is lesser than the Max VDS rating of mosfet, do we still need snubber?
2) If we implement snubber at primary winding of flyback transformer, do we need to add protection across the switching mosfet?
3) While designing snubber if we calculate peak voltage that would appear across switching mosfet.So after applying snubber ckt, Can we choose mosfet with lower voltage ?
4) how can leakage inductance be minimied? Or what factors influance leakage inductance of flyback transformer?
5) Wish to understand what is deltaVs (delta snubber voltage). (in application notes its said to be allowed ripple but cant get idea what do they mean by it.)
 

Hi I_think !
1) If snubber is to protect the switching transistor, then if the max voltage during turnoff of device is lesser than the Max VDS rating of mosfet, do we still need snubber?
Click to expand...
An snubber network will have two aim in that case :1-shifting the dissipation . 2- your mosfet won't need ability of tolerating high voltages !
2) If we implement snubber at primary winding of flyback transformer, do we need to add protection across the switching mosfet?
Click to expand...
What do you mean by protection in this case ??!
3) While designing snubber if we calculate peak voltage that would appear across switching mosfet.So after applying snubber ckt, Can we choose mosfet with lower voltage ?
Click to expand...
If you use a right snubber network , of course you can ! ( for example if it needs to be able to tolerate 2vm you can use a mosfet with ability of Vm !
4) how can leakage inductance be minimied? Or what factors influance leakage inductance of flyback transformer?
Click to expand...
Most important issue in this case is how to wind it's transformer ! ( sandwich winding ! )
Best Wishes
Goldsmith
 
Dear goldsmith,

Thanks for the reply.

2) If we implement snubber at primary winding of flyback transformer, do we need to add protection across the switching mosfet?
What do you mean by protection in this case ??!
Click to expand...

Here I mean that , do i need to connect RC snubber across switching mosfet , after i connect RCD snubber to the primary of Flyback transformer?

One more query which I did not ask in previous post and also I could not find answer on net.

While calculating flyback voltage we use Vo = V1 (with single secondary), as the secondary voltage is reflected on the primary. But when multiple secondaries with say, V1 , V2 , V3 then what should be value of Vo?
 

A snubber is used to protect the transistor from too high Vds, reduction of unwanted emission and to divert some of the switching loss from the transistor to a resistor.

If the snubber across the primary winding assures that the transistor is happy, you don't need an additional snubber or clamp across DS. In case of large physical distance between snubber across primary winding in combination with high dI/dt, you may need some snubber action close to the mosfet.

Vds voltage, I think you should need to have some margin to handle input voltage variations, inrush/transient phenomena etc. If you can reduce Vds max (with sufficient margin left), you can do that. Mosfet performance (think of Rdson) is better for low voltage devices.

Reducing leakage inductance is a combination of selecting the correct core material and using interleaved windings. interleaving increases the capacitance between primary and secondary.
Modern ferrite material can accept higher flux density at certain power loss, resulting in less turns, hence less leakage inductance.
 

Here I mean that , do i need to connect RC snubber across switching mosfet , after i connect RCD snubber to the primary of Flyback transformer?
Click to expand...
Hi again
In the general words an snubber network is an optional choice ! it is up to you ! yes when you use an snubber in primary you can use another one across the DS .
While calculating flyback voltage we use Vo = V1 (with single secondary), as the secondary voltage is reflected on the primary. But when multiple secondaries with say, V1 , V2 , V3 then what should be value of Vo?
Click to expand...
Don't forget that one of the out puts will have feedback path ! and of course higher current ! on the other hands you will have just one main out put ! of course currents will be Iv1+Iv2+Iv3+Ivn . and will be in primary with K ratio . but about voltages you should just consider highest voltage which will give you more currents .
Best Wishes
Goldsmith
 

WimRFP, Thanks for the words here.

you may need some snubber action close to the mosfet.
Click to expand...

A very good input which most designers might miss.


I have created a mathcad worksheet request you to review it.

Attached: Mathcad - Snubber DCM.pdf

- - - Updated - - -

Thank you Goldsmith.
 

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