What chip to use for a constant current step-down (peltier)? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: December 27, 2014, 05:36:36 pm
content: In short:uC-controlled 6A Peltier - any advice how to do it properly?In detail:I need to provide zero to 6A to a Peltier (cooling only), from no more than 24V.Exactly "how much" should be controlled by a uC.Normally i'd try TL494 with some shenanigans, but i heard people here say to dump this old beast and usesomething modern.So, what sort of modern chip/PWM controller is good for that?Another thing i considered is to use an attiny85 to PWM a FET directly.However, remembering how well it worked the last several times (https://www.eevblog.com/forum/projects/sensing-in-attiny85-driven-buck-converter/msg506894), i'd rather not try again.And inductors would have to be quite large.Something akin to MAX1969 would have been perfect, only with an external MOSFET.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2014, 06:22:32 pm
content: Mind that the Peltier is pretty much resistive, so it doesn't matter how you control it (CV or CC). You could also do blind PWM (TL494, MCU timer/counter?), as long as it's still short-protected (don't want to be careless!) and controlled by whatever outer loop you need (temp reg?).You do want high frequencies, to minimize inductor size and losses, versus no filtering at all. Peltier efficiency drops sharply at rated current, because resistive losses go as I^2, while the Seebeck effect only goes as I. So just going on and off is a lot worse than smooth filtered DC.Tim

author: Artlav
author_rank: Frequent Contributor
timestamp: December 27, 2014, 07:18:03 pm
content: Quote from: T3sl4co1l on December 27, 2014, 06:22:32 pmHm, but how consistent would they be?If i had two of the same model, but bought from the different sides of China, would they produce the same effect at the same voltage?Perhaps it is solvable by calibration.Actually, the idea of just PWMing the thing from uC directly still feels the neatest here.It does not have any runaway modesor sudden changes, so no fast loop response is needed.Not even feedback is needed, given a known input voltage.What sort of measures can be taken to make it fail safe (disconnect->voltage spike->FET shocked to death, and short circuit->all the current->FET fried to death)?Quote from: T3sl4co1l on December 27, 2014, 06:22:32 pmAFAIK it's because the resistive losses are by RMS, while the cooling is by average, and RMS of a PWM is much worse than the average of the equivalent filtered output?Quote from: DiligentMinds.com on December 27, 2014, 06:15:39 pmThe "boost the output" part is much, much more important than "0.001*C" part. I only need to tell within a few *C apart, and using tiny parts at the limit of their dissipation is best avoided.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 28, 2014, 12:08:00 am
content: You wouldn't want to run it completely open loop; of course, that will kill you on ambient alone, nevermind manufacturing differences. But if it's a temperature controlled outer loop, the only effect a variation in resistance has is changing the loop dynamics (how fast/slow it can respond, and over what range of voltage/current) and what the maximum temperature difference is (which will be limited by supply voltage, current, DCR of the device, and whatever max delta-T it supports).You'll still want short protection or something like that, just because it's a switching app, and leaving the inductor completely alone to itself is dumb. You'll also need some startup for the output filter, and yes you can simply soft-start the whole thing, but if it gets interrupted and freezes or jumps suddenly, you still end up with the same problem.One example: set up two transistors as a monostable timer. Turns on until inductor current reaches threshold, then ticks off. Restart it with a pulse. You can have the pulse simply be a constant frequency (short duty cycle, or filtered so it's edge sensitive), and vary the peak current -- so you're making a half-MCU, half-discrete UC3842 basically. To control power, you need a DAC output (to adjust the peak current threshold), and/or the adjustment of frequency. Peak current mode, of course, works best in DCM (discontinuous conduction mode), which means a fair bit of ripple (relatively large capacitors and inductor).You can run PWM from the MCU, but gate it through a missing pulse detector (so if the output accidentally latches high somehow, the output fails safe). And put a peak current detector on there to shut it down for the rest of the cycle (thus integrating the flip-flop action of the previous case), or for some time (in which case it acts like a self-resetting fault). Follow up with a gate driver and transistor.You could do worse than an average-current-mode controller, but this is much more involved. You need an independent PWM generator, and run an op-amp to control output current. But running the current setpoint signal from a DAC leaves the least attention to the MCU -- it only needs to update the setpoint when an event occurs (presumably, the temperature PID will be on the order of one or a few samples per second).Conversely, the most software-intensive version might use the internal analog comparator (tends to be slow, but at a low enough frequency, this isn't terrifically bad) or software polling (and external comparator) to control PWM per cycle. Or the cycle is just a software counter, who needs timer-counters. I wouldn't suggest average current mode control, because you need an ADC that can sample at least once per cycle (something an ATtiny or ATmega won't do -- at least, not with rated accuracy; though if only a fraction of the bits are important, it could be quite doable).Tim

author: Andreas
author_rank: Super Contributor
timestamp: December 28, 2014, 08:33:51 am
content: Quote from: Artlav on December 27, 2014, 07:18:03 pmHello,keep in mind that the current ripple has to be well below 10% (better below 1%)otherwise the element will heat more than cool.http://www.meerstetter.ch/compendium/pwm-vs-direct-currentPerhaps it is easier to drive the peltier with constant power and use a small heater to control temperature.with best regardsAndreas