Fun with positive feedback
in [Design]
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From: Tim Williams on 11 Jan 2010 23:17 I'm having fun with these self excited oscillator thingies... http://webpages.charter.net/dawill/Images/LED_Buck_Supply.gif ....I think it will work... The traditional charge-the-base-bias-cap approach would probably work as well, but I wanted to try something slightly different. The main motivation is making current continuous, which the average circuit isn't so great at, you have to "overdrive" it. Since I can't test how much is needed, I went with a time delay instead. Too bad I can't test it here, lab doesn't have the parts. Any thoughts on making one that turns *on* and off at specific current thresholds? Seems to me turn-on requires inverting a current signal, which is costly (10mA bias at 160V = 1.6W wasted). Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: dagmargoodboat on 14 Jan 2010 00:29 On Jan 11, 11:17 pm, "Tim Williams" <tmoran...(a)charter.net> wrote: > I'm having fun with these self excited oscillator thingies...http://webpages.charter.net/dawill/Images/LED_Buck_Supply.gif > ...I think it will work... > > The traditional charge-the-base-bias-cap approach would probably work as > well, but I wanted to try something slightly different. The main motivation > is making current continuous, which the average circuit isn't so great at, > you have to "overdrive" it. Since I can't test how much is needed, I went > with a time delay instead. > > Too bad I can't test it here, lab doesn't have the parts. > > Any thoughts on making one that turns *on* and off at specific current > thresholds? Seems to me turn-on requires inverting a current signal, which > is costly (10mA bias at 160V = 1.6W wasted). Why the outboard oscillator? The main switch and the current sensor can be made to oscillate. If your main switch is a MOSFET you can start it up nearly for free, then run its gate drive from your feedback winding. That's pretty decent and widely used. Two transistors. -- Cheers, James Arthur
From: Tim Williams on 14 Jan 2010 00:58 <dagmargoodboat(a)yahoo.com> wrote in message news:4c53ca30-cbb3-4323-b39c-e256a356b72a(a)r5g2000yqb.googlegroups.com... > Why the outboard oscillator? The main switch and the current sensor > can be made to oscillate. They can, but it's usually boundary conduction mode (BCM). Away from the Bench, I don't want to take a stab at brute force driving it to points unknown. I'd much rather have it oscillate around a Q point. > If your main switch is a MOSFET you can start it up nearly for free, > then run its gate drive from your feedback winding. > > That's pretty decent and widely used. Two transistors. That's true, and if the savings is worthwhile I might explore MOSFETs. BJTs are still cheaper though. Even if I'm not likely to create a circuit that winds its way into mass production (if it's not there already), I'd like to investigate the kind of circuits they use. And they don't use MJE13003's (or less!) in CFLs for nothing. This is exactly what I wanted: http://webpages.charter.net/dawill/Images/CC_Buck.gif (which works amazingly well in simulation, by the way), except the 1k burns total supply voltage, which is no good at 160V, and even worse when hFE = 5. If I had a way to communicate inductor current down to ground level, it would be easier. Maybe I'll have to go with a transformer. Not a terrible idea, and a CT can have arbitrarily low voltage drop, even if it has to handle DC. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: dagmargoodboat on 14 Jan 2010 14:59 On Jan 14, 12:58 am, "Tim Williams" <tmoran...(a)charter.net> wrote: > <dagmargoodb...(a)yahoo.com> wrote in message > > news:4c53ca30-cbb3-4323-b39c-e256a356b72a(a)r5g2000yqb.googlegroups.com... > > > Why the outboard oscillator? The main switch and the current sensor > > can be made to oscillate. > > They can, but it's usually boundary conduction mode (BCM). You could use a current-limited on-time with a constant off-time. Might possibly even be done with two transistors. That topology can run continuous-mode over a wide range, especially at the higher loads where you want that (to maximize output for a given peak inductor current). > Away from the > Bench, I don't want to take a stab at brute force driving it to points > unknown. I'd much rather have it oscillate around a Q point. > > > If your main switch is a MOSFET you can start it up nearly for free, > > then run its gate drive from your feedback winding. > > > That's pretty decent and widely used. Two transistors. > > That's true, and if the savings is worthwhile I might explore MOSFETs. BJTs > are still cheaper though. Even if I'm not likely to create a circuit that > winds its way into mass production (if it's not there already), I'd like to > investigate the kind of circuits they use. And they don't use MJE13003's > (or less!) in CFLs for nothing. 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. > This is exactly what I wanted:http://webpages.charter.net/dawill/Images/CC_Buck.gif > (which works amazingly well in simulation, by the way), except the 1k burns > total supply voltage, which is no good at 160V, and even worse when hFE = 5. > If I had a way to communicate inductor current down to ground level, it > would be easier. > Maybe I'll have to go with a transformer. Yep. > Not a terrible > idea, and a CT can have arbitrarily low voltage drop, even if it has to > handle DC. Don't need a CT--it's easier sensing emitter current with a resistor. Cheaper too. -- Cheers, James Arthur
From: Tim Williams on 14 Jan 2010 15:59
<dagmargoodboat(a)yahoo.com> wrote in message news:f8380bc1-cc31-4da2-a5a8-02f1d67792a8(a)p8g2000yqb.googlegroups.com... > 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, 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. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |