Help on MOSFET and buck converter

Discussion in 'The Projects Forum' started by MatheusLPS, Jul 21, 2011.

  1. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
    34
    0
    Lets slowly....So, I made this circuit:


    [​IMG]



    It is working but not has I want.


    Now I can thrigger the FET with 5V, but let me show something:
    This is the input of 5V and the output with 1Khz:


    [​IMG]



    Hum.. It is fine, hun? Ok, let me show now with a 10Khz 5V input:


    [​IMG]



    Still working...


    But now with 50Khz, something strange starting to occur:


    [​IMG]



    Here you can see better:


    [​IMG]



    And finally with 400 or 300Khz:


    [​IMG]


    The problem is that the mosfet is still conducting even when I put 0V on the gate. This is taking some time to "discharge" the energy on the gate.


    I ploted some cicles of the input to see the voltage on the gate:

    5V on the lower transistor and 300Khz:

    http://img.photobucket.com/albums/v222/ahhh/Eletronica/vg.jpg

    Why? This is causing my problems now.

    The project that I using is this one:


    http://www.bineco.dk/ellert/smps.htm


    The project is not in english, but I can see the circuti and interpret it...

    Hope some help.

    Thanks again.

     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You are using a TIP122 for Q2 and a TIP127 for Q3, which are far too slow for this application. The author even said that about the TIP121 and TIP126 on his site; if you use Google Translate to go from Danish to English you will see that.
    http://translate.google.com/transla...&tl=en&u=http://www.bineco.dk/ellert/smps.htm

    Those TIP122 and TIP127 transistors are pretty much out of gain by 1MHz. Non-Darlington transistors might have a gbw of 300MHz or more; your mileage will vary.

    If you want to be serious about this circuit, you will find a much better MOSFET driver.

    Using a P-channel MOSFET might seem more convenient, but P-channel gates are ~2.5x as large as equivalent N-ch MOSFETs, thus requiring far more current to charge/discharge the gate as quickly as a comparable N-ch.
     
  3. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
    34
    0
    Thanks for your reply.

    I changed the Transistores to a 2n2222 and to 2n2907.

    But the result was the same.

    Do you have any driver made with discrete components?

    Consider that I need to trigger with 5V because I will use a PIC on my simple buck circuit.

    bye
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The regulator the Danish fellow was using had an internal oscillator of 52kHz fixed.

    I made some changes to the circuit. Plug them into your simulator and experiment with it. I have it running at 50kHz with a 25% duty cycle.
     
  5. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
    34
    0
    Thanks for your help.

    I took your circuit and simplified it to just to test. The ideia is the same:

    http://img.photobucket.com/albums/v222/ahhh/Eletronica/buck-1.jpg

    So, output with 10% duty and 15Khz:

    http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_10_duty.jpg

    So, output with 50% duty and 15Khz:

    http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_50_duty.jpg

    So, output with 80% duty and 15Khz:

    http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_80_duty.jpg

    Let see the ripple:

    http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_50_duty_10m_30m.jpg

    High? No, it is not:

    http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_50_duty_29m_30m_ripple.jpg

    Only 16mV.

    So now, lets connect the PIC and try to read the output voltage with a resistor divider and adjust the duty to obtain the Vref of 9V:

    Sweet:

    http://img.photobucket.com/albums/v222/ahhh/Eletronica/vout_50khz_com_pic.jpg

    This last test was with a 50Khz pwm into the lower transistor.

    Well, it looks like that is working! Hun?

    Coments?
     
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