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CPLD P-Mosfet Output Circuit - Will it Work?

oldmanraskers

Could someone please offer their opinion on whether my circuit would work well for turning on an output via a CPLD. The output voltage at PIN0 will be 3.3V. I want logic 1 to turn on my Q1.

Regards, oldmanraskers.

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    In addition to the comments from others I'll add these:

     

    1)      When the CPLD powers up or is reset  the output pin will have a default state - you must decide if you want the power switch to be on or off at this point and design the biasing accordingly.

     

    2)     I'm not sure why you are using an opto isolator to drive the MOSFET - is this really necessary, what is the load you are switching  ?

     

    3)     Is additional protection of the switching device required, like inductive spike suppression or current limiting ?

     

    MK

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  • 0

    In addition to the comments from others I'll add these:

     

    1)      When the CPLD powers up or is reset  the output pin will have a default state - you must decide if you want the power switch to be on or off at this point and design the biasing accordingly.

     

    2)     I'm not sure why you are using an opto isolator to drive the MOSFET - is this really necessary, what is the load you are switching  ?

     

    3)     Is additional protection of the switching device required, like inductive spike suppression or current limiting ?

     

    MK

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  • 0 in reply to michaelkellett

    I'm using the opto because I'm experimenting with stuff, I guess I could use a standard NPN transistor - I am using the circuit to drive small solenoids that will eventually form a lock - they draw about 240mA. It appears to me now, that I have my FET in upside down, and in regards to spike suppression I will need some kind of flywheel diode I think.

    Regards, oldmanraskers.

  • 0 in reply to oldmanraskers

    oldmanraskers wrote:

     

    I'm using the opto because I'm experimenting with stuff, I guess I could use a standard NPN transistor - I am using the circuit to drive small solenoids that will eventually form a lock - they draw about 240mA. It appears to me now, that I have my FET in upside down, and in regards to spike suppression I will need some kind of flywheel diode I think.

    Regards, oldmanraskers.

    For driving that kind of load, I would suggest using NPN Darlington pairs, such as the TI ULN2003A or similar.  Those can sink a lot of current and have built-in flyback diodes.

     

    To get a lot of current through a FET, you need a gate voltage that's higher (or lower for PFET) than your normal logic voltages.  For example, you get a small IC called a "high side switch" that uses a charge pump to boost the an NFET's gate voltage.