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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi all, I made a flyback PSU for a 1W LED which drops around 3V @350mA.
Here are my transformer specs: ca. 20mm^2 cross-section, 200 pri. turns, 10 sec. turns, on EE core with paper spacers (one thickness). The primary inductance is 6mH, resistance 6R and drive frequency 100kHz. Driving with an "all-in-one" SMPS chip (TOP223P). Primary duty cycle seems to be around 15-20%.
The problem is that the inverter starts squealing/hissing when I try to push more than one watt through the transformer. Decreasing the gap makes it hiss even sooner, which means the xformer is saturating. Increasing the gap makes the output diode conduct less time and increases primary peak current and switch losses, which is undesirable.
Now the weird part: a 5W telephone charger uses the same size of a transformer with quite similar specs, just the primary resistance is half and inductance too. Turns unknown.
How on earth can they push 5W through the same transformer size out of which I can only get 1W ???
Edit: I think the problem was partially in my current sensing circuit... It will push now a bit over a watt without problems which is all I need, so mods can delete this thread if they want.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Maybe one of the flyback supplies operates in the continuous current mode and the other one operates in discontinuous current mode.
AS you found out, discontinuous current mode causes high peak current and voltage stresses on devices for a given output power. On the plus side the discontinuous operation gives the flyback diode time to recover gracefully.
High power commercial flyback supplies usually operate at least slightly into the dreaded continuous current mode. This is where the net flux in the core never falls to zero between switching cycles. This gets more power through a given size of core, and the peak current stresses on the switch and diode are lower. The downside is that the flyback diode now undergoes forced reverse recovery when the switch turns on. The other reason I said "the dreaded continuous current mode" is that it can introduce a right-half-plane zero into the transfer function which makes closed loop control compensation much more tricky!
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Flyback inverter with not enough power; transformer questions
