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?
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Look for Vgs diagram in datasheet.
This MosFet is absolute unsuitable for a gate voltage of 5V.
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.
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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