A reasonably simple three transistor buck converter. - Page 1
Forum section: electronics / projects

author: Refrigerator
author_rank: Super Contributor
timestamp: August 31, 2017, 08:07:44 pm
content: Since my boost converter turned out so well i've decided to make a buck converter.It started out as a simple PNP transistor based flip-flop oscillator circuit and was modified into a reasonably simple and pretty efficient buck converter.Although, i don't think it's quite done yet but it's pretty damn close to being done.The operation is rather simple, imagine a PNP based flip-flop oscillator, where one of the resistors between collector and ground is your load. The oscillator produces a reasonably sharp square wave which is fed through the load resistor and the energy dissipated through the load resistor can be calculated by the duty cycle and the supply voltage.Since the square wave is quite sharp the switching transistor wastes very little energy, which is why i chose to base my circuit off this oscillator.Now to smooth out the square wave i put a pretty standard 33µH inductor between the load and the transistor, a free wheeling diode was also put in it's respective location.The regulation part took me some thinking to do, since my boost converter was ground referenced it was easy to make a feedback circuit to regulate it. Heck all it took was a single zener.This buck converter is a little more complicated, since it's referenced to the supply voltage, which is constantly changing.If i used the same kind of feedback loop as i did in my boost converter the output voltage would be stable in relation to supply voltage but not to ground, which is what i don't want.So i came up with a simple solution to make the output ground referenced.A little more tinkering later i had a three BJT based buck converter, which was about 62% efficient.For me 62% was not a satisfactory figure but, luckily enough, my solution to make the circuit ground referenced made it possible to replace the switching transistor with a P-FET.Replacing the BJT with a P-FET instantly brought the efficiency of my circuit to a tad over 78%, which is a great improvement.The output of the circuit is determined by the zener diode, output voltage being 0,6V lower than the zener voltage.Another great feature of my feedback solution is that it effectively gives my oscillator circuit the ability to range from 0 to 100% duty cycle, which greatly increases the versatility of my circuit.I'll include the SPICE files if anyone wants to play with this circuit.It's not exactly the best thing out there, i'm sure something better can be done with a 555 timer, but i like the occasional puzzle.The part count is not that bad with this circuit coming in at 13 components, only 4 more than my boost converter.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2017, 08:53:57 pm
content: Have you looked at the Black regulator?https://www.romanblack.com/smps/smps.htm

author: Benta
author_rank: Super Contributor
timestamp: August 31, 2017, 09:12:24 pm
content: Refrigerator, I'm sure you've built a great circuit.I do, however, strongly recommend that you think about how to layout schematics. They should be easy to read, with as few crossed wires as possible, and components in places where their function is easily understandable.A tip: start with pencil and paper.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2017, 09:15:45 pm
content: Quote from: Benta on August 31, 2017, 09:12:24 pmI agree, I haven't really looked at the circuit in detail, because of that.The LTSpice shortcuts crtl+R (rotate) and ctrl+E (flip) are vital.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 31, 2017, 09:31:23 pm
content: Quote from: Hero999 on August 31, 2017, 08:53:57 pmWoah, that almost looks like it was chopped out of my circuit.Too bad i didn't find this sooner, might have saved me some time on what now makes me look like i copied someone else's work, bummer.Quote from: Benta on August 31, 2017, 09:12:24 pmWhat's ironic is that i actually tried rearranging the components to make the schematic look neater.Ps: and yes i do a lot of of my schematic drawing by hand, whenever i'm planning on making something, since i can't bring my laptop to my work bench, and i like drawing schematics by hand since it allows me to understand them better.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2017, 10:11:40 pm
content: Eeee, yikes! That gate voltage...Surprisingly, LTSpice shows drain current going more or less to zero. Not very quickly, mind!This sort of has a similar feedback operation, but it switches more cleanly:Actually I lie, this is intended to go between "off" and "linear", rather than "saturation"; but a few minor adjustments (mainly, getting rid of that half-wave voltage doubler output) cleans that up nicely.It does use more transistors, though.Note that a MOSFET can be used, too (in place of the 2N4401). Then you don't need the current limiting 100 ohm base resistors, but you will probably need a complementary emitter follower to drive the gate quickly. (Dropping in a 2N7000 to this circuit, as is, would probably be pretty reasonable. Of course, a 2N7000 isn't good for much more than a 2N4401 is, either.)It's not very "safe", because it's still basically a self-excited oscillator, and if the oscillation gets quenched, it sits there, simmering, like any other such circuit. This is true all the way down to the single transistor blocking oscillator:(Which may be recognized as the "Joule Thief" circuit, among many other things throughout history.)It's also very dependent on supply voltage, because the switching waveform is necessarily part of the oscillator timing. (The switching amplitude goes right into the feedback transistor; if the amplitude is too small, it doesn't sustain, and quenches. If it's too large, it avalanches the transistor and base current runs away, forcing the circuit to full throttle!)I prefer somewhat more elaborate, but much safer, more nuanced circuits:Note that the core, aside from the complementary emitter follower, only needsSeven Transistors...The overall function is broadly similar to a UC3842, lacking some protection features (UVLO and startup, duty cycle limits). It is a peak current mode controller, the safest kind of SMPS controller. A short circuit on the load cannot destroy this, it just throttles back to a low duty cycle, instantly. An odd difference is that the operating frequency depends on load as well (which you could hack into a UC3842 circuit, but it's not usually done), which automatically gives it "pulse-skipping" behavior at light load. (Really, it's not that it's ever "skipping" pulses, it's just waiting so-and-so before the next, and that wait depends directly on the load.)And as a proper current mode controller (unlike a hysteretic controller, as most of Black's designs are), it can be designed for arbitrarily low output voltage ripple, with no dependency on filter capacitor value and ESR.It is still sensitive to input voltage, but not as much as the earlier one; this is easily fixed by adding a few current mirrors, but by then you might as well put the damn thing on an IC, add the UVLO startup circuit, and buy the UC3842 ready-made.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: August 31, 2017, 10:21:39 pm
content: @Refrigerator,Its got problems - turn on the.tran startupoption and it peaks at 15.4A through the MOSFET and the output voltage overshoots to nearly 9.4V.Quote from: Benta on August 31, 2017, 09:12:24 pmI've done a bit of rearranging and tidying up for you and added some node labels for the transistors etc.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 31, 2017, 11:17:58 pm
content: @Ian.MYeah, that looks like some serious overshoot, i guess it's that 100% duty cycle kicking in while it charges the output filter capacitor up, i presume.Looks like i'm not done tinkering with this circuit yet, i'll have to make a quick prototype on some proto-board when i get back home to see how things work in real life.