Forums

TEC for laser diode

Started by mt September 28, 2013
On Wed, 02 Oct 2013 06:07:20 -0500, Tim Williams wrote:

> "George Herold" <gherold@teachspin.com> wrote in message > news:597b4f0e-097f-4a5b-984f-57e50337d3f1@googlegroups.com... >> OK, having your cake and eating it too. >> Hmm, so you roll off the fast loop output at some frequency where the >> slow loop takes over, (high pass filter on the fast one) and then sum >> them. >> The details of that hand-off are -interesting-. >> Is a single pole roll off the way to go? >> (Drawing some PI bode plots I seem to be confusing myself. Do you roll >> the slow loop with a single pole low pass too?) > > Don't think loops side-by-side, or split in parallel: think composition, > putting one loop inside another. f(g(x)), not f(x) + g(x). > > As an example, suppose you have a dumb buck converter: a PWM modulator > driving a transistor. So, whatever control voltage sets PWM%, that's > what it's chopping at. The buck feeds an inductor, which (ignoring > what's after it) integrates PWM into switching current. If you just set > PWM willy-nilly, you'll get all sorts of crazy currents, because for a > fairly small increase in PWM, current just keeps climbing and climbing > and climbing! > > So, we put a current sensor in there (a shunt resistor, or a DC current > probe, whatever), and put a loop around the PWM so as to control > current. Since PWM is first order to current, this loop can be pretty > fast: generally, the error amp's pole will be near maybe half the > switching frequency, so as to filter out switching ripple, without being > needlessly slow, or so fast as to be unstable. > > But a buck converter is usually for constant-voltage output, so we need > *another* loop. This one sets the current setpoint for the inner loop, > and monitors the output voltage on the filter capacitor. The cap (in > and of itself) first-order integrates current, so again, we can put a > fairly simple error amp around it. Typically this pole will be 1/3 to > 1/10th the inner loop, and you want to have extra on there in case the > load has extra bypass caps or whatever. > > One might ask, if voltage is directly proportional to PWM, why not > control PWM by voltage in the first place? And the problem is that > pesky inductor inbetween. You always want to control inductor current > first, since it's also the switch current, and limiting that keeps the > magic smoke in!
There are PWM controller chips that go one farther than that, and use the sensed current as the trip point for the PWM, with the trip point set by the outer loop. I think they're called current control regulators, and I seem to remember that they're National parts, but they might be from Linear. I _do_ remember that they seemed to work well, in the hands of the guys using them. (I mostly do switching stuff to make 3.3V for a processor, or from the processor that I just made 3.3V for. So when I get fancy it's with the processor, not with some off-the-shelf chip). -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
"Tim Wescott" <tim@seemywebsite.really> wrote in message 
news:mfOdnc1SS-j01tHPnZ2dnUVZ5gmdnZ2d@giganews.com...
> There are PWM controller chips that go one farther than that, and use > the > sensed current as the trip point for the PWM, with the trip point set by > the outer loop. I think they're called current control regulators, and > I > seem to remember that they're National parts, but they might be from > Linear. I _do_ remember that they seemed to work well, in the hands of > the guys using them.
What I described above (snipped) is average current mode control -- you can do the same thing with peak mode control, in which case the inner loop uses a comparator and flip-flop. UC3842 is a classic example. (You could further put an average current sensing loop around that, I suppose, but I don't know why.) The main downside to peak-mode control is, it only works well in DCM (inductor current goes to zero each cycle), which costs ripple and noisiness. You can run at higher duty cycles (over 50%, inductor current not returning to zero), but you have to compensate for it, otherwise the loop goes unstable[1]. They call it "slope compensation", which just adds some of the oscillator slope with the current slope, which gets you a hybrid between the lone-pulse-width-modulator case and pure current mode operation. [1] The loop exhibits a zero in the right half plane at 1/2 F_sw, or something like that, so as power output is turned up, it starts oscillating at half the operating frequency. As it grows, the small-signal approximation stops being valid, and the pulse widths saturate, resulting in a chaotic system. In fact, it exhibits a chaotic map identical to the Logistic Function. Time domain intuition shows that, if inductor current doesn't discharge to zero by the time the oscillator kicks the switch back on, the amount of time until it reaches peak becomes a free variable -- current is still limited in magnitude, but now the pulse widths can see-saw from one pulse to the next, with the average pulse width held in place by the setpoint. At low power, it operates smoothly, but as you turn it up, it starts alternating, short pulse, long pulse; then it bifurcates again: very short, medium long, medium short, very long; and so on. Over most of the range, it hisses because it's jumping between so many pulse widths that it's approximately random. Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
On 10/02/2013 08:30 AM, Bill Sloman wrote:
> On Wednesday, 2 October 2013 00:56:45 UTC+10, George Herold wrote: >> On Saturday, September 28, 2013 9:20:34 AM UTC-4, Bill Sloman >> wrote:
<snip>
>> >> Well as Bill says current control is a bit better. But if you are >> wrapping a control loop around it (PI) then you can do it with just >> voltage too. > > Keeping the control loop critically damped becomes an issue. As the > voltage difference across the TEC changes the gain around a simple > loop change also changes. > >> The most important thing (IMHO) for a TEC is sizing the heat sink. >> If it's free air cooled then you need a pretty big heat sink >> compared to the size of the TEC. It's just a matter of energy >> conservation, all the heat that flows through the TEC has to also >> leave the heatsink. > > Worse, the current you are driving through the heat sink to move the > heat generates more heat in the resistance of the heat sink. > > Big heat sinks can be vital. >
You generally need to supervise the loop with a MCU, because otherwise it'll need a bunch of circuitry to do thermal cutouts and other sanity checks. (Do put a thermal clicker on the heatsink as a last-ditch defense.) With marginal heatsinking and no safety cutouts, turning the temperature setpoint down can cause the whole thing to turn to lava rather suddenly. Some multistage TECs will melt if you just apply power too quickly, because the thermal spreaders are too slow. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Wednesday, October 2, 2013 10:35:22 AM UTC-4, Phil Hobbs wrote:
> On 10/1/2013 10:45 PM, George Herold wrote: >=20 > > On Tuesday, October 1, 2013 2:51:53 PM UTC-4, Tim Wescott wrote: > >> On Tue, 01 Oct 2013 09:37:49 -0700, George Herold wrote: > >> > > <snip.. go upstream to read previous stuff=20 > >> In general when you do a two-loop control system you don't try to roll=
=20
> >> off the control of the outer loop or any of that razzmatazz -- you tun=
e=20
> >> the inner loop for good behavior in a conventional way, then you say=
=20
> >> "gosh, look at this plant that I have" and you tune the outer loop to =
the
> >> inner loop's characteristics. You may do some iteration between inner > >> loop and outer loop tuning (usually to relax the inner loop's gains -- > >> see below), but in general you want to tune the inner loop in isolatio=
n,
> >> then tune the outer loop as if the inner loop's characteristics are fi=
xed.
> >> > >> You will, in general, find that the outer loop's bandwidth is > >> significantly less than the inner loop's bandwidth -- but you will als=
o
> >> often find out that the control system as a whole is much better behav=
ed,
> >> and often faster, with two loops. >>> > >> You often don't want to tune the inner loop as tightly as you might if=
it
> >> were the only loop -- having an inner loop that is Really Well Damped =
is
> >> usually better than having one that has the World's Highest Bandwidth. > >> The best compromise is between these two extremes, but leaning toward > >> well damped. If you want better bandwidth from the inner loop it's > >> usually better to use some judicious feed-forward; that usually gives =
you
> >> better bandwidth and phase roll-off characteristics at the cost of > >> accuracy, but since you're supplying accuracy with the outer loop that=
's
> >> usually OK=20 > >> > >> -- > >> > >> Tim Wescott > >> > >> Wescott Design Services > >> > >> http://www.wescottdesign.com >=20 > > > > Gee that was nice, thanks Tim and Phil, > > I think I'd have to build/(or tweak) one to really understand. > > >=20 > > I guess I'm trying to see this in the frequency domain. (let's stick w=
ith thermal control with a PI loop, and no long thermal paths) I picture th= e gain as like the open loop response of an opamp. (Maybe that's wrong?) S= o is the DC gain of the fast/inner loop always less than the slower/outer? = And I can then add them together with 'no worries'?
>=20 > > George H. > Tim and I are talking about wrapping the outer loop around the inner=20
> loop, like using a closed-loop buffer inside an op amp feedback loop. > =20 > Adding them is a more difficult problem, but not impossible as long as=20 > the slow loop can be made to have lower gain at all frequencies.=20 > Otherwise you get whoopdedoos where the gain curves cross. > =20 > For instance, I made a laser frequency locker a couple of years ago that =
=20
> used current and temperature in a single loop. It locked up using=20 >=20 > temperature, and when the (much narrower range) current-tuning loop came =
=20
> into range, its higher gain immediately dominated. (The temperature =20 > loop kept the current loop centred.) > =20 > Cheers > =20 > Phil Hobbs >=20
OK Thanks, two Tim's and Phil. You've at least identified my problem. I w= as thinking of two side by side loops and not one inside the other. (I kno= w you were both saying inner and outer loops.. but that's not what I 'heard= '.) =20 I'll have to draw some pictures tonight and see if I can make sense of it= .=20 =20 OK, no integrator on the inside loop because any offset error will be clean= ed up by the outer loop.=20 Then the outer loop see's the reference (temperature), defines an error sig= nal, processes that(PI), and then sends this control 'information' onto the= inner loop. The inner loop defines it's own error signal (based on temperature measurem= ent and information from the outer loop.) Gains it up, and tells the plant = (TEC) to get there asap. What exactly the 'information' is, is a bit foggy= . It's gotta look like a reference (temperature) for the inner loop. =20 You know that does sound fairly straight forward. =20 George H. =20
> --=20 >=20 > Dr Philip C D Hobbs >=20 > Principal Consultant >=20 > ElectroOptical Innovations LLC >=20 > Optics, Electro-optics, Photonics, Analog Electronics >=20 >=20 >=20 > 160 North State Road #203 >=20 > Briarcliff Manor NY 10510 USA >=20 > +1 845 480 2058 >=20 >=20 >=20 > hobbs at electrooptical dot net >=20 > http://electrooptical.net
On Wednesday, October 2, 2013 1:19:13 PM UTC-4, Phil Hobbs wrote:
> On 10/02/2013 08:30 AM, Bill Sloman wrote: > > > On Wednesday, 2 October 2013 00:56:45 UTC+10, George Herold wrote: > > >> On Saturday, September 28, 2013 9:20:34 AM UTC-4, Bill Sloman
<lazy snip>
> > > > Worse, the current you are driving through the heat sink to move the > > heat generates more heat in the resistance of the heat sink. > > > > Big heat sinks can be vital. > > > > You generally need to supervise the loop with a MCU, because otherwise > it'll need a bunch of circuitry to do thermal cutouts and other sanity > checks. (Do put a thermal clicker on the heatsink as a last-ditch defense.) > > With marginal heatsinking and no safety cutouts, turning the temperature > setpoint down can cause the whole thing to turn to lava rather suddenly. > Some multistage TECs will melt if you just apply power too quickly, > because the thermal spreaders are too slow. >
I guess I've always done very conservative designs. I set all the knobs to their 'worse position' and let it run. The diode laser TEC I did, sits on a ~3" cube of aluminum, which bolts to a 1/2" thick by 2' by 4' aluminum bread board. Now that's a heatsink you can count on! (I tortured the thing without the bread board.) George H.
> > Cheers > > > > Phil Hobbs > > > > > > -- > > Dr Philip C D Hobbs > > Principal Consultant > > ElectroOptical Innovations LLC > > Optics, Electro-optics, Photonics, Analog Electronics > > > > 160 North State Road #203 > > Briarcliff Manor NY 10510 > > > > hobbs at electrooptical dot net > > http://electrooptical.net
On Wed, 2 Oct 2013 12:44:01 -0700 (PDT), the renowned George Herold
<gherold@teachspin.com> wrote:

>On Wednesday, October 2, 2013 1:19:13 PM UTC-4, Phil Hobbs wrote: >> On 10/02/2013 08:30 AM, Bill Sloman wrote: >> >> > On Wednesday, 2 October 2013 00:56:45 UTC+10, George Herold wrote: >> >> >> On Saturday, September 28, 2013 9:20:34 AM UTC-4, Bill Sloman ><lazy snip> > >> > >> > Worse, the current you are driving through the heat sink to move the >> > heat generates more heat in the resistance of the heat sink. >> > >> > Big heat sinks can be vital. >> > >> >> You generally need to supervise the loop with a MCU, because otherwise >> it'll need a bunch of circuitry to do thermal cutouts and other sanity >> checks. (Do put a thermal clicker on the heatsink as a last-ditch defense.) >> >> With marginal heatsinking and no safety cutouts, turning the temperature >> setpoint down can cause the whole thing to turn to lava rather suddenly. >> Some multistage TECs will melt if you just apply power too quickly, >> because the thermal spreaders are too slow. >> >I guess I've always done very conservative designs. I set all the knobs to their 'worse position' and let it run.
Boy am I tempted to relate recent experiences, but discretion being the better part of valo(u)r, let me say you're not the only one that sets up instrumentation that way. ;-)
>The diode laser TEC I did, sits on a ~3" cube of aluminum, which bolts to a 1/2" thick by 2' by 4' aluminum bread board. Now that's a heatsink you can count on! (I tortured the thing without the bread board.) > >George H. > >> >> Cheers >> >> >> >> Phil Hobbs >> >> >> >> >> >> -- >> >> Dr Philip C D Hobbs >> >> Principal Consultant >> >> ElectroOptical Innovations LLC >> >> Optics, Electro-optics, Photonics, Analog Electronics >> >> >> >> 160 North State Road #203 >> >> Briarcliff Manor NY 10510 >> >> >> >> hobbs at electrooptical dot net >> >> http://electrooptical.net
Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
On Wednesday, October 2, 2013 4:13:10 PM UTC-4, Spehro Pefhany wrote:
> On Wed, 2 Oct 2013 12:44:01 -0700 (PDT), the renowned George Herold > > wrote: > > >On Wednesday, October 2, 2013 1:19:13 PM UTC-4, Phil Hobbs wrote:
<snip>
> >I guess I've always done very conservative designs. I set all the knobs to their 'worse position' and let it run. > > > > Boy am I tempted to relate recent experiences, but discretion being > the better part of valo(u)r, let me say you're not the only one that > sets up instrumentation that way. ;-)
Ya gotta save some stories to go with the beer. It's weird I always want to torture apparatus and my colleagues become worried, "Careful, you might break it." I scratch my head, "You want the students to break it?" (We sell lab stuff that college students use.) We recently had a set of coil forms made of Teflon, that 'melted' after ~20 minutes of full power. A recent bit of electronics had a series RLC circuit. With a power opamp in the same apparatus. It has a Q a bit over 30 and when driven by the low impedance of the opamp (at ~20 Vp-p)I was getting >600 V p-p at the output.. Fun, but not for students. George H.
> > > >The diode laser TEC I did, sits on a ~3" cube of aluminum, which bolts to a 1/2" thick by 2' by 4' aluminum bread board. Now that's a heatsink you can count on! (I tortured the thing without the bread board.) > > > > > >George H. > > > > > >> > > >> Cheers > > >> > > >> > > >> > > >> Phil Hobbs > > >> > > >> > > >> > > >> > > >> > > >> -- > > >> > > >> Dr Philip C D Hobbs > > >> > > >> Principal Consultant > > >> > > >> ElectroOptical Innovations LLC > > >> > > >> Optics, Electro-optics, Photonics, Analog Electronics > > >> > > >> > > >> > > >> 160 North State Road #203 > > >> > > >> Briarcliff Manor NY 10510 > > >> > > >> > > >> > > >> hobbs at electrooptical dot net > > >> > > >> http://electrooptical.net > > > > > > Best regards, > > Spehro Pefhany > > -- > > "it's the network..." "The Journey is the reward" > > speff@interlog.com Info for manufacturers: http://www.trexon.com > > Embedded software/hardware/analog Info for designers: http://www.speff.com