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Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
I need to convert 12 volts to 24 volts at about 10 mA. A single-transistor flyback is the simplest. In reasearching it, I've found a couple of design choices. This link has a converter circuit at the bottom where is says "floating supply inverter." (You'll probably have to click expand.) This link has its flyback wired a bit different.
I am not going to use a secondary, just take the pulses at the collector using a fast diode -- a simple boost converter with single inductor and a feedback winding, as it were. I will use the output of the booster to drive a high side mosfet as a barrier rectifier with very low power dissipation, in other words syncronous rectification.
Which circuit will commence oscillations with the most reliablity? I don't want to find out it only works some of the time. This isn't for an experiment, it goes in a vehicle's voltage regulator so reliablity is critical.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
For low power, and provided you don't need an isolated output, just use a simple boost converter circuit. Just requires one switch (i.e. MOSFET) and an inductor - no transformer or anything complicated.
National Semi evens has complete solutions (complete parts lists) for many different times of boost converters.
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
I said I wasn't going to use a transformer. "I am not going to use a secondary, just take the pulses at the collector using a fast diode -- a simple boost converter with single inductor and a feedback winding, as it were." And this won't require an IC.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Neither of those circuits you posted links to have buil-in regulation. The flyback driver doesn't have regulation at all, and the other one uses external voltage regulators to do it.
Quote from the page:
* 92% efficiency (from 70mA to 300mA out) * excellent powerband, (from 30mA to 500mA) * good 12.5v regulation! * very cheap circuit (around 75c in parts) * input: 22v to 30v * output: 12.5v @ 700mA maximum
Looks like it works best when you draw a bit more from it than 10mA, but I guess that wouldn't be too hard to fix.
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
Don't think I'll use the flyback after all because I remembered the circuit has a quad comparator with a spare. I can make a multivibrator to drive my little baby booster. At the few tenths of a watt it will produce, a zener will serve to regulate it.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Steve Conner wrote ...
IIRC, synchronous rectification is not worth using for any output voltage greater than about 12V.
I depends more on current than voltage as synch rectification is typically done because of poorly inefficient output diodes in power supplies. We frequently use synch rect on supplies with output voltages up to 50V with large gains in output efficiency vs. using output diodes.
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low power dc boost
