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LTC3786 3V to 18V boost converter

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Siwastaja:

--- Quote from: Hamed_ta on November 16, 2019, 12:31:24 pm ---
--- Quote from: Miti on November 14, 2019, 04:20:22 pm ---You may be dangerously close to the Vdss max of the power transistors. It is only 20V.

--- End quote ---

yes thank you but

I'm going to limit the output voltage to 18 volt do you think still it can be dangerous ?

--- End quote ---

No matter how perfect your layout will be, you will have parasitic ringing on the switch node which will exceed the 20V rating. By how much? Hard to say. As some kind of rule of thumb, a very well laid out converted might achieve 15-20% overshoot voltage, while a decent one gets around 40%, and a poor beginner project could closer to 100% overshoot.

I strongly recommend on designing so that the ringing won't exceed the Vds rating, ever.

Still, it may work: it depends on the avalanche rating of the MOSFETs, and the ringing energy in the LC parasitics. This energy is now converted as extra heat in the avalanching MOSFET. You will see it in the scope trace as a flattening SW node voltage somewhere around 22-23V or whatever the avalanching voltage of your 20V MOSFET is.

It's easier to analyze when you don't hit the avalanche rating. You don't necessarily need to add much extra voltage headroom, but do leave a little. Then the avalanche capability of the MOSFET works as your safety margin. Use a high bandwidth scope (100MHz as a very bare minimum, preferably more) to look for the maximum voltage of the SW node.

An RC snubber may be used to reduce the overshoot, but a properly sized MOSFET that can handle the overshoot is likely simpler.

Vds max = 25V would be a good starting point, and even that doesn't have much excess, only some 38%.

Hamed_ta:
what does it mean ?

Hamed_ta:
it is designed to work with only single cell batteries to avoiding balance charging and discharging trouble.

Hamed_ta:
yes good point, thank you I will change the Mosfets, so there isn't any other problem in design and layout ?

Hamed_ta:
I was thinking of adding precise output current adjustment, is it possible you think by just adding another voltage divider in the circuit of inductor current sensing to be able to controll inductor peak current to adjust output current precisely?

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