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> Another Seven Transistor Special, Boost Converter
Sch3mat1c
Posted: May 09, 2017 11:27 am
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Enjoy wink.gif

Interesting features:
- Q6 can be pretty much any stupid transistor. I've tried everything from the slow TIP31C to a blazing fast 2SC4821 (2GHz!). The slower ones switch more slowly of course, but they do switch.
- Even Darlingtons work. They go fast, too -- about 1/3 the rated switching time!
- Also works with MOSFETs. Needs logic level type (notice 5V supply, as shown), and remove D4 and R11 (and all the spooky junk -- C1, C3, L3, R6-R8).
- Topology is common ground, so it can be used for boost or buck configurations with little change (main downside to buck: the whole thing is floating and needs a bootstrap supply).
- Output power is varied by two currents: one shown (I1, more current --> shorter off time --> higher frequency --> more power), the other into Q7 base (node IS2), which puts an offset I * R12 in series, thus reducing the turn-off peak current threshold (this is a peak current mode controller).
- Supply operating range is designed for 5V, but it should be pretty flexible, give or take what ends up happening with the voltages and currents around R2-R4.

Calling it "Seven Transistors" is kind of a lie, because you'll need two more transistors -- for those current sources -- and one more for voltage error amp (preferably a TL431, an "ideal transistor", nevermind what's inside it wink.gif ), to make a proper regulated voltage supply.

Tim


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