Electronic – How to calculate gate charge time for a mosfet

chargemosfetpower electronics

enter image description here

I am designing the circuit shown above using mosfet NDS355AN. MCU is 5 V.
However, I have become stuck as I am unsure of how to calculate the gate charge time of the mosfet. Hence what is the formula to calculate the charge time for the gate of the mosfet?

Is it just essentially tau = RC = 1500 * Ciss?

Note LED contains resistor inside (is a very large all in one)

Best Answer

The step by step time constants

enter image description here

I coloured zones for dynamic losses.

When using a driver with matched impedance to the gate resistance, Rg ;

  • During turn-on Ids almost peaks THEN Vds falls
  • During turn-off Vds almost peaks THEN Ids falls.
    • during these dynamic switching times
    • Pd= Vds*Ids is greatest.

As once written, seek and ye shall find. or re-Search, next time DIY.

From your datasheet...

spec: VGS = 4.5V, RGEN = 6 Ω You are driving gate with 1k5 instead of 6 Ω , so we use total Gate Charge equation and datasheet specs instead using worst case.

So effectively with a high source R you are driving with a current source Ig=Vgs/R then from $$ I_g = ΔQ_g/Δt_s , (ΔQ=Q) , Δt_s = Q_g/I_g = Q_gR_g/V_{gs} = 5nC_{max}\cdot 1k5/5V=1us$$

Yet if we simply used T=RC=1k5*195pf=293ns (typ) (0.3us) is only 30% typ of the more accurate worst case gate charge times.

The Miller plateau for a current source gate drive means that from C=Q/V is Vgs is flat and Q jumps to the right, that means C rises until Vds drops to minimum. This is a negative feedback effect.