Multiple independent TECs on one heatsink -- potential instability? - Page 1
Forum section: electronics / projects

author: shridmaster
author_rank: Contributor
timestamp: June 27, 2025, 07:37:52 pm
content: I have multiple lasers being held at a steady temp with a TEC control loop, type of assemblies you can get off Thorlabs and the like. The temperature very directly affects every performance metric so it's absolutely crucial these be held as steady as possible. For space and design constraint reasons, we are putting one heatsink w/ heat pipes on all of them and putting a fan on that heatsink.My concern is that I'm taking what could originally be modeled as 4 independent single-input single-output (SISO) control loops, and now coupling them together through the heatsink to create a multiple-input multiple-output (MIMO) system, where the heatsink acts as a zero and creates a phase lead which makes the system unstable overall or leads to some oscillations.Is my concern valid? Is this a potential problem, or am I overthinking it? If it is a problem, can I fix it by simply moving up my gain crossover frequency (using an analog controller from Analog Devices with adjustable PID) or do I need to switch to a digital control loop that can handle a MIMO system?

author: dietert1
author_rank: Super Contributor
timestamp: June 27, 2025, 07:55:15 pm
content: I think your concern is valid if the thermal mass of each laser module is more than the thermal mass of the heat sink. But i would guess that the heatsink is pretty massive and works as a low pass limiting interaction to a frequency range covered by the TEC PID controller with enough loop gain. Also you want the heat-sink to be large since otherwise the fan will couple it to rapid ambient temperature change.Recently i made a TEC oven for a metrology standard where the oven is a massive aluminum enclosure with two TECs, one attached to the main body and the other one attached to the enclosure lid. The TEC cooler heatsinks are rather small with slow running fans (China kit). There are two separate PID controllers and no interaction was observed at all. Of course inside the oven there are two temperature sensors each close to "its" TEC. So the loop is pretty fast.Regards, Dieter

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 27, 2025, 09:03:39 pm
content: Quote from: shridmaster on June 27, 2025, 07:37:52 pmHow much does the laser power change between each of the 4?A bigger problem than control loop stability here, could be crosstalk, where a 'hot' laser disturbs the ones next to it.A 'big' heatsink will certainly add thermal inertia and reduce thermal resistances, but there will always be skews along the heatsink with 4 unbalanced sources.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2025, 05:46:56 am
content: Your concern is valid because thermoelectric coolers are only capable to produce a temperaturedifferencerelative to their input power. So if one cooler needs to do more work, the heatsink gets hotter, and the others need to start doing more work as well - and there is delay in the process due to thermal mass.Maybe the heatsink can handle their combined power, and maybe your control loop is good enough for this to not become a problem. But the concern is valid.Thermoelectric coolers have poor efficiency which drops even more with increasing hot side temperature, so I would recommend putting engineering effort to minimize the hot side temperature as much as you can - often that means liquid cooling.

author: Simmed
author_rank: Frequent Contributor
timestamp: June 28, 2025, 07:30:22 am
content: if you can simplify the pump character of the tec/tecs (2nd order equation ?)it can be simulated to certain extentthe opamp is to simulate drivethe RC stuff are the thermals

author: shridmaster
author_rank: Contributor
timestamp: June 30, 2025, 03:50:57 am
content: I suppose part of my worry is that the TEC itself operates on a pretty non-linear curve and control of it assumes two values of three (hot side temp, cold side temp, heat exchange) are given. In typical single TEC equilibrium, hot side temp is handled by the rest of the thermal system, heat pipes + fans whatever; cold side sees fluctuations that the loop controls through heat exchange which is determined by current drive which is what the electronics actuate. Basically, the TEC becomes a LTV system rather than LTI.I agree if there are two TECs/thermal loops coupling between each other, as long as the heatsink is sufficiently larger than the thermal mass of the lasers then pretty much by definition each loop should have phase margin. As we increase the number of independent loops sinking heat into this heatsink, I think this changes. I'm having a hard time figuring out how to even approach modeling this situation. I'm considering looking at it as an LTV state space problem, which would take a lot of effort that would be worth it if it needs but I'd prefer to avoid if we can make reasonable assumptions that collapse the problem to simple independent SISO loops.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 30, 2025, 06:16:38 am
content: How much power is involved anyway?Given that the efficiency of Peltiers drop nearly linearly as dT increases, one usually wants to do everything they can do reduce the hot side temperature. So you pretty much want the "infinite heatsink", not just for control loop stability, but for the thing to function at all. So I think if your shared heatsink is heating up significantly from the contribution of multiple units (vs. just one), you might have bigger issues than just the loop...Exception to that would be if only little power is involved. Say, a 60W Peltier unit used to cool a laser dissipating 1W. Then you would have significant hot side headroom for regulation, which you could sacrifice by letting the hot side become hotter with multiple units, and then you could indeed have issues with control loop.

author: miro123
author_rank: Frequent Contributor
timestamp: June 30, 2025, 07:22:00 am
content: I've run to the similar problem for my big oven. I went digital control route.Disadvantage - Slow start of project. Need a lot of extra components. ADC frontend, high-res multichannel ADC. Microcontroller. Last but not least software.Advantages.- flexible soution.- easy to tune - open loop ADC trace data allowed me easy to find plant parameters of the system.- allow implementing complex control - my control code grow to 2000+ line of codeI have chosen the following approach1. de-couple the mimo to multiple siso - they serve as inner loop2. outer loop control the common AL-block temperature3.many feed forward path- Based on plant parameters- Outside temperature- DUT power consumtionFinally the system runs 95..99% at feed forward.PS: final code went so big because it has many control blocks - like overtemparature, power or current limiters, state machines. ramp-up/down limiter - just to name few of them

author: ajb
author_rank: Super Contributor
timestamp: June 30, 2025, 05:36:38 pm
content: It's a reasonable concern, but I don't think it's likely to cause a problem in practice, as long as you have enough headroom in the heatsink capacity and the TEC's Q/ΔT curve for your highest power/temp operating point. The heatsink temperature will naturally change much more slowly than the laser temperature due to the higher mass, so a loop that is optimized to regulate the cold side temperature shouldn't have much of an issue. With well-placed temperature sensors there shouldn't be much control lag to allow for much overshoot, as long as you don't overdo the integral component of the control loop. As far as any one TEC control loop is concerned, the change in hot side temp caused by the other TECs is equivalent to a change in ambient temperature -- albeit probably faster, since the other TECs have better coupling to the heatsink than the ambient environment does -- so you can model it the same way. As Simmed shows, you can translate the thermal system to electronic components and simulate the whole system in ltspice if you want.Quote from: miro123 on June 30, 2025, 07:22:00 amThis is a good point. MCUs with ~16 ADC inputs are widely available, so it would be very easy to monitor TEC current, cold side temp, and hot side temp, for all four channels plus heatsink, ambient temps, etc. The time constants of the system are slow enough to easily stream all of that, plus whatever internal variables, into a serial console or plotting tool in real time or load into a spreadsheet for later analysis, and you can use a generous sampling time to get the best out of the MCU's ADC, and still have plenty of time for control loop calculations. It's also very easy to implement multiple control modes, for example you might start up in a simple fixed power mode to get the system close to operating conditions before switching to PID mode, or set a total system power limit based on temperature, etc.

author: shridmaster
author_rank: Contributor
timestamp: June 30, 2025, 06:13:35 pm
content: It's important to note that the interface is not a big heatsink, it's heat pipes that wick the heat away to a heatsink. The heatsink is relatively large, yes, but the heat pipes are quite fast and I don't think have the low-pass filter effect people are noting. You can take a heat pipe that's burning hot, put it in some ice water and in under a second it'll freezing cold to touch, it's really fast.

author: ajb
author_rank: Super Contributor
timestamp: June 30, 2025, 08:12:03 pm
content: So is it an independent heatpipe from each TEC to the heatsink? Or a single heatpipe that joins all of the TECs to the heatsink? If the latter you will certainly get more interaction between the TECs than in the former, but it will still be moderated by the mass of the heatsink in either case. A first order approximation of the system might look like one of the below, where the current sources represent the total heat produced by the lasers and TECs, voltage represents temperature with ground being ambient, and capacitance represents heat capacity. Clearly the coupling is more complex in the latter/right case, but the heatsink still forms part of an RC network attached to the TECs.

author: shridmaster
author_rank: Contributor
timestamp: July 02, 2025, 02:53:36 pm
content: Thanks for the suggestions! I'll try simulating it this way, the TEC controller I'm using is one that's in LTSpice so that may help. I'll have to simulate the properties of the TEC as well.The lasers all share one heatpipe. Thinking about it, the thermal mass of the heatpipe may be less than that of the laser, the thermal resistance from cold side to the actual seed + NTC is likely higher than thermal resistance from hot-side to heatpipe, which means it's higher than TEC to TEC.I'll simulate with some reasonable values to see whether it's worth investigating, and if it's within reason make measurements to figure out the cold side to seed laser thermal resistance.