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Forum: Analog Circuits What is the reason for the slow switching speed of mos?


von Sunny Y. (Company: utsource) (sunny2020)


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The single-chip microcomputer drives the IO port, and the reaction time 
of the top and bottom edges of the optocoupler 3 pins is basically 
within 1ms, but the top and bottom edges of the mos tube switching 
waveform are basically about 40ms. The drain in the figure has been 
connected to the +KM and 10K resistors through the wiring terminals. The 
switching response time Why is it so long?

: Moved by Moderator
von Abdul K. (ehydra)


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Look for Vgs diagram in datasheet.

von Andreas B. (bitverdreher)


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This MosFet is absolute unsuitable for a gate voltage of 5V.

von W.S. (Guest)


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Sunny Y. schrieb:
> Why is it so long?

Please keep im mind, that the gate of the FET is good insulated, so it 
does not consume much current while it is held in a constant state, but 
on the other hand it has a remarkable capacity to source and do drain. 
Therefore to drive the gate from one voltage to another, it needs a 
rather big amount of current to do it fast. When your driving circuit 
cannot deliver this current, your switching speed will be slow. So the 
best is to use a dedicated gate driver. Such a chip is capable to 
deliver a suitable current to the gate to speed up the switching. For 
example: the Tc4420 can deliver up to more than 1 ampere when driving 
L->H and up to 6 ampere when driving H->L (both just only for a short 
time).

W.S.

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