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help needed for a buck converter

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Patrick

Tue Jul 19 2011, 12:00AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

haxor5354 wrote ...

and whats different between mosfets and IGBTs? they both voltage controlled, maybe IGBTs can handle more voltage.

IGBT's have voltage drops instead of on-state resistance, and IGBTs are slower to turn off, but generally IGBT's are favored above 500-600 V, while Mosfets are favored below 400V. Youll have to study the differences in detail.

Mattski

Tue Jul 19 2011, 01:52AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...
ok yeah.... maybe substituting parts and using random parts isn't such a good idea

Substituting parts is a fine idea, especially given that a p-channel device will simplify the circuit greatly, there's nothing particularly special about the IRFP260N that means you can't use any of dozens of other FETs with only somewhat minor performance differences. The only difference and it's quite minor is that with a PMOS FET a high signal to the gate would turn the FET off, while a low signal turns it on. It means that with the same input signal an 80% duty cycle is now 20%, so you have to change your 555 timer values up a bit, or insert an inverter.

N-type IGBTs or even BJT's will suffer from the exact same problem in that you need to drive the gate/base with a voltage higher than your input supply voltage.

haxor5354

Tue Jul 19 2011, 02:30AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

I just used a random inductor that i've found somewhere in an ATX PSU, i did no calculations at all and I dont even have an inductance meter. is it still a good idea to use random parts?

Patrick

Tue Jul 19 2011, 03:42AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Mattski wrote ...

N-type IGBTs or even BJT's will suffer from the exact same problem in that you need to drive the gate/base with a voltage higher than your input supply voltage.

How about this, just temporarilly, run the 555 off 18Vdc while running the input to the power circuit at the usuall 12v? if two 9volt batteries is to much for the 555, then put two or three silicon diodes in series with the 18 Volts. This will give you enough voltage to see if its the gate drive thats the problem, then youll make your decision on a p channel device...

haxor5354

Tue Jul 19 2011, 05:34AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

OKAYYY ITS ABOUT TIME I GOT THIS DAMN PoS "WORKING"

the buck converter schematic that I posted before sucks

what a boat load of fail that was..

Buck5

I followed this schematic and now it works awesome!
but it has some rather "loud" ringings.
should I use an inductor of larger values?

20110719012407771

Patrick

Tue Jul 19 2011, 05:39AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

NICE!!!!!!!!!!!!!!!!

Use a pi type filter like a normal PS, that will solve your problem ( i think) .

pi

haxor5354

Tue Jul 19 2011, 07:19AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

so the output of my buck converter goes in to the input of the pi filter?

Patrick

Tue Jul 19 2011, 07:21AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

haxor5354 wrote ...

so the output of my buck converter goes in to the input of the pi filter?

Yes, and the final load is attached to the output of the PI filter.
This is how the AT, ANd ATX PS'es do it, so maybe you can use those powder metal inductors the yellow and white ones with magnet wire on it...

haxor5354

Tue Jul 19 2011, 07:59AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

OUCHH %^&$&^$*%(%
i just touched the diode it so so hot, the leads 5mm away from body was hot enough to melt solder!!! imagine how hot the body was.

i just found out that..
slow switching frequency = cool diode/lots of ripple/no ringing.
high switching frequency = hot diode/very-low ripple/lots of rining.

are my diodes to slow or what? I had 4 of them in parallel, so they can handle more than enough current

haxor5354

Tue Jul 19 2011, 08:00AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

*%&#^$(%())@&$&)#*&*(*#^(@%^($^)*$&^>?<~`*#>|^#(*&@&@&#^$*$
time to invest some cash into an IR thermometer instead to touching things to see if its or or not.

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