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[SOLVED] Suggestions for converting BJT Ib value to equivalent MOSFET Qg value

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d123

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Hi,

I want to use a BJT instead of a MOSFET in a DC-DC converter circuit, the type is even described as suitable for DC-DC due to its very low Vce(sat) in the datasheet blurb at the top. First need to see how well or not it fits into circuit with the maths. Also want to use same BJT to replace diode in converter circuit as Vf of BJT is ~150mV, Schottky is ~300mV, see this is possible and standard practice, synchronous rectifier it was called, if I remember term correctly.

Anyway, trying to plug the BJT into the formula for switch PD requires MOSFET Qg as one of the values - see attached image for formula.

Does anyone know if, as Coulumbs = current multiplied by time, it will be valid to change Qg/t = IG into IG * t = Qg > Ib * t = ~nC equivalent value as an approximate but useful/valid number to plug in to the PD formula?

If not, any suggestions on how to turn Ib and delay + rise time (or whatever is appropriate) into an equialent of MOSFET Qg?

Thanks.

Turn BJT turn-on requirements to MOSFET Qg and IG.jpg
 
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There are several terms which are part of Qgg. The
Miller Qdg, the channel Qgs+Qgb primarily.

Qdg is probably close enough to Qcb (Ccb*Vcb(off-on)).
Ccb is nonlinear but maybe "close enough for now"; an
integral of C(dV)*V would be a bit better.

Qgs+Qgb might be expressed by Ib*t(on).

When you say Vf=150mV, that might be true for the
Vce(sat) but the third terminal wants some controlling
current to "make it so". It's not a 2-terminal replacement
diode-for-diode.
 
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    d123

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Hi,

There are several terms which are part of Qgg. The
Miller Qdg, the channel Qgs+Qgb primarily.

Qdg is probably close enough to Qcb (Ccb*Vcb(off-on)).
Ccb is nonlinear but maybe "close enough for now"; an
integral of C(dV)*V would be a bit better.

Qgs+Qgb might be expressed by Ib*t(on).

That's super, thank you.

When you say Vf=150mV, that might be true for the
Vce(sat) but the third terminal wants some controlling
current to "make it so". It's not a 2-terminal replacement diode-for-diode.

Yes, thanks, I did mean controlled turn on and turn off to fit circuit function, not as a diode-connected BJT :).

Thanks for the formulas and explanation, what I found so far today reading was all simplistic BJT switching descriptions or device design-level highly detailed equations about base width and so on that didn't help/was too complicated.
 

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