From: Chris Carlen
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Subject: 87% All that vector calculus paid off
Date: Sat, 21 Dec 2002 19:44:40 GMT
NNTP-Posting-Date: Sat, 21 Dec 2002 11:44:40 PST
I built my first new flyback power supply prototype using my new
Ferroxcube 19/8/5 E cores in 3C90, and a UC3525A PWM chip.
I have yet to run it closed loop, so I'm just manually setting the duty
cycle right now. This chip sure produces better gate rise/fall times
than my useless function generator.
I had my wife wind 32 turns of 26 ga. on the spool last night, and she
did a real nice job for her first inductor. This morning I soldered the
leads onto the bobbin pins, and gapped the core with several layers of
packing tape, measured to exactly 0.18+/-0.01mm with a digital caliper.
So that gives an effective gap of 0.36mm. That should produce an
Al=100 and ue=140. So I figured I can sustain 1.14A before exceeding
160mT, and with switching around 180kHz, I'd need 47V input to pump 12W
through the thing, which is my goal.
Later I'll have to re-design the inductor to get the thing to produce
the power throughput I want with only 11-25V input. But for now I just
wanted to put the 3C90 through some tests, and see if my gapping would
yield results close to what is predicted.
At the moment the thing is driving a 10k resistor with 351.5V, pulling
0.315A from a 45.1V supply, for 87.0% efficiency. I'm running at 150kHz
with 40% duty. Not a bad efficiency for a flyback boost converter on
a breadboard, huh? Sure better than the results that stupid toroid
inductor gave. I'll update my www with this design once the whole thing
The core, coil, and FET are very slightly warm to the touch. The whole
thing is built on a breadboard with moderate attention paid to
minimizing parasitics, and providing ample bypassing reservoirs.
Oh, my analog scope shows the drain spiking up to the 600-700V range
before the HER105 diode turns on. Is this bad? The inductor only has
about 65uJ per cycle, and the IRF730a is able to handle repetitive
avalanche energy of 7.4mJ, so I can't imagine that the thing will break
under these conditions. But I don't know much about this avalanche
stuff. I'd appreciate some lectures on the subject. Should I try a
I will try closing the loop next, and then think about what additional
features will be needed in order to make the supply usable as an
adjustable output voltage supply, with some open circuit and short
Oh, I'll probably experiment with the TL3842 current mode controller
before I settle on a final design to commit to PCB. That might be a
good thing for making a supply with short circuit protection.
Christopher R. Carlen
Suse 8.1 Linux 2.4.19