Negative version of Roman Black's two-transistor discrete buck regulator - Page 1
Forum section: electronics / projects

author: KhronX
author_rank: Frequent Contributor
timestamp: April 16, 2018, 09:42:27 pm
content: This is a bit of a shot in the dark, but apart from a few minutes of my time, i figure i've got nothing to lose.http://www.romanblack.com/smps/smps.htmA while ago, i took the liberty to tinker with the component values both in LTspice as well as the real-life circuit, and it works well enough for some of my applications.Recently though, i "finally" decided to overhaul the power electronics in my active studio monitors. I particularly wasn't overjoyed with the heavy-handed way the powering of the line-level stages was done (series dropper resistors and a 78/7915 combo, supplied from the +/-40v feeding the power amps), since the thing ends up running a bit (or actually, MUCH) more "toasty" than i'd like.So i decided to start in LTspice, and basically "mirror" the two-transistor regulator, to make it a negative buck (by, of course, flipping the polarities of all the silicon parts). Trouble is, though, that despite it working seemingly fine in theory / in simulation, the real-world circuit seems to disagree.Because i had some to spare, i went with small MOSFETs as the main pass/switching elements, and stuck with BJT's as the drivers (or however they should be called).When supplied with 20-30v (with the positive end on the ground plane, and the negative at the regulator's input), there doesn't seem to be any oscillation getting triggered. The PNP's Vbe starts off around 0.7, but then just drops slowly (reaches 0.4v within a few seconds), and the N-channel's Vgs has a "blip" up to around 0.5v on startup, then fades to nothing. This was with a 50mA current-limit set on the bench supply feeding this; i ramped that up to see how much it would draw if it wasn't limited, and found that point to be around 110mA. All this, with no load on the output.Attached you'll find a screenshot of the LTspice sim i've used, and the actual circuit uses the same components & values. If you might find the time to read through this, and perhaps throw an idea or two back, i'd dearly appreciate it, but otherwise, i'm somewhat stumped.

author: KhronX
author_rank: Frequent Contributor
timestamp: April 17, 2018, 07:34:42 pm
content: Anyone..?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 17, 2018, 08:35:24 pm
content: Should probably start with DC supplies, so you don't have to wait for rectified AC to swing into action. You may need a stimulus to kickstart the oscillation (such as an initial voltage on a capacitor, different from its calculated DC operating point).I don't recommend MOSFETs, because the gate charge is so much larger and the gain is so much lower. With only two transistors, you need every bit of gain you can spare, to keep the waveforms sharp.Note also that you are applying Vgs(on) = (+V) - Vout (and the complement), which is likely to blow up those transistors during initial startup when V(C3, C6, C8, C9) = 0.Note that this circuit has no short circuit protection, no current limiting, no compensation, it's only a hysteretic control. Actually what's worse is, if there isn't enough loop gain to achieve hysteresis, it's a linear regulator, still without current limiting, and using parts wholly unprepared for that kind of power dissipation! This effectively happens during startup when charging the output capacitors.A final note about the control, hysteretic controls depend upon capacitor ESR. If you haven't set it (it's not displayed on the schematic, so I have no idea), you will find it's very important to performance. This also means that the circuitmust havehigh output ripple. (The output filter helps, but a better controller would need less.)Tim

author: KhronX
author_rank: Frequent Contributor
timestamp: June 08, 2018, 03:06:56 pm
content: I got my hands on some BCP53-16 and BCP56-16 to use as pass-transistors (to give some extra wiggle-room power-wise, at least).I did indeed add the capacitor ESR's in the simulation, both for the electrolytics and even the ceramics (the 1nF and 10nF caps there).As mentioned in the initial post, i'm more concerned with the efficiency of such an arrangement; the ripple can be filtered out, if need be.Either way, the negative regulator still refuses to start oscillating, in real life (while it does so readily, in LTspice).The positive one's steaming along just fine, with 70-75% efficiency.I've tried multiple combinations of resistors between the two transistors, but to no avail. Even replaced the R7 with a 10k trimmer, went from end to end, and still nothingAt best, it stays in linear mode; at worst, with 30v input, the output ramps up to 25v or so, at which point i switched off the bench supply.That pretty much confirms that old tech-proverb: amplifiers oscillate, and oscillators amplify...

author: iMo
author_rank: Super Contributor
timestamp: June 08, 2018, 03:38:09 pm
content: QuoteDouble check your wiring..

author: KhronX
author_rank: Frequent Contributor
timestamp: June 14, 2018, 11:15:26 am
content: I already have, at least a dozen times (or two).Quote from: imo on June 08, 2018, 03:38:09 pm