mosfet BSS89

[jorjo]

hi every body.
In below circuit why bss89 mosfets breakdown?
at the first, this circuit worked properly,but after 30 minute when I switched power off & on
it didn't worked.
secondary mosfet breaked down.
it never would harmed.

 

[smanches]

First guess is the 21V at the gate of the second mosfet. Mosfets can usually only handle 20V MAXIMUM before the gate breaks down. 10V is the usual turn on voltage. Usually it's best not to go over 15V.

 

[MikeMl]

Putting on my teacher hat:

That circuit has several problems besides the voltage applied to the gate.

There is no snubber diode on the relay coil. That could blow the mosfet.

There is no current limiting resistor on the LED.

The relay driving the LED is totally not needed; why not just drive the LED directly from the FET?

Why not turn the FET into a constant-current driver for the LED?

I could go on and on...

 

[jorjo]

i should have 24 v , beuase it is needed for special purpose.
pls guide me how can i reduce voltage of second mosfet gate.
where does diode place?
that LED is only indicator for showing that circuit work correctly.
I attached my proteus file for currection.
pls help.

 

[jorjo]

Dear smanches
how can i reduce gate voltage ?
i attached my file.
pls currect and post it.
notice: i can't change BAT1"24 v" to another content.

 

[MikeMl]

i should have 24 v , beuase it is needed for special purpose.
pls guide me how can i reduce voltage of second mosfet gate.
where does diode place?
that LED is only indicator for showing that circuit work correctly.
I attached my proteus file for currection.
pls help.

I don't do Proteus. Look at the attached, especially V(g) and V(d2). Note that the input Fet must be a "logic level" Fet because of low gate drive. Your 100K pull-up on the gate of the second FET is way too high, resulting in slow gate switching.

 

[smanches]

You reduce the voltage by creating a separate power supply. When you are designing circuits, you need to figure out your power requirements first, and make sure you have a power supply for each voltage that is needed.

Don't try to just reduce the voltage, create a new 10V supply for driving the mosfets.