Help with optocoupler mct2e !!

Discussion in 'The Projects Forum' started by thebenman, Mar 27, 2014.

  1. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    Hey,
    I am trying to implement a DC-DC converter and there seems to be a problem with the circuit. Bascially,i checked the PWM from PIC and it seems to be fine.However i was wondering if it was normal to connect the 1k resistors with the optocoupler.I was browsing when i noticed the most of you had suggested lower values for resistance in similar cases.Please go through the attached schematic :)

    Regards

    Bennet
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Hi,
    Using a 5Vs PIC supply and the Vfwd of the opto emitter is 1.1V means you have only
    5v - 1.1v = 3.9v to drive the emitter resistor.
    The opto requires 10mA, so the Rser= 3.9/0.01 =390R.

    The 1K is too high a value.

    For a faster opto response you could run at 20mA

    E
     
    thebenman likes this.
  3. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    Hey,

    Thanks for the quick reply.So a 390 ohm resistance should suffice ? and is the 1k resistance between the emitter and gate of the mosfet too high as well ?
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    If I am reading your circuit correctly, you appear to have the MOS Gate connected directly to 0V via the 1K.!! How will this control the Gate.?

    E
     
  5. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    I believe that the PWM switches on the led and that in turns drives the transistor in definite pulses .So,when the tansistor is off the emitter is at ground potential and when the transistor is high the emitter output is given to the MOS gate.Correct me if i am wrong :)
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    The drawing shows that the Gate never goes High its directly to 0V via the 1K.

    The 12V is also shorted to 0V when the opto is On.

    Some of +V sources are set to 0V.

    Is this only a sim design.?

    E
     
    Last edited: Mar 27, 2014
  7. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    hey,
    Oh i see the problem now...anyway will not connecting the emitter to ground trigger the MOS Gate as per the PWM signal ?
     
  8. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    Actually i have fabricated the entire circuit in PCB :(
     
  9. ericgibbs

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    Jan 29, 2010
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    If the opto emitter output is not connected to 0V, what do you think will happen at the MOS Gate when the opto is switched OFF.??

    The opto out to Gate needs to be redesigned.

    EDIT:
    Its important in a MOS FET inductive switcher that the Gate be driven Hard On and Off
     
  10. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    The circuit actually is to implement ZCS in the converter.Anyways when i measured the voltage across pin 4 and pin 5 of the optocoupler MCT2e it showed 12 volts.Do you think that it might work if i change the series resistance to around 390 ohms ?
     
  11. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Its not the resistor value thats the problem at the moment.

    When the opto output is switched ON ,the Gate will be pulled towards +12V, but when switched OFF the Gate will be 'floating', so there is a possibility that the FET will not switch OFF.

    You Must actively 'drive' the Gate ON and OFF, ie: pull the Gate up to +12V and pull it down to 0V to remove the 'charge' on the Gate.
     
  12. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    hey,
    Any suggestions on how ensure the the mosfet is either on or off and not floating in there ? As of now since i have made the PCB i would not be able to change it too much :D
     
  13. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    I would guess that you have written the program for the PIC and that a Hi on a pin drives the opto ON and that you expected the opto output to go Hi to +12V, to switch the FET On.??

    Also you intended the use the opto as voltage level shifter.??

    I would not have used a opto for this, a simple transistor would be my choice or a 74LS07 non inverting buffer.

    Are you prepared to modify your circuit.?
     
  14. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    hey,
    Since i have gotten the PCB done it would be difficult to make any major changes as you have suggested :) Anyway do you think it would be a good idea to change the series resistance to around 390 ohms and try-like you had suggested earlier ?
     
  15. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    The only suggestion I have made that would help you is changing the 1K from the PIC to Opto driver, down to 390R.

    Removing the 0V connection on the opto output would still mean the Gate would be 'floating' when the opto is Off...
     
  16. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    Thanks a lot,i will try that out and get back to you :) and just to be clear-there is no point in changing the 1k resistor from opto to the MOS Gate right ?
     
  17. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    I would only reduce the value from 1K to say 100R IF the driver stage was correctly designed.

    Sorry if I sound negative about this,;) but you will not get the performance you require from the present design of the driver.
     
  18. Experimentonomen

    Member

    Feb 16, 2011
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    I would use a proper mosfet gate driver, like the IR2111.
     
  19. thebenman

    Thread Starter New Member

    Jan 26, 2014
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    thanks for the help,since I am on a very tight schedule I would not be able to implement your suggestion.Anyway,I would try to change the series resistor to 390R and the MOS gate side resistor to 100R and try the output as it is pretty much the only thing I can do.

    Regards

    Bennet
     
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