H-bridge vs op amp control for peltiers - Page 1
Forum section: electronics / projects

author: leonhart88
author_rank: Regular Contributor
timestamp: May 15, 2013, 04:28:57 pm
content: Hi All,Just curious, but what are the main differences between using mosFETs and an H-bridge design, versus power op-amps for motor control or for peltier control?I've only seen H-bridge designs before but I stumbled across the idea of using power op-amps which can be used to output a linear voltage.With an H-bridge design, for a peltier cooler, I would pwm the FETs and then filter out the signal to get a nice, smooth voltage. It seems that using a DAC and power op-amps would also be another way of doing this.What are the advantages/disadvantages of each?Thanks!

author: Paul Price
author_rank: Super Contributor
timestamp: May 15, 2013, 06:27:34 pm
content: Power op-amps are expensive, slow in response time, easy enough to kill by overload, transient inductive spikes, can oscillate at high loads, are very inefficient (since they are linear), and are limited to relatively small currents compared to H-bridge controls which overcome all of these shortcomings. They can be used to advantage if your interested in only controlling low current motors with very simple circuity.Peltier junctions don't need smooth voltages to work well, they are inherently low pass filters in temperature response to their control voltages.

author: free_electron
author_rank: Super Contributor
timestamp: May 15, 2013, 06:46:02 pm
content: why a bridge ? you cant change polarity. all you need is a single powertransistor like mosfet that you control by pwm . no smoothing required. the peltier doesn;t care.

author: cuban8
author_rank: Contributor
timestamp: May 15, 2013, 07:25:12 pm
content: Well, you might want to be able to use the peltier for both cooling and heating. Thus, use an H-bridge.

author: penfold
author_rank: Frequent Contributor
timestamp: May 15, 2013, 07:52:10 pm
content: Your main advantage using a high frequency PWM driven h-bridge to give you a variable heating/cooling control is efficiency.With a linear circuit, you can still bridge two amplifiers to give you a positive and negative polarity to the peltier if you only have a single rail supply.The linear circuit will give you a ridiculously low efficiency figure, in one application I was using a peltier to cool a semiconductor device to a very precise temperature, killing the noise was a prime concern for which I ended up using a linear amp.

author: Psi
author_rank: Super Contributor
timestamp: May 16, 2013, 01:56:23 am
content: If you change from heating->cooling or cooling->heating without giving the peltier time to equalise the temperature it will crack.Normally you change polarity with a mechanical switch as its not something you need electronic control of.

author: leonhart88
author_rank: Regular Contributor
timestamp: May 16, 2013, 08:12:05 am
content: Quote from: Paul Price on May 15, 2013, 06:27:34 pmAh I see. I thought it might be some of those reasons, but always reassuring to hear it from someone else. Thanks for the info guys. Electric polarity control is useful if you need to maintain something at a very precise temperature and also be able to heat or cool.Quote from: penfold on May 15, 2013, 07:52:10 pmSo using a linear amp gave you better accuracies in terms of controlling the temperature? Just curious, but what precision were you able to maintain at (1, 0.1 degrees)?I also see a lot of people saying you don't need to filter the PWM, or that you should filter it using an LC filter. Which is more efficient and why? I originally thought that PWMing is worse because of increasing joule heating. However, say I apply a 10V PWM at 50% duty cycle with a 2 second period. The joule heating is effectively the same as if I applied 5V DC over 2 seconds. So why do people suggest filtering (assuming we are using a suitable frequency for the PWM)? I understand that at larger delta Ts, the peltier resistance actually increases, and becomes more inefficient. Is this a possible reason?

author: SeanB
author_rank: Super Contributor
timestamp: May 16, 2013, 08:28:50 am
content: The reason for using a smooth supply is injected noise, as the junction is going to be capacitively coupled to the heatsink. Also using a smooth supply is going to give better cooling, as the low points on the ripple generate a lower heat flow while still generating about the same heat internal to the heat pump.Using low frequency PWM ( about 10 secs or so cycle time) will give a fast enough response, as the thermal time constants of the transfer surfaces are much longer than this. Switching rapidly from heat to cool will cause thermal stress on the junctions from differential expansion and this will crack the weakest link which is the ceramic device.

author: free_electron
author_rank: Super Contributor
timestamp: May 16, 2013, 01:56:39 pm
content: Go readhttp://www.tellurex.com/technology/

author: penfold
author_rank: Frequent Contributor
timestamp: May 16, 2013, 04:05:58 pm
content: As somebody else just said, the only real reason I went linear was to cut down on injected noise with everything being quite closely mechanically coupled. Well it was complicated, but it was pretty much only due to the noise.Temperature stability was around the 0.1 degrees mark, so pretty tight. That could have been achieved if i was using a high frequency PWM (perhaps above 1 second period) but on the lower frequency side approaching the 10 second period there would have been a reasonably significant ripple in the temperature. It gets very difficult to measure such variations both at what is a temperature level pushing the resolution of most temperature sensors but also at what is a very high frequency for thermal effects.If it was practical for the setup I had I would have gone for the biggest thermal mass possible enclosing the device with a very slow temperature controller. It just so happened that other constraints meant I had to go for a much skinnier design and reduce thermal time constants.

author: leonhart88
author_rank: Regular Contributor
timestamp: May 17, 2013, 04:45:29 am
content: Thanks for all your replies.I realized that I made a mistake in my joule heating calculations, which was why I was getting so confused. Peltiers are less efficient at higher currents (ie. doubling current doesn't mean doubling its' heat moving capabilities). Filtering the PWM keeps the current and joule heating at a more efficient level.Now I've found a OEM TEC controller which outputs a 1kHz PWM output and I was hoping to filter it using an LC filter. I did a SPICE simulation on it with 100uH and 1000uF values, but the simulation showed the output would still oscillate. I'm guessing I will need bigger inductors/caps?

author: leonhart88
author_rank: Regular Contributor
timestamp: May 17, 2013, 07:50:12 am
content: I did some more simulations and using a 10mH inductor will smooth out the signal nicely (0-12V, 1kHz PWM with ~4 ohm load). I couldn't find any fixed inductors that have high current capacity and have 10mH values, so I simulated a common mode choke (744825510) and only connected one side of the inductor.I am not familiar with common mode chokes, but will using them this way cause any harm?

author: penfold
author_rank: Frequent Contributor
timestamp: May 17, 2013, 05:58:03 pm
content: A common mode choke is designed such that the large current flows forwards in one of the coils and backwards in the other and magnetically cancel out leaving only the small amount of flux due to currents that travel 'common mode', chances are a real common mode choke will just saturate when connected as you stated.If you can't find a regular inductor that matches your current and inductance requirements, your best bet is probably to wind your own. If you struggle calculating the inductance then you can always just get a pre-gapped core in a form such as an ETD49 (relatively large) and just keep winding on a few turns until you get the desired result. Not exactly good engineering, but gets the job done for one-offs.

author: leonhart88
author_rank: Regular Contributor
timestamp: May 19, 2013, 05:21:14 am
content: I decided to forgo filtering in my case because it will be a little impractical to have huge inductors and caps. I know it will reduce my efficiency, but I guess it is something I will have to live with for this specific application.Thanks for the help everyone.

author: peltierseebeck
author_rank: Newbie
timestamp: October 03, 2018, 01:48:11 pm
content: I can confirm operating a Peltier module with PWM puts the Peltier cooler in a position to operate less efficiently. Direct current will be more efficient.The reason the Peltier becomes less efficient is that relative to Peltier cooling, the Joule heat generated within the module is exponentially higher at higher currents. Peltier cooling is a linear function of current. Joule heat generated within the Peltier module is calculated as I^2R. Even if the average PWM current comes out the same as steady DC current, the efficiency (COP) will be lower. The time the current spends pulsing above the nominal DC value increases that Joule heat per I^2 R. The average Joule heat is higher than DC even though the average current is the same with DC. Sounds counter intuitive until you do the math side by side.See these white papers about using current pulses for thermoelectric Cooling. Scroll down to the bottom of the page for the downloadable PDF paper.Thermoelectric Pulse Cooling 1Thermoelectric Pulse Cooling 2Also, this company suppliesDC Peltier controllers.Something I would be interested in and have not investigated is the efficiency of the controller itself. Is a DC controller more efficient than a PWM controller for the same reason as above? Or are there other factors? Case by case?

author: coppercone2
author_rank: Super Contributor
timestamp: October 03, 2018, 02:14:00 pm
content: keep in mind, if you have some kind of heating effect in your peltier module, resistance from anything, it lowers its throughput and maximum cooling because your heating the thing.you want thick wires too, so your not heating around the module with your wires. If you are using peltiers you don't really care about the electrical efficiency, but you want the best thermal performance, so you don't want any waste heat being made there that does not result in the thermal flow, more heat sinking per unit area.you also need a fairly fast PWM to eliminate thermal stressing and early life failure on these modules if you go with a strait PWM, if you go with DC you want ramp-rate-control locking.commercial I have seen to be >1KHz, unfiltered, filtered whatever but keep the inductor size in mindgiven that their high current systems, your DC controller is gonna be a switching power supply. I never saw them pulsed with constant current, just voltage -h bridge. feedback is from process thermometer. this is on big systems with lots of modules. maybe you can do something fancy with a single module.and about using a linear amp (P) control, if your controlling temperature process you really want PID.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2018, 04:51:55 pm
content: Unfiltered PWM is a really bad idea - compared to plain PWM even linear control (e.g. a power OP or OP with emitter follower) is more efficient for cooling, even with the higher power loss on the control side. It sound counter intuitive, but with just PWM the extra voltage is lost in the peltier element instead of the linear driver. So this is a really bad idea because even more average current is needed for cooling.A higher frequency does not help with unfiltered PWM - however it makes sense with filtering, as smaller inductors / caps can be used.PWM with a kind of LC filtering with free wheeling diode makes it a kind of buck converter. This is usually the preferred version. The filter cap can be relatively small, as there is no need to bring the ripple very low.Besides special chips one can also use µC based PID control, ideally with taking into account the other side temperature too (as a kind of feed-forward).

author: jbb
author_rank: Super Contributor
timestamp: October 03, 2018, 07:28:54 pm
content: I would suggest a switching requncy higher than 1 kHz! With a 100 kHz PWM stage you only need 1/100th the filter size (e.g. 100 uH and a few uF)There is a control chip, theLTC1923, which is intended for H Bridge with output filtering. Maybe it would be helpful? (Maybe you could just buy a dev board and tweak it...)Edit: I also guess that applying unfiltered PWM directly to a Peltier over some wires could cause EMI problems ie conducted and radiated emissions.

author: coppercone2
author_rank: Super Contributor
timestamp: October 03, 2018, 08:36:09 pm
content: Well the PWM frequency has some kind of minimum related to the thermal time constant of the peltier junction and the process being controlled and the heat sink because if you PWM too slow, it looks like bang bang control, and you cause thermal stresses because some kind of significant thermal gradient develops in the module that can cause its premature failure. They are usually pressed hard between two heat sinks using bolts anyway (for good heat transfer of the grease material) so you definatly want to try to treat them as nice as possible with regards to thermal shock causing additional strain. They are made of ceramics and silicon after all. Very very inflexible, You can break them just trying to pry them off the baked-thermal grease. They behave very similar to oreo cookies, but much more fragile in terms of how they break.But, things like thermal grease have significant pressure sensitivity in regards to thermal transfer, since peltiers are rather inefficent you want good thermal transfer, so you wanna press them between the rigid (this is imporant) heat sinks hard, you can't have a floppy heat sink. When their already tensioned like this, I believe the thermal loading becomes more significant from slow unfiltered PWM. Eventually the PWM gets high frequency enough that it does not really matter as much if you filter or not, but obviously filtering will lead to better results.What threw me off at first was the large contact size. To get proper contact pressure you need to look at the contact size. A processor chip contact size is still kinda small compared to a peltier. One of those modules is bigger then a intel I7, but its made of stiff ceramics.If you have a few peltiers next to each other... even more annoyingYou need very flat heat sinks too.https://www.innovationcooling.com/case-study-contact-pressure-related-thermal-compound-performance/And obviously I recommend high quality thread (using a serial tapping system is recommended, where three progressively larger taps are used to make a smooth thread that is low in asparities), a high grade bolt (you can buy bolts of various grades), or high grade nuts. You might want to actually use the proper reamer for the tap. And real thread cutting fluid.If your heat sink is thick or reinforced you can kinda estimate the force based on screw torque, so long your threads are high quality and clean and you use alot of screws.I don't know what the proper way to do this is, some kind of thin strain gauge maybe, to run a test on, but obviously you can't really include a strain gauge in your finished product.