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    Power MOSFET as a switch

    Hi,

    How do I use a N channel power mosfet as a switch like we do with NPN BJTs?

    I mean the configuration I know but how do I calculate gate resistor, and so on?

    I'll use a CEB6030L (N-CHANNEL POWER MOSFET)very common in mother boards DC/DC converters to switch on and off a 55Wx6V halogen lamp in a special camping flash light.

    Regards,

    Fernando

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    Re: Power MOSFET as a switch

    It is absolutely easy and comfortable to use the power MOSFET as a DC switch as in your case, it must be much easier than using a BJT with the better characteristics.

    You need to calculate nothing, just care of the MOSFET specs itself. In most case, you need only a resistor of about 1M to 10M ohm between G and S (you may omit this resistor), a resistor of about 100 ohm to 1M ohm (meanings that you have a very wide range of free choice of resistors but always remember that these 2 resistors also make a voltage divider and your resulted Vgs may be lower than needed level).

    About the MOSFET specs, you should know about the maximum Vds, Vgs, Id, and maybe Rds ON. Of course, for n-channel type, you have to ensure that voltage at G must be more positive in respect to S to make MOSFET works.

    For most of MOSFET, Vgs = 5... 10V is good enough to make MOSFET open saturately. For logic-level MOSFET, it will work at logic-level of 5V, this type of MOSFET will have the letter L as the suffix of symbol, maybe your CEB6030L is that type, please check with its datasheet.

    With your simple use, you can pick out the appropriate circuit from MOSFET datasheet to use.

    Goodluck
    nguyennam



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    Re: Power MOSFET as a switch

    just tried this scheme in Proteus:




    •   Alt31st January 2007, 14:36

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    Power MOSFET as a switch

    hi,m using MOSFET as a switch in my circuit.m designing a SEPIC converter for photovoltaic application.the problem is ,MOSFET gets heated up very fast when i give pulse from PWM control circuit.any idea what could cause it ?



    •   Alt7th April 2010, 17:28

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    Re: Power MOSFET as a switch

    Hi, in your diagram above i can see that you are using AC as your supply, remember p=I*IR. you are to expect heat losses due to this factor,therefore you need a heat sink to minimise the losses and use mosfet with a low rds.



    •   Alt7th April 2010, 18:03

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    Re: Power MOSFET as a switch

    Quote Originally Posted by sron
    hi,m using MOSFET as a switch in my circuit.m designing a SEPIC converter for photovoltaic application.the problem is ,MOSFET gets heated up very fast when i give pulse from PWM control circuit.any idea what could cause it ?
    We would need to see the circuit to be able to help.

    Keith



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    Power MOSFET as a switch

    i am unable to upload the circuit....its SEPIC Converter...u can google search it...so so so sorry ...



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    Power MOSFET as a switch

    Sorry sron,
    is it not possible for you to sketch your circuit & scan it & attache to your comment/ topic-or its a nonpublic circuit pls!?
    You can draw the part of interest with a paint SW, & as png/jpg or pdf attache to your comment...
    K.



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    Re: Power MOSFET as a switch

    Quote Originally Posted by sron
    i am unable to upload the circuit....its SEPIC Converter...u can google search it...so so so sorry ...
    I know what a SEPIC converter is - I have built a few. But unless you can post a circuit of what YOU have built it is going to be impossible for anyone to help.

    Keith



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    Power MOSFET as a switch

    hello,i have attached the SEPIC converter design.my aim is to get a constant 9V output by adjusting the duty cycle.currently i am getting an output but its not stable,it keeps on increasing up to 23.4V den it reduces.The problem m facing is that i m unable to change the duty cycle when the output from the PIC microcontroller is connected to the gate of the MOSFET.on top of it the MOSFET gets heated up very fast.do i need to design separate MOSFET drive circuit?any idea how can i do dat ...m running out of time ....help needed ...

    Added after 53 seconds:

    http://images.elektroda.net/89_1270795681.jpg



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    Power MOSFET as a switch

    Are you trying to use a PIC to close the feedback loop? If so, that is probably why it doesn't work. I doubt the PIC has enough speed and processing power to execute a suitable feedback loop. That is why your output voltage overshoots and the transistor gets hot - it is unstable. I would suggest you use a SEPIC controller chip.

    Keith.
    I started life with nothing and I've still got most of it left. (Seasick Steve)



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    Power MOSFET as a switch

    ok,so i dont need a mosfet gave drive ?

    Added after 1 minutes:

    what is this SEPIC controller chip...i cant find any info on the net ...so sorry ...



  13. #13
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    Power MOSFET as a switch

    LT1308 is one I have used http://cds.linear.com/docs/Datasheet/1308i.pdf

    There are plenty around, but often they aren't specifically listed as SEPIC. The may just be listed as a boost converter then the details are in the data sheet. It makes it tricky to find them, but most boost converters can probably be connected as SEPIC - it doesn't really require anything extra from the controller, it is more about what you conenct to it and how.

    Keith.
    I started life with nothing and I've still got most of it left. (Seasick Steve)



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