dc-dc boost converter

Discussion in 'Embedded Systems and Microcontrollers' started by hertz.12, Aug 30, 2015.

  1. hertz.12

    Thread Starter Member

    Apr 25, 2015
    i am building dc dc boost converter.....volt in 5v...vout 8v .....load 2ohm...n-ch Mosfet irlz44 ...mcu pic16f628a to drive the mosfet...inductor 3.3u 18A Current - Saturation 32A..........computer power supply...now the problem is that: when start with 25khz 1% duty the voltage step up a little ...25khz 10% duty the voltage step up a little.....25khz 20% duty the voltage step up a little....25khz 30% duty the voltage start to fall!...25khz 40% duty the voltage fall more!! what is the problem with my setup? i had tried 20khz and 30khz and 60 khz!...at low duty the voltage step up little then its begin to fall and fall and fall:)

    hope any one help me

    my inductor http://www.digikey.com/product-detail/en/0/SRP1250-3R3MCT-ND
    Last edited: Aug 30, 2015
  2. ScottWang


    Aug 23, 2012
    Have you measure the waveform for the positive and negative of diode?
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Try reducing C1 to, say, 100uF and adding a low-pass LC filter (L=3.5uH, C=1000uF) between D1 cathode and the load.
  4. hertz.12

    Thread Starter Member

    Apr 25, 2015
    i am using 2 parallel diode FR307...is it slow diode for my project?
  5. John P

    AAC Fanatic!

    Oct 14, 2008
    I see that reverse recovery time of that diode is 250nsec, which is way slower than a Schottky diode. And it also has around 1V of forward drop at a 2A current--that's a lot to be throwing away.

    But the major issue seems to me to be that inductor. How did you arrive at 3.3uH? That inductance gives you a rate of current change of 1/3 A per microsecond per volt, so if you had 5V pushing the current during the "on" phase, you'd be up to the 32A saturation level in 20usec or so. And at high current levels, you start to lose some voltage in the transistor too. I'd have thought that you'd want to keep a more steady current flowing in the inductor, which implies a larger value there. Um, I mostly do software these days. Maybe I've got the details wrong.
    Last edited: Sep 5, 2015
  6. HULK28

    New Member

    Sep 6, 2015

    Have you take capacitor with low ESR?
    if not, your dc/dc doesn't work.