Timeline for Is there an error in the dissipation calculation of a mosfet?
Current License: CC BY-SA 4.0
11 events
| when toggle format | what | by | license | comment | |
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| Sep 29 at 14:32 | comment | added | hennep | I think the difference is in the rounding when LTspice calculates the average. It is too small to be of concern | |
| Sep 29 at 13:11 | comment | added | Hearth | @hennep The source current should be exactly equal to gate current plus drain current; if there's a difference, it's either a problem with your model or a numerical error. | |
| Sep 29 at 12:22 | comment | added | hennep | I would like to see I(drain)*( U(drain)-U(source) ). Now I(gate) is used twice, in I(gate)*U(gate) and it is also part of I(source)*U(source). I ran a simulation to check that, My schematic shows a difference in the average current between I(source) and the sum of I(gate)+(Idrain), of only a few nA. Possibly it will be impossible to calculate it exactly right because we don't know all the properties of a mosfet. | |
| Sep 29 at 12:22 | comment | added | hennep | I see that there is a current going into the "capacitor" of the gate and that a large part of that is returned. The difference in power going in and going out is probably caused by resistance of the gate circuitry. Also my plan to abs() the source current was wrong but also the calculation of power in LTspice is not accurate. The way LTspice would calculate dissipation in your right simplified schematic above, that would be: I(R2)*U(C)+I(R2)*U(D). That would be correct in this simplified case but in case of a mosfet where the source current is the sum of drain current and gate current. | |
| Sep 28 at 16:52 | comment | added | Hearth | @hennep Is that better? | |
| Sep 28 at 13:42 | history | edited | Hearth | CC BY-SA 4.0 |
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| Sep 28 at 13:04 | comment | added | Hearth | @hennep Returning power from the gate capacitance to the circuit means that the transistor's gate is sourcing power, the capacitor is discharging. This energy is not dissipated within the MOSFET, but it was counted earlier when the gate charged up. You can think of this returned energy as correcting an earlier miscount (counting stored energy in addition to dissipated energy), if you want. || Σₙ V(n)*Ix(n) is a general formula that correctly calculates the power dissipated in any device; if this is your issue, give me a moment and I'll expand on this answer. | |
| Sep 28 at 12:53 | comment | added | hennep | What I meant was, returning power from the gate charge to the circuit lowers the power consumed by the circuit, not the mosfet. Not the complete gate charge is dissipated in the mosfet. A part of the dissipation that is lost in the gate depends on the resistance of the bond wire/internal gate circuit. I think that V(G)*Ix(U1:G) is not correct. Also in V(D)*Ix(U1:D)+V(S)*Ix(U1:S) the gate current is part of Ix(U1:S), so it is calculated twice. A better way would be Ix(U1:D)*(V(d)-V(s)) for the drain-source part. | |
| Sep 27 at 13:51 | comment | added | Hearth | @hennep I'm not sure what you're asking, honestly. | |
| Sep 27 at 12:20 | comment | added | hennep | Wouldn't that mean a lower overall dissipation of the circuit, but still extra dissipation in the mosfet? | |
| Sep 27 at 11:59 | history | answered | Hearth | CC BY-SA 4.0 |