Do MOSFET datasheets usually specify the switching speed? - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: August 19, 2019, 06:37:25 pm
content: This one:https://www.diodes.com/assets/Datasheets/ds30937.pdfsays "fast switching speed" in the features, but I don't see any switching speed listed in electrical characteristics. The only one related to time is "Forward Transfer Admittance".

author: TimNJ
author_rank: Super Contributor
timestamp: August 19, 2019, 06:49:52 pm
content: You can switch a MOSFET at any speed that you'd like...but more precisely, up to the maximum speed as dictated by the MOSFET'sparasitic capacitance.A MOSFET has a few intrinsic capacitances, as byproducts of the way they are constructed. The higher those capacitances, the slower the MOSFET turns on and off, for a given drive signal. As a simple model, think of the gate of a MOSFET as a capacitor (CISS on the datasheet). To change the voltage across that capacitance enough to reach the gate-threshold voltage, it will take some finite amount of time.Depends what you are doing, but high capacitance MOSFETs can incur big switching losses.

author: magic
author_rank: Super Contributor
timestamp: August 19, 2019, 08:12:05 pm
content: I have no idea how fast it switches, it's a function of gate to source capacitance, gate to drain capacitance, drain voltage and also internal gate resistance plus any external resistance.Otoh, transfer admittance is not related to speed and the unit of ms is a mistake. It should be mS, which is mA/V. That's how much drain current increases per one volt increase of gate voltage, in some stated test conditions.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 19, 2019, 08:21:50 pm
content: Datasheets don't provide numbers, because basically MOSFETs are not switches, they are transistors.To use them as switches, you need additional stuff to drive them. Switching speed can only be determined with what's around the MOSFET (how you drive it, the characteristics of the load...)In the DS you linked, the two following parameters: "Low Input Capacitance" and "Low Gate Threshold Voltage" are indicators that it will be easier to get fast switching speed if you use it as a switch, but that's pretty much all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2019, 08:40:22 pm
content: Yes, they often do; but it's not very useful. There are also some quirks, like Fairchild datasheets often preferring R_G = 25Ω and having rather long (50, 100+ ns) times as a result.The relevant factors are Rg (internal gate spreading resistance) and Qg (total gate charge). Most transistors have small Rg to begin with (some exceptions being older types like 2N7002, which vary between manufacturers), so the dominant factor is the R_G (generator resistance) you drive it with.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2019, 02:04:53 am
content: T3sl4co1l basically said everything I might have.If you knew the equivalent resistance in series with the gate, and at higher speeds the equivalent inductance, then you could calculate the switching times from the gate threshold voltage, transconductance, input capacitance, reverse transfer capacitance, and drain voltage. But in most applications the resistance or current from the driver will limit performance before the gate resistance and inductance so the gate resistance and inductance can be considered zero.It is interesting to design a test circuit for measuring the switching characteristics but in practice, most people can get by with the MOSFET driver of their choice and measuring the result.