Mosfet replacing, Gate-Source Threshold Voltage?

[goodpsusearch]

Does it need to be the same value?

Take for example an SMPS that uses this mosfet:

https://cdn.badcaps-static.com/pdfs/...9c266ad419.pdf

Gate threshold voltage VGS(th): min:3 typ:3.75 max:4.5 V

If I replace it with a mosfet like that:



Gate-Source Threshold Voltage VGS(th): min:2.0 - max:4.0 V

it wont work?

What will happen? Will it blow?

[paulstef]

Re: Mosfet replacing, Gate-Source Threshold Voltage?

It depends very much on how this is driven.

Vgs (th) is also always given with a corresponding current, usually very low. Meaning this is where it juuuust starts to conduct.

Now in an SMPS you will probably not want to drive a MOSFET in the linear region, meaning in order to turn it fully on you will need at least 10V VGS (unless it's a logic level MOSFET).

You will see that the power supply of most gate drivers and therefore the voltage on the gate drive output will be 12V or more.

Then , in a gate driver or smps controller chip the gate drive output actually consists of a two transistors in a push-pull configuration. That means the output is either ON or OFF, near 0V or VCC of the chip.

In short, I would not worry to much about gate threshold voltages (unless they are VERY low). Important is continuous and peak current and of course VDSmax.

[budm]

Re: Mosfet replacing, Gate-Source Threshold Voltage?

The sub does not have Gate protection Zener so unless the circuit you are putting in has one.
The input capacitance of the original is 1370pF, the replacement has 2300pF so it will not turn on as fast due to higher capacitance and also will not turn off as fast also due to the higher capacitance, so hopefully the Gate discharge R-D network is there to make sure that the MOSFET will be off when it needs to be off. If it is used in the bridge drive circuit, that can be real bad problem.

[goodpsusearch]

Re: Mosfet replacing, Gate-Source Threshold Voltage?

Thank you for your input!

After reading carefully your replies, I think it would be better to replace the original mosfet: https://cdn.badcaps-static.com/pdfs/...9c266ad419.pdf


with this:

https://cdn.badcaps-static.com/pdfs/...6a3e8e0dfb.pdf

The STP10NK60Z had something glued on its Source leg:

https://www.badcaps.net/forum/attach...1&d=1429967707

Can I just put it on the STW12NK80Z?

[ben7]

Re: Mosfet replacing, Gate-Source Threshold Voltage?

Yes you can put it on there if it will fit. It's just a ferrite bead, but you should put in on there, since it was probably put there for a reason (to reduce ringing?).

Your replacement choice has slightly slower turn on/off characteristics (some are faster than the original as well - interesting) and higher gate charge/capacitance. I have no clue if it is big enough of a difference to cause an issue ... as budm said it could.

[goodpsusearch]

Re: Mosfet replacing, Gate-Source Threshold Voltage?

Thank you very much! I replaced it and it works!!!!

I used mica instead of the original silicon pad. I hope it is ok.

[Longbow]

Re: Mosfet replacing, Gate-Source Threshold Voltage?

Differences in the G-S threshold voltage is not a big deal, since the gate drive will usually be a pulse or step function. The reason to stay with the original part is due to the parasitic capacitance in that MOSFET. The circuit design provides for charging a certain amount of capacitance as seen whilst looking into the gate. Different devices switch at different rates and have differing "on" resistance both of which can affect the heat generated by it. Also the D-S breakdown voltage will be important in a flyback switching arrangement, which is compensated for a particular device. Another MOSFET may work fine - for a little while. At least, that's what usually happens. The manufacturer has already blown up a number of MOSFETs to find the right one. Take their word for it!