Solving MOSFET Issues | 180 Ohm Resistor

[Raisintoe]

I am new to using MOSFET transistors as load switches. Here, I am trying to trigger the P-channel MOSFET to run a current through the 180 ohm resistor. I can not figure out what I am doing wrong. It is supposed to turn on for a small moment (at the zero-crossing moment of the AC phase), then turn back off.

 

[Svein]

Your circuit turns on the PMOS at all times except for a small moment (at the zero-crossing moment of the AC phase).

 

[Raisintoe]

Your circuit turns on the PMOS at all times except for a small moment (at the zero-crossing moment of the AC phase).

I know it does, but why is it doing that? It is a p-channel MOSFET, why doesn't it turn on when a positive voltage is present? Also, when it does turn on, it has a very small current, (240uA in this LTSpice simulation).

 

[Svein]

It is a p-channel MOSFET, why doesn't it turn on when a positive voltage is present?

A p-channel MOSFET turns on when the gate is negative relative to the source. You need to invert the driving signal.

 

[Raisintoe]

OK, that's confusing. My next question is, why is the current so small? What do I have to do to increase the current?

 

[Svein]

OK, that's confusing. My next question is, why is the current so small? What do I have to do to increase the current?

I do not think the optocoupler is able to drive the PMOS gate as far down as necessary. You need to amplify and invert the output of the optocoupler.

 

[Raisintoe]

I do not think the optocoupler is able to drive the PMOS gate as far down as necessary. You need to amplify and invert the output of the optocoupler.

Hmmm, I tied the gate directly to ground, and that didn't change the current through the 180 ohm resistor. I finally get the proper current (27.8mA) when I set the gate to negative 1 million volts!

 

[Svein]

You have not specified what PMOS you are using. I checked just one supplier and found several possibilities. One of them is here: http://www.onsemi.com/PowerSolutions/product.do?id=MCH3374.

 

[meBigGuy]

It could be the gate threshold for the PMOS is too high. Try setting V2 to 10V and ground the gate.