Thermal Design Considerations Linear and Buck Converters - Page 1
Forum section: electronics / beginners

author: InsertUsernameHere
author_rank: Contributor
timestamp: May 03, 2020, 12:54:52 pm
content: OK I've been trying to figure this our for weeks now, I don't know how many data sheets I've read through to try and source parts for my application but I must be missing something.My power requirements are as follows:12V @ 250mA5V @ 2A3.3V @ 2AI'm using an ambient temp of 50 degrees Celsius as this is an outdoors application in a sealed box.My board is fed 12V DC from an external PSU.I need to convert the 12VDC down to 5VDC and 3.3V DC both at 2 Amps (max).When working my thermal calculations with a linear regulator to deal with that drop I sometimes get junction temps in excess on 1,000 degrees so obviously that wont work.So my current plan is to use a dc-dc converter to drop the voltage to 6.5V then use my LDO voltage regulators as a post switching regulator with a beefy heat sink to manage the temperature.There is one thing i cant get my head around however, how do you work out thermal dissipation for the dc-dc switching regulator, for example using TI's Webbench Designer Software, it spits out a circuit below that is apparently capable of 4A over the input voltage range of 3.8 to 36V.The problem is the thermal properties for the switching IC are below, so I don't know how this will work as its SOIC package and I cant really heat sink it, what am I missing here?If i need to reduce 12v to 6.5V at say 3 Amps then that roughly 16 Watts and at that junction resistance is 700 degrees?Do i use the normal calculation or does the efficiency factor come into things?Sorry for the probably pretty dumb question but I'm at a loss here.Cheers.

author: Etesla
author_rank: Regular Contributor
timestamp: May 04, 2020, 01:38:20 am
content: Only dumb questions the one you don't ask. I am unfamiliar with calculating junction temperatures, and I think it's fairly uncommon to do so... I think I may have found one of your misunderstandings in your post though, and I'll try to explain it here.You said "If i need to reduce 12v to 6.5V at say 3 Amps then that roughly 16 Watts and at that junction resistance is 700 degrees?"I think this shows that you're thinking about DC DC switching converters the wrong way...That 16 watts of power dissipation is true with a linear regulator, but not true with a switching regulator. That's the whole point of switching regulators with efficiencies around 90-95%, meaning the power out of the converter is .9 to .95* the power in. In other words, voltage*current out is .9 to .95* voltage*current in.For a 90% efficient regulator the produced 6.5V with a maximum output current of 3A, only 6.5*3*(1-.9) = 1.95 watts would be dissipated by the entire converter when the converting is operating at a full load current of 3 amps. That power dissipation would be spread between the switching mosfet, the inductor, and the diode for a standard buck converter topology. Spice simulations with good models are the best way to see what components dissipate what amount of power. You don't even need the model for the integrated circuit you are using to do this. Just use the standard textbook layout of a buck converter, and get models for your diode and mosfet. Then just feed the gate of your mosfet with a fixed duty cycle square wave to give the correct output voltage, for buck thats duty cycle = Vout/Vin*100%. I highly recommend LTSpice for all your spice needs.From there you might get that the mosfet will dissipate 1 watt, the diode half a watt, and the inductor half a watt. Then you just look in the datasheets for your chosen components and see if the wattage they are expected to dissipate is within the 'maximum power dissipation' rating. You will also want to check that currents through, and voltages across your chosen devices are within their respective current and voltage ratings.Lots of parts won't give a 'maximum power dissipation'. In this case, just make sure that you stay well within the current ratings and you'll probably be alright if you have a reasonable heat sink.I took a class on DC to DC converters in college and didn't worry about 'thermal resistance' ratings even a single time. I might have had a bad professor, or it might just not be the normal way to go about it...

author: InsertUsernameHere
author_rank: Contributor
timestamp: May 04, 2020, 02:18:59 pm
content: Cheers mate, yeah i thought I must have been missing something with the efficency, I still have alot to learn.I havent had a play around with LTSpice yet, I'll have to download it and have a bit of a look.I'm thinking something like this should to the job then.https://au.mouser.com/ProductDetail/Texas-Instruments/TPSM84424MOLR?qs=sGAEpiMZZMt6Q9lZSPl3RQsOIly2NLEkAcMqE2lQaUZy7B9M7zoBtQ%3D%3DThanks again.

author: thinkfat
author_rank: Unknown
timestamp: May 04, 2020, 02:31:21 pm
content: The converter in your example seems to be a "synchronous buck" type. You can expect an efficiency of over 90%. You'll still need to sink about 1W of power somewhere. But it won't be a problem to dissipate that through a generous ground plane connected by thermal vias to the center pad of the chip.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2020, 04:05:13 pm
content: What's the 5V powering? There are few applications where a switching supply won't suffice. And an LDO doesn't fix very much, on account of having poor PSRR at switching frequencies. Likely common mode noise is the bigger offender anyway, which cannot be fixed by LDO.In other words, toss on some more LCs (input andoutput -- buck input is noisier!) and be done with it.Common mode noise is avoided by following good layout. For a common-ground DC-DC converter, it takes an almost willfully bad layout to be a problem.An LDO for 3.3V, from 5V, isn't unreasonable, but at 2A, is kind of a lot. If you don't mind dissipating the 3.4W, that's fine, but a switching reg again here will save >2W on that.Will this box be exposed to sunlight? Your estimate of 50C ambient may be woefully optimistic! Design accordingly, put in a shaded area or add shades or ventilation as needed.Tim

author: InsertUsernameHere
author_rank: Contributor
timestamp: May 10, 2020, 08:01:34 am
content: Sorry for the late reply guys, I really appreciate all the help so far; thanks!The 5V rail is powering a few ADC's for interfacing with various sensors 4-20mA mainly, an LTE modem, Level Shifters, 2 quadrature encoder counter IC's, an LCD and driving a few relay coils and LED's.The 3.3V rail powers a microcontroller, SD Card, GPS Module, Level Shifters, and a few I2C LED Driver IC's etcI've since watched a few videos from Dave about how Linear regulators are not actually that good at smoothing ripple, so thanks for the heads up there.There has also been a new development in my design in that I now need to be able to accept AC power.So with that in mind I've now gone back to the drawing board and i'm thinking of using the Mean Well PCB mount IRM-60-12http://www.meanwellusa.com/productPdf.aspx?i=687with 12VDC @ 5A output.The ripple on this AC/DC converter is 120mVp-p and I want to power a few 4-20mA sensors directly off this 12V rail plus feed it into the dc-dc buck converter, should I filter this output with a capacitor multiplier circuit maybe?I am going crazy over working this one out but its been a great learning experience so far, I thought the other parts of the circuit would cause me dramas but the power side has been the hardest to grasp.I'm about to pull the pin on my first oscilloscope so once I get that I'm hoping to get a few bits and pieces to bread board different options and understand the ripples/noise of each typology.My thought process at the moment is ac/dc -15V (for headroom) then RC filter capacitor multiplier circuit using a darlington pair to get the current requirement, DC-DC buck Converter -> 6.5VDC then another capacitor multiplier RC filter, LDO for 5V and another LDO for the 3.3V.It just sounds god awfully complex for what it is but I don't know what the "normal" way of doing something like this is.Am I on the still right track here, I cant help but wonder that I'm making this so much more harder than it needs to be.Yes I can have this protected from direct sunlight so hopefully 50 degrees is ok to use.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2020, 12:45:23 pm
content: Okay so nothing needs to be particularly quiet, that makes sense. The ADC can be separately filtered with little burden, and its reference voltage is the most important part, which can be provided by a separate component entirely.Just how many LEDs is that? The 3.3V load doesn't sound like more than... 100mA maybe even is a lot? Might also shift those LEDs to the 5V rail (often the I2C logic can still be powered by 3.3V for logic level compatibility, while the LED supplies can be higher), which puts more load on an already beefy rail (guessing one or more of: modem, LCD, or LEDs, is what's dominating that load?).Also, with big, transient loads mostly removed from the 3.3V rail, it becomes cleaner, and it may be of interest to run the ADC from it.What are the 4-20mA sensors, what kinds?Any more filtering than this (i.e., using mostly raw SMPS for most of these 3.3 and 5V loads) requires justification, in my book. You haven't shown any need, so your insistence, just... looks completely superstitious!Tim

author: InsertUsernameHere
author_rank: Contributor
timestamp: May 12, 2020, 11:16:54 am
content: Number of LED's powered from 5v rail is 6 and 1 LED powered from 3.3v as a power indicator only.The module im using G01 from Pycom indicated is need a minimum of 950mA at 3.3V the I have the other minor loads.The LED's are powered from the 5V rail they are just controlled by an IC that uses 3.3V for the I2C bus etc.The ADC's ADS1115 need to be run off the 5V rail as the analoug inputs maximum input voltage is VCC +0.3V and I'm feeding a 1-5V signal (4-20mA passed through a 250ohm resistor) into them.The 4-20mA sensors are mainly liquid level sensors like thishttps://www.sensorsone.com/2m-depth-diesel-ip68-tank-level-transmitter/I just thought the ADC's, possibly the quadrature decoder IC and the microcontroller would need lees than the 100+mV ripple specified in both the AC-DC output and the DC-DC outputs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2020, 12:25:13 am
content: Hah, this is sounding a lot like a project I worked on last year... they're using similar sensors (as far as I know) but requested 24-bit ADCs, ultimately to obtain weights-and-measures approval, for the application was fuel metering and sales.Yes, you'll probably want additional filtering, but nothing crazy should be needed. POL converters should be quiet to the tune of ~mV, unless they've been chosen, designed, or tested, poorly.Tim

author: InsertUsernameHere
author_rank: Contributor
timestamp: May 23, 2020, 03:39:25 am
content: OK so i think I've finally settled on a dc-dc converter solution for the 5 and 3.3v rails, hopefully they will be able to feed straight into the adc and microcontroller without much issue.The one thing I'm banging my head on here is do i need to worry about the capacitor multiplier on the output of the ac/dc converter to feed into the dc-dc converters or is it probably a waste of time or can I just replace it with a large electrolytic cap?See what i have below now:https://ibb.co/DtLr4Mm

author: james_s
author_rank: Super Contributor
timestamp: May 23, 2020, 04:10:19 am
content: Personally I would just use an electrolytic cap following a bridge rectifier, that will work fine on either AC or DC input. Calculate the capacitor just like you would for a linear power supply, in practice I usually just pick something that seems "about right", slap together a prototype, load it up and poke at it with a scope to see how everything is behaving. Don't forget to sweep it through the full input range it is specified to handle, and a bit beyond to make sure it's not on the ragged edge.