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Terrible Idea Boost Converter To Be Tested On Gocart
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i_am_fubar:
Very fair point on the fast turn off. Hadn't really considered that turn on is much less critical.

Inductor wise, I'm hoping these Wurth ones do the job, the spec is very tidy looking, but I'm also expecting more leakage than on the toroidal one I wound.

I wish I had an Iso probe. Just measuring the ground, that was also the ground point I was grounded to, is giving me high levels of pickup :(

Am going to build up another boost phase over the weekend and play around with different snubber arrangements, decoupling options, slew rate control.... etc. Drive it from Sig-gen as opposed to PIC so I can have a more consistent drive signal.

Got my battery isolator, meter & fuse box on the go to make it a bit safer and be able to operate the whole thing floating.


Oh, and I'm slowly relenting..... 10Ah 22.2v Lipo may be a better option for me :p


T3sl4co1l:
So what, ballpark 20nH there?  Mostly from lead length.  And Coss around 1nF?  Resonates at 35MHz and impedance 4.5 ohms.  So, if turnoff is faster than about 30ns, peak voltage will be noticeable.

Note that the part of turn-off below Miller plateau is what's relevant: Vds has stopped moving (the diode has turned on) and current falls (until this moment, the transistor is still carrying all the inductor current).  So this can still turn off fairly quickly even if your gate resistor is relatively large.

The maximum overshoot is delta V = I_pk * Zo.  So it's pretty significant that the impedance is quite high, compared to the switching impedance (which if it's say 100V and 50A peak or about 2 ohms).  Preferably it should be the other way around (Zo < Zsw).

No, there's no good way to bring it down -- you could load the drain with some capacitance, but for one it's not going to be any "closer" to the transistor itself (there's still the 10nH or so of the TO-247 package), and it's only going to increase losses (namely at turn-on).  Increasing L or C only increases the reactive power and thus switching loss.

Note, no source resistor, or current clamp on the inductor, is a disaster waiting to happen.

Tim
i_am_fubar:
Need to have a proper read through this reply when I've got the thing in-front of me.

For reference here, what do you mean by source resistor or current clamp? (in this instance).

Cheers.
T3sl4co1l:
Anything to sense switch or inductor current.  So, a source shunt resistor, or one referenced to the input (positive or ground, as, and if, applicable).  Or a current sensor, like a Hall effect chip or clamp.  Or etc.

Tim
i_am_fubar:
Ahhh, I've got a separate 100A hall sensor on another PCB that also generates the PWM. It's shared across phases but should be able to detect OC and failures to force shutdown.

Ditto a 12A floating shunt on the output, shared across.
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