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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
haxor5354 wrote ...
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
Lets scope the waveform of voltage across the diode, simply increasing diodes doesnt nesscarilly help, we need to know more. It sounds like youve shifted the heat out of the transistor to the blocking diode, which is fine. This new circuit is way better than the old one, so dont feel too bad. Yes IR thermometers are a blessing, i use mine all the time.
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
12v ish.. excluding ringing.
could it be the ringing thats causing the heat, because when I lower the switching frequency, the diode doesn't heat up as bad as high switching frequency. theres also less ringing if I use a slower switching frequency. datasheet of my diode mine is a 1n4508
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
The datasheet doesnt say a Trr. Maybe too slow? Rectron and Fairchild dont list a Trr, only a 60cycles chart, that usually means its nota fast diode, though i thought the 1N5408's were better. I think its too slow, thats why it doesnt over heat till you ramp up the freq.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
wrote ... 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...
Yes Patrick, that is a technique which can get around the high side drive issue. Thinking back a bit more I think part of the past problem is the battery trick was making it so that the FET did not turn entirely off, since when the 555 was outputting 0V there was still 3V going to the gate.
wrote ... the buck converter schematic that I posted before sucks
It was the classic buck converter, but as it requires a high gate voltage it is easiest with a p-type FET. The circuit you post next has the FET source pin grounded so it is much easier to drive the gate appropriately.
haxor5354 wrote ...
I followed this schematic and now it works awesome! but it has some rather "loud" ringings. should I use an inductor of larger values?
I'm confused by the schematic, the way it looks is that when the FET is off the filter capacitor will charge to 12V, then when the FET is on current will build up in the inductor. Then when the FET turns off again the only path for that current to flow is to break down the FET (leading to high loss) or break down the diode and discharge the capacitor. This circuit is similar to a boost, but not quite. What does your output voltage look like?
In power supply design there are always lots of little things that can go wrong :)
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help needed for a buck converter
