dc-dc boost converter

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hertz.12

Joined Apr 25, 2015
40
hi
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
 

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Alec_t

Joined Sep 17, 2013
10,372
Try reducing C1 to, say, 100uF and adding a low-pass LC filter (L=3.5uH, C=1000uF) between D1 cathode and the load.
 

John P

Joined Oct 14, 2008
1,758
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.
 
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