| Electronics > Beginners |
| P-Channel Mosfet VGS |
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| Nikos A.:
Hi everyone learning about P-Channel Mosfets. Looking at the datasheet below, I can see that in order to turn the Mosfet on you have to apply -1V to -3V. I a little bit confused.. It says that the max Vgs should be -3 in order to turn ON (allowing 250mA drain current) but on figure 2 there are a variety of Vgs greater than -3V.. From figure 2 I understand that I could also supply Vgs = -10V without any problem.. Can anyone explain to me these details? What is the range of Vgs in order to turn the Mosfet ON and what is the ideal Vgs? https://eu.mouser.com/datasheet/2/308/FDD6685-1122838.pdf |
| Psi:
The mosfet is considered to be 'on' when the gate is at 3V The mosfet will be damaged if the gate voltage exceeds 25V |
| Nikos A.:
Hi, thank you for your answer. You mean -3V isn't it? |
| Siwastaja:
Hi, I very classical trap for the young players: VGS(th) is a completely meaningless number for switching applications, and is, in any case, varying all over the place. This number tells you where the boundary of not conducting "at all", and starting to conduct "a tiny bit" lies. For switching applications, you want to switch "fully". This information is available in the curve set: for example, the Vds vs Id curve, or the Rds vs Id curve set you posted is fine as well: look at your maximum worst case Id, and see if the resulting Rds(on) is good enough for your purpose. Note that Rds(on) also rises significantly with the die temperature. I typically multiple the datasheet room temp values by 1.5x for the first order estimate. Always calculate for power dissipation: P = I^2 * R, then look at the thermal resistance numbers; for example, 40 degC/W is reported on a typical PCB layout shown on page 4. This means that dissipating 1W increases the die temperature by 40 degrees C over the ambient. You need to understand that if the PCB sits in a closed box, for example, the "ambient" is the local ambient inside that box. |
| Psi:
yeah, I dont like using negative voltage convention for p channel gates, but yes, -3V. It's a bit confusing because they are trying to say the mosfet has a 3.3V logic level gate. But the more voltage you put into the gate the lower the on resistance because that's just how mosfets work. They have to draw the line somewhere and say Y Vgs and X Rdson is considered 'on' so they can claim it has a logic level gate. |
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