Switch mode LED driver for a peltier supply? - Page 1
Forum section: electronics / projects

author: orion242
author_rank: Unknown
timestamp: May 03, 2015, 11:45:46 pm
content: I'm looking for a adjustable constant current source to drive a small peltier cooler. A switch mode supply is needed as the input voltage will be 20-30vac and I want to be able to either PWM or voltage signal to control the current to the peltier. While I saw there where a few ICs specifically for this, they seemed silly expensive in small quantities (max1968, max1969). So I started looking at constant current switch mode drivers for LEDs, specifically the LM3409HV.The peltier is a TEC1-04905 which is 5v 5A, and I would like to drive it ~2A. Peltier will not have any real load on it, just looking to maintain 30-40F dT with the cold side insulated (ie thermistor mounted to it and insulated) and a CPU heatsink on the hot side. Running through the datasheet it seems with a low enough switching frequency 250-300khz this IC should work. Vo should be around 2.3v @ 2A.Any major pitfalls I'm not seeing using a switch mode LED driver to drive a Peliter?Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2015, 12:48:15 am
content: You do want to use a filter choke and all that, as a proper switching supply. Otherwise you get full ohmic losses and terrible performance, using pulsed DC. That'll also mean the controller gets to do its job (presumably it's sensing inductor current and switching based on that).Control method doesn't really matter; voltage mode would be bad if it's ever possible to short the output, though most controllers are at least current limited, if not full on current mode control, these days. You don't really need to regulate the current, just as long as it's consistent enough for your outer loop (which I assume will ultimately have regulation around the thermistor or whatever).In fact if you get a controller with the error amplifier exposed (usually they have a VFB or -IN pin and a COMP pin), you can do the temp loop all in one chip.Anyway, an LED controller as such, should be fine; probably any random controller would be fine, as long as it can be controlled as you need.Tim

author: ajb
author_rank: Super Contributor
timestamp: May 04, 2015, 02:52:37 am
content: Not a bad way to go. You've picked a controller that can be adjusted with an external control voltage, which is good, since full scale PWM is definitely not the way to go for TECs! You'll want to pay close attention to the ripple current through the TEC to maintain efficiency.Really, you could go even simpler if you wanted, and just drive a half bridge straight from a sufficiently high frequency PWM output--you don't necessarily care what the precise current through the TEC is, as long as it's appropriate to the current heat load and not going to blow up, so you don't, strictly speaking, need a current regulating device like an LED driver. Of course some sort of overcurrent protection would be advisable to prevent catastrophe. This is basically what you wind up looking at if you want to drive much higher power TECs--with correspondingly larger inductors in the output filter.

author: orion242
author_rank: Unknown
timestamp: May 05, 2015, 12:53:48 am
content: Thanks for the replies, seems like I'm not totally out in the weeds.I don't really require any temp control, I'm going to using this to teach PID tuning and plan on interfering with the temp regulation with a uC in the middle to simulate several scenarios.I liked the fact with CC any shorts wouldn't be an issue, though thinking about it your right almost all switchers seem to have O/L protection. I have this guy drawn up so I may as well roll with it.To PWM or not to PWM.... Seems that there are two very divided camps there. These will not see alot of usage, but when they do it may be students trying to beat the living crap out of them. Even if I use the analog control with the LM3409, someone could drive it from 130F to 50F constantly just to see what happens. I would assume this could cause pretty good thermal stresses on it and may be something I simply prevent in the uC.

author: ajb
author_rank: Super Contributor
timestamp: May 05, 2015, 05:35:01 pm
content: Quote from: orion242 on May 05, 2015, 12:53:48 amThat sounds neat! Wouldn't it be simpler and more effective, though, to control the heat load on the TEC with your MCU (assuming the student is controlling the TEC)? Or is the TEC intended to interfere with a heater that the student is controlling?As far as PWM, a TEC will always be more efficient if it's run at a steady state current that is appropriate to the heat load versus being switched between zero and full power. In other words, you don't want to actually PWM the TEC at all. Of course for efficiency's sake you probably do want to use a switching topology, but you want to filter it sufficiently that the TEC sees only DC. You can do this with a dedicated buck controller like you've chosen, or you could directly drive a switching stage from an MCU with a suitable output filter--in either case, you're using an inductor to convert PWM into DC. So if you go with the LM3409, you're definitely better off using the control pin as an analog control for the sake of efficiency. Thermal cycling will also be reduced, but as you suggest may it be unavoidable given that these are essentially guaranteed to see unstable control loops, so. . .probably best just to choose cheap TECs

author: orion242
author_rank: Unknown
timestamp: May 07, 2015, 02:30:13 am
content: Quote from: ajb on May 05, 2015, 05:35:01 pmThe smaller the load the better, low thermal mass is inherently trickier to tune a PID loop with, not to mention reduces the heatsink and current I need for peltier. That's why on one side I plan on nothing more than a couple of thermistors and insulation. One thermistor will be for the student Pv on the their loop, the other to my uC. This is basically going to simulate HVAC heating and cooling coils, and I'm planning on generating a range of something like 50-130F which seems easily obtainable with this setup and current level. As for the software interfering with their control I have several ideas but working on the hardware at the moment. I'm more a software guy by day and a hardware tinkerer at nite...I can always adjust the "load" by scaling the students drive signal, or even drive the peltier in the opposite direction as needed to create overshoot, Pv time lag, etc. My uC will read their heating and cooling signals, apply whatever I wish, and then drive the peliter they are measuring the temperature of.Didn't think of using the uC as the switcher.... I should have the speed to do it, but I want to get this done sooner rather than later. Maybe something I need to pursue in a later rev. Thanks for the idea!

author: ajb
author_rank: Super Contributor
timestamp: May 07, 2015, 07:02:23 am
content: Quote from: blueskull on May 07, 2015, 04:24:55 amAs you increase the current through the TEC, ohmic losses increase with the square of current, but the heat moving capacity increases at best linearly. So 50% pwm will be substantially less efficient than 50% DC. Compounding the problem, every watt that you apply to the TEC appears as heat at the hot side, which raises the hot side's temperature (assuming you're regulating the cold side) and increases the temperature delta across the TEC which reduces its coefficient of performance (watts of heat moved per watt of applied electricity). That makes the extra power you're burning during the on-period of the PWM waveform a double hit to your system efficiency.And on top of all that, there's thermal cycling to consider.