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# MOSFET ON time

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#### abhishek.2138

##### Full Member level 2
To calculate MOSFET turn ON time do we consider (rise time + turn ON delay) or only rise time?
silmilar for OFF time = Fall time + turn OFF delay?
What exact differnce between rise time & turn ON delay for MOSFETs?

Hi,

To my understanding, yes. There is a delay while the MOSFET capacitance is charged until the device turns on (gets enough energy to make electrons and holes start to bounce around and cross over the (band)gap, so to speak) and then also the time for the output to go from e.g. '0' to '1' or '1' to '0', like charging or discharging another capacitor.

Another loose, inaccurate, but hopefully comprehensible way, of seeing it: Gate is one RC network, Source-Drain is the other RC network.

Have a search for MOSFET gate drive and such like, many documents will explain it in more or less complicated way depending on if written at component user level or transistor designer level.

### abhishek.2138

Points: 2
Thats right....the delay bit, is where there is not yet any switching loss.
Laszlo Balogh, in his doc about "high speed mosfet driving", speaks more on this.

### abhishek.2138

Points: 2
The on time is when the Vds is ~ < 1% of Vds max, plus any current pick up time just before the Vds falls,

the off time is all other times.

To calculate MOSFET turn ON time do we consider (rise time + turn ON delay) or only rise time?
silmilar for OFF time = Fall time + turn OFF delay?
What exact differnce between rise time & turn ON delay for MOSFETs?
For MOSFET switching characteristics -

MOSFET total turn ON time = Rise time + Turn ON delay time + Time delay due to external gate resistors
MOSFET peak gate source current = Total gate charge / Total turn ON time
Average MOSFET gate drive power = Average voltage * Average current

Are above calculations correct?

If gate voltage = 15V, MOSFET total turn ON time = 100 ns, gate drive on time = 4.8 us, gate drive OFF time = 5.2 us, then how to find average and RMS gate voltage?

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