Fet for turning on high voltage.

[HenrikOlsson]

Hi,
Thank you for posting the schematic Ioannis, that's the circuit I tried to describe earlier.

Looking at the datasheet for the P-channel MOSFET we can see that it has a specified gate threshold of -2 to -4V (ie it starts to conduct at a Vgs of -2 to -4V) and a maximum Vgs of +/-20V. In this case simply grounding the gate of the P-channel thru the N-channel switch would have put 24V between the gate and the source of the P-channel MOSFET which is out of spec for it - the voltage divider takes care of that.

When the N-channel MOSFET is "off" the gate of the P-channel is pulled up to 24V by the 1k resistor, the P-channel MOSFET is off. When the N-channel MOSFET turns on it grounds the lower side of the voltage divider which makes the voltage at the gate of the P-channel MOSFET ~7.5V. 7.5V at the gate minus 24V at the source gives a Vgs of -16V which is well above the threshold yet within the +/-20V maximum.

Obviosuly, using a 74A MOSFET to switch 100mA is overkill but the principle of switching the high side is the still same. Using a solid state relay as suggested or perhaps even an optoisolator capable of supplying the current would of course work nicely too.

/Henrik.

[Ioannis]

Hi Henrik.

You are right about the power mosfet, but the schematic was ready from another project. I mentioned that, so one can of course select a lower amp one.

I just needed the low Rds for my project. P-Mosfets with low Rds, are hard to find.

The idea of optocoupler is great! Vishay SFH619A is 125mA capable and sure there must be stronger devices.

Ioannis

[jmgelba]

Thanks for the responses guys. Good stuff. This is what I tried in simulation and it works. The NFET used will be a STN3NF06.

[HenrikOlsson]

Hi,
If I'm looking at the correct datasheet the SI4401 P-channel MOSFET has an absoulte maximum Vgs rating +-/20V. If wired as in your schematic the MOSFET will see 24V (or 45V if that's the supply voltage) between the gate and the source which is above its rating - in real life it will likely self destruct even though it doesn't in the virtual world.

You need a second resistor between the gate and the collector to form a voltage divider as in Ioannis schematic.

/Henrik.

[jmgelba]

Henrik, Im not using the SI4401 in the schematic, it was a random device picked from a list of available parts.