Solving MOSFET Issues | 180 Ohm Resistor

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Discussion Overview

The discussion revolves around troubleshooting issues with a P-channel MOSFET used as a load switch in a circuit involving a 180 ohm resistor. Participants explore the behavior of the MOSFET during operation, particularly at the zero-crossing moment of the AC phase, and the implications of the gate voltage on current flow.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes difficulty in triggering the P-channel MOSFET to run current through the resistor, specifically at the zero-crossing moment of the AC phase.
  • Another participant confirms that the circuit keeps the PMOS on except for the zero-crossing moment, prompting questions about the underlying reasons.
  • A participant expresses confusion about why the P-channel MOSFET does not turn on with a positive voltage, suggesting that it requires a negative gate voltage relative to the source.
  • Concerns are raised about the small current (240uA) observed in simulations, with suggestions that the optocoupler may not be driving the PMOS gate sufficiently.
  • One participant humorously notes that grounding the gate did not change the current, and mentions achieving the proper current only when applying an extreme negative voltage.
  • A suggestion is made to check the specific PMOS model being used, as different models may have varying characteristics.
  • Another participant proposes that the gate threshold for the PMOS might be too high and recommends adjusting the voltage settings in the circuit.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the operation of the P-channel MOSFET, with some agreeing on the need for a negative gate voltage while others question the current levels and the role of the optocoupler. The discussion remains unresolved with multiple competing views on the issues presented.

Contextual Notes

Participants have not specified the exact PMOS model being used, which may affect the discussion. There are also unresolved questions regarding the gate threshold voltage and the adequacy of the optocoupler's output.

Raisintoe
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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.
Opto with MOSFET.jpg
 
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Your circuit turns on the PMOS at all times except for a small moment (at the zero-crossing moment of the AC phase).
 
Svein said:
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).
 
Raisintoe said:
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.
 
OK, that's confusing. My next question is, why is the current so small? What do I have to do to increase the current?
 
Raisintoe said:
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.
 
Svein said:
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!
 
It could be the gate threshold for the PMOS is too high. Try setting V2 to 10V and ground the gate.
 
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