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I'm currently using N-channel MOSFET's my hobby circuit, and have had two go out on me. As a result, I'm studying them a little more and making sure I'm not doing anything that might harm them.

On this product page, the description of the item says

"The threshhold voltage is also very low, less than 2.5V so you can control it directly from a microcontroller running on 2.8V, 3.3V or 5V logic."

However, on the datasheet, V_gs(th) is listed as 1.35V min, 2.35V max.

Am I interpreting the data sheet correctly? If not, what exactly does this rating represent?

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  • \$\begingroup\$ Your MOSFETs may be dying due to static issues, or forgetting the bypass diode on a motor driver. \$\endgroup\$ – pjc50 Nov 19 '12 at 9:54
  • \$\begingroup\$ @pjc50, you're right; I'm using an LED for the bypass diode, and it appears to be lighting up the mosfet is turned off, so that's good. I only have two and I've messed around with them so much there could be a number of things I've done to them. A multimeter diode clearly shows that both are either duds or broken. I have a pack of 10 coming that I'll be more careful with! \$\endgroup\$ – cemulate Nov 19 '12 at 10:25
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The two descriptions do not contradict each other, but some explanation of Vgs(th) may be useful.

  • Vgs(th) of MOSFET devices is the voltage at its Gate, at which the device barely begins to switch on - this is not the voltage to drive the Gate at, if the device is to be used in its fully conducting, therefore low dissipation "on" mode.
  • The "fully effective" Vgs voltage for a low Rds(on) is much higher, and is often not explicitly mentioned in many MOSFET datasheets. Note that this particular datasheet states "Very Low RDS(on) at 4.5V VGS", so that should be the minimum gate drive voltage to aim for.
  • The minimum and maximum Vgs(th) ratings reflect the fact that there is often a large variation in threshold voltage of MOSFETs in manufacturing, even within parts from the same batch. The range indicates the best case and worst case Gate voltage for a given product, at which the device barely begins to conduct. This "maximum" is not the maximum voltage that can and should be applied to the Gate.
  • The maximum Vgs the part can tolerate is separately listed in the Absolute Maximum Ratings, ± 20 Volts for this datasheet.

MOSFET datasheets are often confusing for this reason, that the ratings provided do not highlight what voltage range is ideal to drive the gate at: "Vgs for good conduction: 4.5 Volts to 20 Volts" would have been a very useful line for this datasheet, for instance.

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  • \$\begingroup\$ Thank you very much for clarifying that for me. It makes perfect sense now. \$\endgroup\$ – cemulate Nov 19 '12 at 10:26

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