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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.

von fun with magic smoke (Guest)


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To switch the MOSFET you need to charge/discharge its gate. For 
simplicity, think of it as a capacitor.
Use the parameter "Total gate charge" in the MOSFET datasheet, which is 
56nC for 10V. That is a lot, meaning your MOFET needs a lot of current 
to switch fast.

Your rising edge is limited by the current driving capability of your 
photo-coupler. There is a ratio "CTR" (=current transfer ratio), which 
limits the photo-transistor current of your photo-coupler, depending on 
how much current the photo-coupler LED gets.
Your photo-coupler has a CTR of 50 to 600%.

your falling edge is limited by the time constant of Cgate and the 10k 
resistor (which is in the µs-range).

As others mentioned bevore, your MOSFET is not usable for 5V gate 
voltage.
Don not use the gate threshold as indication for your gate drive 
voltage. Instead, look for the voltage the Rdson is rated. In this case, 
this would be 10V.

For 5V, your RDSon will be terrible. Switching on will be very slow, 
since the photocoupler transistor also has an saturation voltage you 
will not get 5V anyway. Which might be the problem for your rising edge 
in the ms-range.

to sum it up:
With this type of MOSFET, you may want to use gate driver of some sort 
and a higher gate drive voltage, e.g. 10V.
You may be able to use a logic level Mosfet without gate driver, 
depending on your application.

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