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Peltier element controller method, 20Amps bidirectional current

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Flyback

Well-Known Member
Hello,
We wish to do a Peltier cooler for a Laser diode.
The current will be up to 20 Amps, bidirectional, so we cannot use off-the-shelf switch mode peltier controllers.
Do you think our following method is the most lean?...

We will have two synchronous bucks either side of the peltier element. We will have a single control signal which feeds into both bucks. This control signal will increase the energy throughput of one buck whilst simultaneously decreasing the duty cycle of the other one (and vice versa). We will also use two LTC6101 current monitors as per page 29 of the following to get a current monitor signal which is unipolar but represents current flowing in either direction.
http://cds.linear.com/docs/en/applic...te/an105fa.pdf
The error signal between demanded temperature and actual temperature will feed into the current error amplifier to control the peltier current accordingly, and thus control the temperature of the laser diode.
Do you agree that this is the best way forward?
 

Flyback

Well-Known Member
Why bidirectional?
Thanks, its because the current might have to be regulated down to the zero level, and as you know, even though the reverse current may only need to be small, we still need to be able to get it...and if it is unidirectional, then the current can keep "bouncing off the zero point", instead of smoothly regulating.
 

alec_t

Well-Known Member
Most Helpful Member
Like Ron, I don't see why you need bidirectional current. You're surely not trying to heat the diode? It should be possible to design a regulator which cuts the current cleanly and doesn't 'bounce off the zero point'. Indeed, current reversal in your method would inevitably result in 'bouncing around zero'.
 

Flyback

Well-Known Member
You're surely not trying to heat the diode?
As far as i know its because the characteristics of the laser diode are those that are required at whichever temperature, and if the ambient temperature may sometimes be high, then we will have to pick a high regulation temperature.........in other words, its constant temperature that we seek, and as low as possible....but as you know, the peltier cannot cool stuff below the ambient temperature, or below the temperature of the place to where heat is being removed to..

so we just want the lowest temperature that we can always get to, and as discussed, if sometimes the ambient temperature is say 35degC max, then our chosen temperature must always be that....because if we picked 30degC, then that would be reachable when ambient is 25degC, but not during those times when ambient is 35degC.
I got into a mess explaining that but i hope i unravelled it in the end.
 

JimB

Super Moderator
Most Helpful Member
the peltier cannot cool stuff below the ambient temperature, or below the temperature of the place to where heat is being removed to
Not correct.
The "hot" side of the peltier device will be hotter than ambient so that it can dissipate the energy extracted from it "cold" side.

If your statement above were true, there would be no advantage of a peltier device over a big fan blowing air across the thing to be cooled.

JimB
 

Flyback

Well-Known Member
thanks, but if a device is mounted on a peltier which sits on a heatsink, over which cooling air is blown, then i dont see how the device can be cooler than ambient.
 

Flyback

Well-Known Member
Thanks, but with a conditioner it uses latent heat of vaporization.....it effectively shovels the heat energy elsewhere.....with a peltier system, there is no "elsewhere" for the heat to be sent to.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks, but with a conditioner it uses latent heat of vaporization.....it effectively shovels the heat energy elsewhere.....with a peltier system, there is no "elsewhere" for the heat to be sent to.
Not at all, a peltier does exactly the same - it moves the heat from one side of the device to the other.

I also agree with the rest, there's no reason or requirement for a bidirectional design.
 

alec_t

Well-Known Member
Most Helpful Member
with a peltier system, there is no "elsewhere" for the heat to be sent to.
Then think of those Peltier-based mini-fridges. Surely heat from inside the box transfers to a heatsink outside the box?
 

Flyback

Well-Known Member
I also agree with the rest, there's no reason or requirement for a bidirectional design.
Thanks, but Maxim give an explanation of why bipolar is needed…

Quoting from page 1 (LHS) of the MAX1968 datasheet shows why a full bridge with bipolar current capability is essential for all peltier controllers…

The MAX1968 operates from a single supply and pro-
vides bipolar ±3A output by biasing the TEC between
the outputs of two synchronous buck regulators.
Bipolar operation allows for temperature control without
“dead zones” or other nonlinearities at low load
currents. This arrangement ensures that the control
system does not hunt when the set point is very close
to the natural operating point, requiring a small amount
of heating or cooling.
MAX1968 datasheet:
https://datasheets.maximintegrated.com/en/ds/MAX1968-MAX1969.pdf
 

tcmtech

Banned
Most Helpful Member
Thanks, but Maxim give an explanation of why bipolar is needed…
Not needed. 'Could be used' would be the more fitting description. :rolleyes:

Fist off what is the as designed thermal limits of the laser diode?

Unless It's some extremely special one of a kind device that has a tiny working temperature range high precision thermal control techniques are a total waste of time, money and component resources.

Just because something can be done certain way doesn't necessarily mean its the best and most correct way to do it if the working conditions do not justify it. :oops:
 

alec_t

Well-Known Member
Most Helpful Member
Maxim give an explanation of why bipolar is needed…
I can see that being useful if you are trying to both heat and cool an object when its temp can go below or above a target temp, but in your case the object temp will always be above or at the target, so I don't see the Maxim approach being necessary. It would be like switching your air-con to 'heat' during the summer, to raise the inside temp deliberately to 21C when the outside temp is 40C and you are trying desperately to get the inside temp down to 20C :rolleyes:.
 

crutschow

Well-Known Member
Most Helpful Member
First, as everyone has noted, Peltiers do indeed cool below the ambient temperature, otherwise they would be of no use .

Second, also as everyone has stated, there's absolutely no need for bipolar current through the TEC for your application.

So if if you want to continue to muck around with "alternate facts" about using bipolar current and ignore the good advice in this thread, have at it, but you asked our advice and that's it. :rolleyes:
 

Flyback

Well-Known Member
Thanks, Due to the thermal time constant of the peltier system being so long, an analog feedback loop would need very big capacitors in it. Therefore, I believe that a simple microcontroller based slow iterative feedback loop would be best. Do you agree?
 

alec_t

Well-Known Member
Most Helpful Member
we just want the lowest temperature that we can always get to
Then why not just run the Peltier flat out and forget controlling it? What justifies throwing an MCU and a precision control loop at the 'problem'? What is the application of this fussy laser diode?
 

tcmtech

Banned
Most Helpful Member
Thanks, Due to the thermal time constant of the peltier system being so long, an analog feedback loop would need very big capacitors in it. Therefore, I believe that a simple microcontroller based slow iterative feedback loop would be best. Do you agree?
No.

A simple and cheap PID loop controller or just a proper PID loop control IC would easily do the job.

Until you define the lasers operating temperature range, intended system working temperature range and ambient air temperature range nothing can be figured out let alone justified.

That and a number of us would love to know what company you work for and how you have yet to ever be fired for gross incompetence of your current job. :(
 

JonSea

Well-Known Member
Most Helpful Member
It seems unlikely that the peltier junction could cool the diode too much. Seems like turning it on any time the laser is operating will suffice. Perhaps a simple temperature on/off switch is plenty.
 
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