feedback and stability
in [Design]
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From: Hammy on 24 Dec 2009 21:21 On Thu, 24 Dec 2009 19:00:30 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: >On Thu, 24 Dec 2009 20:51:05 -0500, Hammy <spam(a)spam.com> wrote: > >>On Thu, 24 Dec 2009 17:31:22 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: >> >> >>>>Jim Thompson wrote: >>>>> On Wed, 23 Dec 2009 21:53:45 -0500, ehsjr <ehsjr(a)nospamverizon.net> >>snip >>>> >>>>> >>>>> To do it correctly you want a FIXED FREQUENCY oscillator with variable >>>>> duty cycle. Have the TL431 vary the duty cycle. >>>>> >>>>> ...Jim Thompson >>> >>>Repeat my last paragraph over and over until it sinks in. Don't be a >>>hacker, do it right ;-) >>> >>> ...Jim Thompson >>Or just save some components and time and pick from one of the >>hundreds of PWM controllers available. >> >>This is begining to sound like one of them chinese smps's anything to >>save 2 cents. >> >> I guess it makes sense labour is cheap in China. > >Alternator SMPS ?? > > ...Jim Thompson Maybe I missed something but isn't this about a regulator that goes haywire when he placed a cap across the Cathode ref of the TL431. From the OP. "I built a voltage regulator for an old truck I own. The circuit uses a voltage controlled oscillator with a square wave output to drive the gate of a mosfet that controls the alternator's field. The voltage regulation feedback loop uses a voltage divider from the alternator output/vehicle battery, connected to the reference pin of a TL431. The TL431 controls the vco." I'm just assuming it's a hack PFM using a TL431 and maybe a KA331 VCO.
From: Jamie on 25 Dec 2009 18:14 Michael Robinson wrote: > "ehsjr" <ehsjr(a)nospamverizon.net> wrote in message > news:hgp586$r12$1(a)news.eternal-september.org... > >>Michael Robinson wrote: >> >>>"Tim Wescott" <tim(a)seemywebsite.com> wrote in message >>>news:K56dnfGL45LFBLPWnZ2dnUVZ_vRi4p2d(a)web-ster.com... >>> >>> >>>>On Sun, 20 Dec 2009 15:32:04 -0500, Michael Robinson wrote: >>>> >>>> >>>> >>>>>I built a voltage regulator for an old truck I own. The circuit uses a >>>>>voltage controlled oscillator with a square wave output to drive the >>>>>gate of a mosfet that controls the alternator's field. The voltage >>>>>regulation feedback loop uses a voltage divider from the alternator >>>>>output/vehicle battery, connected to the reference pin of a TL431. The >>>>>TL431 controls the vco. >>>>> >>>>>Now, for a bit of a digression. Years ago I built a boost converter with >>>>>a similar feedback loop, the major difference being that it used a bjt >>>>>instead of a TL431. But it worked on the same principle, with a voltage >>>>>divider from the output to the base of the bjt, which controlled a >>>>>blocking oscillator. The output voltage wandered up and down and was >>>>>very unstable, so I put a capacitor from the collector to the base of >>>>>the bjt. That fixed the problem and made the output voltage stable. >>>>> >>>>>So, having had this experience, I put a cap from the cathode to the >>>>>reference of the TL431 in my truck regulator. When I installed it in the >>>>>truck, the alternator output regulation was unstable; the voltage jumped >>>>>up and down like crazy. After much head scratching, on a hunch I took >>>>>wire clippers and removed that cap from the circuit board. Then the >>>>>regulator worked GREAT. System voltage perfectly stable, good load >>>>>regulation. >>>>> >>>>>Perhaps I could be forgiven for thinking the boost converter and the >>>>>vehicle voltage regulator would operate on the same principle and that a >>>>>cap would have the same effect, in view of the fact that in both >>>>>circuits the cap provides negative feedback in the controlling element >>>>>of the feedback loop. But the two circuits have opposite behavior, and >>>>>adding a cap to the controlling element in the feedback loop has exactly >>>>>the opposite effect in each circuit. The boost regulator was unstable >>>>>without a capacitor, and stable with one. Contrarily, the alternator >>>>>regulator was stable without a capacitor, and unstable with one. >>>>> >>>>>At best I have some intuitive sense that the presence of a big >>>>>mechanical device that drives the load and resides within the loop >>>>>changes things dramatically. But I'm lacking in knowledge of theory and >>>>>could use elucidation. Any pointers? >>>> >>>>A schematic would help - either an ASCII art one, or put one up on a >>>>website somewhere so folks could look. >>>> >>>>Yes, the dynamics of your alternator are likely to be different from the >>>>dynamics of your boost converter output stage, so that could certainly >>>>have an effect on stability. Without knowing more about what you did >>>>(and probably more about the alternator's dynamics) I can't say much. >>>> >>> >>> >>>I don't think I can find the diagram for the circuit, but the following >>>should help: >>>Here's a spice file for a vco with the topology of the one I used. I >>>used a different comparator, and component values may be different, but I >>>didn't change the basic design. >>>This spice file was actually posted by John Popelish a few years ago, and >>>I based my circuit on it. >>> >>>Version 4 >>>SHEET 1 880 680 >>>WIRE -16 -80 -128 -80 >>>WIRE 304 -80 64 -80 >>>WIRE -64 16 -80 16 >>>WIRE 32 16 16 16 >>>WIRE 304 48 304 -80 >>>WIRE 336 48 336 16 >>>WIRE 336 48 304 48 >>>WIRE 336 80 336 48 >>>WIRE 288 160 224 160 >>>WIRE 224 176 224 160 >>>WIRE -240 240 -304 240 >>>WIRE -128 240 -128 -80 >>>WIRE -128 240 -160 240 >>>WIRE -16 240 -128 240 >>>WIRE 96 256 96 16 >>>WIRE 96 256 48 256 >>>WIRE 128 256 96 256 >>>WIRE 176 256 128 256 >>>WIRE -80 272 -80 16 >>>WIRE -48 272 -48 176 >>>WIRE -48 272 -80 272 >>>WIRE -16 272 -48 272 >>>WIRE -48 304 -48 272 >>>WIRE -128 320 -128 240 >>>FLAG 432 288 0 >>>FLAG 432 208 p12 >>>FLAG 336 176 0 >>>FLAG 224 272 0 >>>FLAG 16 288 0 >>>FLAG -48 384 0 >>>FLAG 336 -64 p12 >>>FLAG -48 96 p12 >>>FLAG 16 224 p12 >>>FLAG 128 176 p12 >>>FLAG 224 80 p12 >>>FLAG -304 320 0 >>>FLAG -128 384 0 >>>FLAG 336 48 Out >>>SYMBOL voltage 432 192 R0 >>>WINDOW 123 0 0 Left 0 >>>WINDOW 39 0 0 Left 0 >>>SYMATTR InstName V1 >>>SYMATTR Value 12 >>>SYMBOL Comparators\\LT1017 16 256 R0 >>>SYMATTR InstName U1 >>>SYMBOL nmos 176 176 R0 >>>SYMATTR InstName M1 >>>SYMATTR Value BSS145 >>>SYMBOL nmos 288 80 R0 >>>SYMATTR InstName M2 >>>SYMATTR Value FDS6680A >>>SYMBOL res 208 64 R0 >>>SYMATTR InstName R1 >>>SYMATTR Value 1k >>>SYMBOL res 320 -80 R0 >>>SYMATTR InstName R2 >>>SYMATTR Value 1 >>>SYMBOL res -64 288 R0 >>>SYMATTR InstName R3 >>>SYMATTR Value 100k >>>SYMBOL res -64 80 R0 >>>SYMATTR InstName R4 >>>SYMATTR Value 100k >>>SYMBOL res 112 160 R0 >>>SYMATTR InstName R5 >>>SYMATTR Value 10k >>>SYMBOL cap -144 320 R0 >>>SYMATTR InstName C1 >>>SYMATTR Value 220n >>>SYMBOL cap 96 0 R90 >>>WINDOW 0 0 32 VBottom 0 >>>WINDOW 3 32 32 VTop 0 >>>SYMATTR InstName C3 >>>SYMATTR Value 10n >>>SYMBOL res 32 0 R90 >>>WINDOW 0 0 56 VBottom 0 >>>WINDOW 3 32 56 VTop 0 >>>SYMATTR InstName R6 >>>SYMATTR Value 100k >>>SYMBOL res 80 -96 R90 >>>WINDOW 0 0 56 VBottom 0 >>>WINDOW 3 32 56 VTop 0 >>>SYMATTR InstName R7 >>>SYMATTR Value 100k >>>SYMBOL res -144 224 R90 >>>WINDOW 0 0 56 VBottom 0 >>>WINDOW 3 32 56 VTop 0 >>>SYMATTR InstName R8 >>>SYMATTR Value 100k >>>SYMBOL voltage -304 224 R0 >>>WINDOW 123 0 0 Left 0 >>>WINDOW 39 0 0 Left 0 >>>SYMATTR InstName V2 >>>SYMATTR Value PULSE(0 12 0 1) >>>TEXT 184 360 Left 0 !.tran 1 >>> >>>At the input to the vco there's a TL431 with a pullup resistor on the >>>cathode, and the cathode is connected to the vco input. >>>For what it's worth, the truck is a 1994 Dodge 2500 Ram diesel. >>> >>> >>>Putting a cap across the TL431 cathode/reference terminals ought to slow >>>the circuit's response, which could be a problem. >>>From the way the voltage was jumping around, it was something more than >>>just loose regulation -- it's like the cap introduced some kind of >>>nonlinear effect in the feedback loop's dynamics. It wasn't just >>>drifting, it was bouncing around. >>> >>> >> >> >>Drawing it out will make it clear. The first schematic below >>is a "sanity check", just to make sure that the circuit >>you described is something like it: >> >> + ----+----[PASS]------------+-----------------> field >> | | | >> +-----[VCO]---gnd | >> | | | >> +--[R]--+------+ | >> | | | >> __|__/ [C] [R] >> / \ | | >> /___\----+-------+ >> | | >> | [R] >> | | >> Gnd ----------+--------------+ >> >>Assuming the above captures the essence of what you have, read on. > > > No, it looks more like this: > > field > | > mosfet > | > vco batt > | | > +12v---[R]---+------+ | > | | | > __|__/ [C] [R] > / / \ | | > /___\----+-------+ > | | > | [R] > | | > Gnd ----------+--------------+ > > It looks like the C is leading in the circuit, If that is what's going on, you'll get an oscillating effect. That's just my observations. Try putting a R in series with the C or lower the value of C or both.
From: JosephKK on 27 Dec 2009 19:47 On Thu, 24 Dec 2009 14:52:20 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: >Jim Thompson wrote: >> On Wed, 23 Dec 2009 21:53:45 -0500, ehsjr <ehsjr(a)nospamverizon.net> >> wrote: >> >> >>>Michael Robinson wrote: >>> >>><snip> >>> >>>>>Drawing it out will make it clear. The first schematic below >>>>>is a "sanity check", just to make sure that the circuit >>>>>you described is something like it: >>>> >>>> >>>>As I explained from the beginning, the votlage divider at the reference pin >>>>is connected to the battery/alternator output, like this: >>>> >>>> field >>>> | >>>> mosfet >>>> | >>>> vco batt >>>> | | >>>> +12v---[R]---+------+ | >>>> | | | >>>> __|__/ [C] [R] >>>> / / \ | | >>>> /___\----+-------+ >>>> | | >>>> | [R] >>>> | | >>>> Gnd ----------+--------------+ >>>> >>>>Can you see how it would regulate voltage? >>> >>>Yes. I've seen the idea all along, just not sure what >>>was connected to where. Doesn't change my earlier answer. >>> >>> >>>>Any variation in the battery >>>>voltage will cause the cathode to pull the vco up or down, changing the duty >>>>cycle of the mosfet. It's a big feedback loop that includes the alternator. >>>>I want to know how the cap throws everything off. >>> >>>Check the diagram I drew showing the 431 as an op amp >>>& output xsistor, with an internal 2.5 V ref. The 431 >>>works as a comparator driving a transistor, and your cap >>>feeds the output back into the op amp input, causing >>>oscillation. >>> >>>Your TL431 will operate (turn on or off) in response to >>>a voltage change at the battery. The current through >>>R will suddenly change when the 431 switches on or off, >>>changing the drop across R. *That* change will be fed >>>back by the cap into the input of the 431 at the ref pin >>>as a pulse, and the 431 will react to that pulse. That >>>feedback path is causing your problem. You do not want >>>*that* change (the change at the cathode of the TL431) to >>>appear on the ref pin. >>> >>>Ed >>> >>> >> >> >> TL431's don't SWITCH. They're more akin to an OpAmp. > >Strictly speaking, no, but for usage here, it's simplest to >think of them switching. The switching is like what you see >on a typical zener curve where V goes from sloped to flat >when the zener avalanches and conducts in the reverse >direction. So they can be thought of as on or off. > EUWWW. According to the datasheet is supposed to be used as a linear (read non-switching) device. >Ed > >> >> To do it correctly you want a FIXED FREQUENCY oscillator with variable >> duty cycle. Have the TL431 vary the duty cycle. >> >> ...Jim Thompson
From: Jim Thompson on 27 Dec 2009 19:51 On Sun, 27 Dec 2009 16:47:45 -0800, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >On Thu, 24 Dec 2009 14:52:20 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: > >>Jim Thompson wrote: >>> On Wed, 23 Dec 2009 21:53:45 -0500, ehsjr <ehsjr(a)nospamverizon.net> >>> wrote: >>> >>> >>>>Michael Robinson wrote: >>>> >>>><snip> >>>> >>>>>>Drawing it out will make it clear. The first schematic below >>>>>>is a "sanity check", just to make sure that the circuit >>>>>>you described is something like it: >>>>> >>>>> >>>>>As I explained from the beginning, the votlage divider at the reference pin >>>>>is connected to the battery/alternator output, like this: >>>>> >>>>> field >>>>> | >>>>> mosfet >>>>> | >>>>> vco batt >>>>> | | >>>>> +12v---[R]---+------+ | >>>>> | | | >>>>> __|__/ [C] [R] >>>>> / / \ | | >>>>> /___\----+-------+ >>>>> | | >>>>> | [R] >>>>> | | >>>>> Gnd ----------+--------------+ >>>>> >>>>>Can you see how it would regulate voltage? >>>> >>>>Yes. I've seen the idea all along, just not sure what >>>>was connected to where. Doesn't change my earlier answer. >>>> >>>> >>>>>Any variation in the battery >>>>>voltage will cause the cathode to pull the vco up or down, changing the duty >>>>>cycle of the mosfet. It's a big feedback loop that includes the alternator. >>>>>I want to know how the cap throws everything off. >>>> >>>>Check the diagram I drew showing the 431 as an op amp >>>>& output xsistor, with an internal 2.5 V ref. The 431 >>>>works as a comparator driving a transistor, and your cap >>>>feeds the output back into the op amp input, causing >>>>oscillation. >>>> >>>>Your TL431 will operate (turn on or off) in response to >>>>a voltage change at the battery. The current through >>>>R will suddenly change when the 431 switches on or off, >>>>changing the drop across R. *That* change will be fed >>>>back by the cap into the input of the 431 at the ref pin >>>>as a pulse, and the 431 will react to that pulse. That >>>>feedback path is causing your problem. You do not want >>>>*that* change (the change at the cathode of the TL431) to >>>>appear on the ref pin. >>>> >>>>Ed >>>> >>>> >>> >>> >>> TL431's don't SWITCH. They're more akin to an OpAmp. >> >>Strictly speaking, no, but for usage here, it's simplest to >>think of them switching. The switching is like what you see >>on a typical zener curve where V goes from sloped to flat >>when the zener avalanches and conducts in the reverse >>direction. So they can be thought of as on or off. >> > >EUWWW. According to the datasheet is supposed to be used as a >linear (read non-switching) device. > >>Ed >> >>> >>> To do it correctly you want a FIXED FREQUENCY oscillator with variable >>> duty cycle. Have the TL431 vary the duty cycle. >>> >>> ...Jim Thompson I hate to have to keep repeating myself... a TL431 should be considered as an OpAmp with VOS = VREF and with an open collector output. Is that too complicated for some of you to grasp ?:-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism responsibly!
From: ehsjr on 28 Dec 2009 18:57
JosephKK wrote: > On Thu, 24 Dec 2009 14:52:20 -0500, ehsjr <ehsjr(a)nospamverizon.net> wrote: > > >>Jim Thompson wrote: <snip> >>> >>>TL431's don't SWITCH. They're more akin to an OpAmp. >> >>Strictly speaking, no, but for usage here, it's simplest to >>think of them switching. The switching is like what you see >>on a typical zener curve where V goes from sloped to flat >>when the zener avalanches and conducts in the reverse >>direction. So they can be thought of as on or off. >> > > > EUWWW. According to the datasheet is supposed to be used as a > linear (read non-switching) device. From the datasheet: "Active output circuitry provides a very sharp turn-on characteristic making these devices excellent replacements for Zener diodes..." Where do you see the datasheet indicating it "is supposed to be used as a linear (read non-switching) device" ? How do you define linear as non-switching? I'm sure you have something in mind here, but I'm not getting it. Vin vs Vout is anything but linear when the device is turned on. So you must have something different in mind. As to switching, the datasheet specifies a very sharp turn-on and refers to a zener. How does my description of that fail to capture the gist? Ed > > >>Ed >> >> >>>To do it correctly you want a FIXED FREQUENCY oscillator with variable >>>duty cycle. Have the TL431 vary the duty cycle. >>> >>> ...Jim Thompson |