feedback and stability
in [Design]
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From: Jim Thompson on 24 Dec 2009 11:10 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. To do it correctly you want a FIXED FREQUENCY oscillator with variable duty cycle. Have the TL431 vary the duty cycle. ...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 24 Dec 2009 14:52 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. 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 24 Dec 2009 19:31 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: >> [snip] >>> >>>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. > >Ed > >> >> 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 -- | 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: Hammy on 24 Dec 2009 20:51 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.
From: Jim Thompson on 24 Dec 2009 21:00
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 -- | 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! |