Fun with positive feedback
in [Design]
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From: dagmargoodboat on 14 Jan 2010 23:46 On Jan 14, 3:59 pm, "Tim Williams" <tmoran...(a)charter.net> wrote: > <dagmargoodb...(a)yahoo.com> wrote: > > > You could use a current-limited on-time with a constant off-time. > > Might possibly even be done with two transistors. > > Yeah, that's what I drew, maybe not to that level of simplicity. It's not > hysteretic though, more of a "give me this much run time, and I pray it's > the value I wanted" kind of thing. > > > 5 <= Hfe <= 25 @ Ic = 1A. Which implies ~200mA drive. You can't make > > that efficiently from an oscillator running off +170v; you've got to > > bootstrap startup, then derive operating base-drive current from a > > lower-voltage transformer winding. > > > The CFLs (HV push-pull oscillators) get started either by trickle > > biasing (i(b) < 1mA) both switches slightly linear, or a relaxation > > oscillator that periodically kicks the main osc in the pants. Once > > running, base drive comes from their transformers. > > Yeah. I did that with the other one, but it's a flyback supply:http://webpages.charter.net/dawill/Images/RegBO.gif > the oscillator kicks on sharply by itself, dumping one full-sized cycle of > charge. A few more bumps and there's enough voltage for it to start up > (amazingly, it even starts into a 4 ohm load resistor!). > > To make this general type of circuit self-starting, it would have to start > kicking at a lower current, from a lower bias. That could go on in the > usual self-excited manner for a few cycles. Once the bias supply comes up, > more current is available, which drives the oscillator harder, and gets the > driver/error amp sort of things operating. > > > Don't need a CT--it's easier sensing emitter current with a resistor. > > Cheaper too. > > But that only senses the current to turn off at -- fine for a flyback, The circuit in your 1st post is a flyback, as have all been all the others. Isn't that the goal? > but I > want it to turn *on* at a specific current, too. That seems to be something > fundamentally different. The low voltage circuit does it quite excellently, > but it only does it with a high side shunt. The whole > shunt--inductor--LED--diode circuit is at +V during the off cycle (the > lowest point is the bottom end of the LEDs, which might be +V - Vf, lower > but still arbitrarily high), so the only choice seems to be coupling current > through a transformer, which somehow has to include DC, or a DC bias servo > is required. For hysteretic you'll need a buck arrangement and a high-side current- sensor / mirror thingie for the inductor current. Or a half-bridge oscillator, plus inductor, and some way to throttle it all. Or your CT and split ac/dc feedback paths. Kinda messy compared to a simple current-mode flyback, dontcha think? -- Cheers, James Arthur
From: Tim Williams on 15 Jan 2010 00:25 <dagmargoodboat(a)yahoo.com> wrote in message news:eeb10ddb-71bd-4a2c-9ac7-e03bf7632dac(a)m26g2000yqb.googlegroups.com... > The circuit in your 1st post is a flyback, as have all been all the > others. Isn't that the goal? Noooo! :-( The LEDs are in series with the inductor. The "flyback" is clamped by a diode, forcing discharge current to run through the load. Charging current runs through the load too, therefore it's a buck. Load current is continuous (or it's supposed to be). You could put in a filter capacitor and get a DC output (not very useful since it's bouncing up and down, and not isolated, but that's fine for driving LEDs). This circuit at least: http://webpages.charter.net/dawill/Images/CC_Buck.gif is definitely a buck converter, with constant current control, and hysteretic action (turn on/turn off). The two diodes and transistor look just like a current source, since they are, and +FB makes it oscillate. > For hysteretic you'll need a buck arrangement and a high-side current- > sensor / mirror thingie for the inductor current. Or a half-bridge > oscillator, plus inductor, and some way to throttle it all. Or your > CT and split ac/dc feedback paths. That's what I was thinking. Any elegant ways? > Kinda messy compared to a simple current-mode flyback, dontcha think? Yeah, but discontinuous current isn't what I want. Pulsed LEDs are less efficient, and I want to be able to point at it and say: "I have exactly nnn amps through the load." "How do you know?" "Because the voltage across this current sense resistor is DC plus a little triangle wave, and the average voltage is nnn." "I'm convinced!" It's a lot harder to say that when you've got some intermittent triangle or trapezoid. What should peak voltage be? Peak current? Average voltage? Average current? It also makes more RFI -- like I said somewhere else, this method has exactly one node that's moving a lot. A flyback has the switch side transistor and the load side diode, both of which can make interesting noises. That's acceptable for an isolated, voltage regulated DC power supply, but there's a simpler way for this. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: dagmargoodboat on 15 Jan 2010 18:40 On Jan 15, 12:25 am, "Tim Williams" <tmoran...(a)charter.net> wrote: > <dagmargoodb...(a)yahoo.com> wrote in message > > news:eeb10ddb-71bd-4a2c-9ac7-e03bf7632dac(a)m26g2000yqb.googlegroups.com... > > > The circuit in your 1st post is a flyback, as have all been all the > > others. Isn't that the goal? > > Noooo! :-( > > The LEDs are in series with the inductor.[...] Yeah--I was too hasty. Why the obsession with enforcing a particular inductor ripple current? The "flyback" is clamped by a > diode, forcing discharge current to run through the load. Charging current > runs through the load too, therefore it's a buck. Load current is > continuous (or it's supposed to be). You could put in a filter capacitor > and get a DC output (not very useful since it's bouncing up and down, and > not isolated, but that's fine for driving LEDs). > > This circuit at least:http://webpages.charter.net/dawill/Images/CC_Buck.gif > is definitely a buck converter, with constant current control, and > hysteretic action (turn on/turn off). The two diodes and transistor look > just like a current source, since they are, and +FB makes it oscillate. > > > For hysteretic you'll need a buck arrangement and a high-side current- > > sensor / mirror thingie for the inductor current. Or a half-bridge > > oscillator, plus inductor, and some way to throttle it all. Or your > > CT and split ac/dc feedback paths. > > That's what I was thinking. Any elegant ways? None come to mind. Those always wind up as kludges. > > Kinda messy compared to a simple current-mode flyback, dontcha think? > > Yeah, but discontinuous current isn't what I want. I say, ahh say that's why we have filter caps, son. > Pulsed LEDs are less > efficient, Oh, this is an LED driver? I thought t'was a general-purpose supply, part of your "I've got B.O." thread project. I have to be careful-- this is a potential conflict of interest for me. I've tested pulsed LED efficiency. With a simple filter cap on a flyback (boost topology) the LED efficiency loss is reduced to insignificance even for low flyback duty cycles. Running on rectified mains voltage you can size a flyback winding so the power supply's in 'flyback' most of the time, with the inductor acting as a quasi-constant current source that's driving the LEDs; filtering is then trivial. Even a small cap easily runs the LEDs with minimal ripple current during the short inductor recharging period. > and I want to be able to point at it and say: > "I have exactly nnn amps through the load." > "How do you know?" > "Because the voltage across this current sense resistor is DC plus a little > triangle wave, and the average voltage is nnn." > "I'm convinced!" > > It's a lot harder to say that when you've got some intermittent triangle or > trapezoid. What should peak voltage be? Peak current? Average voltage? > Average current? A sense resistor in series with the LEDs + filter cap. fixes all that. > It also makes more RFI -- like I said somewhere else, this > method has exactly one node that's moving a lot. A flyback has the switch > side transistor and the load side diode, both of which can make interesting > noises. That's acceptable for an isolated, voltage regulated DC power > supply, but there's a simpler way for this. If you're dead-set on hysteretic, you could sense i(L) on the high- side as in your 12v CC_Buck.gif and use that signal as input to a low- voltage-powered base-driver circuit. That's efficient. A continuous-mode current-controlled flyback supply is simple and easy to isolate. Add an "LED current" sensing resistor if you want to. I suggest not worrying about the "zero LED ripple-current" thing-- you'll work too hard solving something that isn't a problem. -- Cheers, James Arthur
From: Tim Williams on 15 Jan 2010 20:53
<dagmargoodboat(a)yahoo.com> wrote in message news:118785a4-4340-4f3f-9302-c381f0cdf2b1(a)v25g2000yqk.googlegroups.com... > Why the obsession with enforcing a particular inductor ripple current? Just 'cuz. I could do it with a flyback (as below), but that would be too easy. :) > Oh, this is an LED driver? I thought t'was a general-purpose supply, > part of your "I've got B.O." thread project. I have to be careful-- > this is a potential conflict of interest for me. Close: it's still transformer feedback and BJT technology, but I'm trying variations now. I'm pretty confident in flybacks, so I'm moving on to other styles. I don't think I'm going to be very successful at forward converters -- well, maybe a half bridge style ala CFLs, but that's for later, and much higher power levels, for which you might as well use a TL494. (Notably, Sony seems to have used self-excited forward converters in their Trinitron monitors -- with saturable reactor control! See this power transformer for evidence:) http://webpages.charter.net/dawill/Images/FunnyTrafo1.jpg http://webpages.charter.net/dawill/Images/FunnyTrafo2.jpg > I've tested pulsed LED efficiency. With a simple filter cap on a > flyback (boost topology) the LED efficiency loss is reduced to > insignificance even for low flyback duty cycles. I did that on my Internet's Largest Joule Thief. It seems to work okay, and it makes the waveforms look nicer. http://webpages.charter.net/dawill/tmoranwms/Elec_JT.html > If you're dead-set on hysteretic, you could sense i(L) on the high- > side as in your 12v CC_Buck.gif and use that signal as input to a low- > voltage-powered base-driver circuit. That's efficient. I drew something new today, but it wasn't running correctly in the simulator, and it uses kind of a lot of parts (~30), one of them a high voltage PNP, which is probably on the expensive side (cents, but in principle, something to do without). Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |