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Just how hot can things get?

Just how hot can things get?

Just how hot can things get?

(OP)
Just designed a switching charger.

Input: 24VDC
Output: 5-30VDC
Output: 10A

My normal designs nurse nanowatts out of coin-cells and run for years on a battery.  This thing funnels 300W thru FETs and blocking diodes etc with 28A thru an inductor.

Anyway, steady state conditions are shaping up to parts on the board under convection cooling hitting 80C.  Nothing seems too mad about it but I am not used to feeling heat radiating off my happily running boards.

The hottest part is the blocking diode. It has the worst forward drop and hence the highest dissipation 80C. The part can run the junction @ 150C so I am well below that.  I am not worried about torching anything, I am more worried about longevity.  On the one hand this is a charger application so I expect maybe 24Hrs charging cycle. Initial time charging some dead battery, running the full 10A, and then tapering later to eventually full cutoff.  Then not charging again except to make up again from an engine start every 2 weeks.

My question is where do you guys consider power electronics to be running too hot for reasonable life expectancy?  A lot of off-line switchers have to do this endlessly when do you guys start reaching for fans and heat sink forests?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Just how hot can things get?

If the parts are 80C in a (presumably) 20C environment, then what would they be in a 40C or 50C environment? Ouch.

RE: Just how hot can things get?

I have seen diodes run hot enough to desolder themselves.

Not by intent, of course.

Cure was 1cm square brass fins soldered to extra long leads between the diode and the pcb.



Mike Halloran
Pembroke Pines, FL, USA

RE: Just how hot can things get?

Running a junction at 150C is inviting problems - why push a component to the edge of the envelope when there is almost certainly a similar component which can do the same job cooler, unless this is really cost-critical and longevity doesn't matter? Can you replace the diode with a TO-220 or TO-247 style and bolt it to a heatsink? Better still at the voltage and current you're looking at you might be able to cut your losses in half by substituting a Shottky diode.
 

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: Just how hot can things get?

Keith,
      if you are saying the case temperature is 80degC but that is ok because the junction temperature is allowed to be 150degC you are missing something. The junction temperature is the case tempertaure PLUS the power dissipation multiplied by the thermal resistance from junction to case (theta j-c).

Suppose 20W dissipation and theta J-C = 5degC/W. That makes the junction 80+ 20*5 = 180degC.

RE: Just how hot can things get?

I would consider the environment the product is going into.  Is the box going to become a shelf for stacking papers or boxes on.  Will the environment have a lot of dust that will coat components and thermally insulate them?  Will a fan filter be maintained and will the fan just bring more dust into the cabinet.  I've seen a lot of products where they stacked the electrolytics right next to hot components, a bad thing in itself, and block airflow.  Location can easily lower temperature several degrees at no cost.

RE: Just how hot can things get?

Hiya-

I too vote for a cooler diode!  A blocking diode failure can lead to catistrophic failure down in the design.  

As mentioned earlier, the junction temp will be much higher than the case temperature and I'm glad that logbook typed in the equasion, rather than me having to go back and look in my notes.

I can envision the supply "letting go", the batteries getting way over charged and going "pop" at an inconvenient time.

Your call of course.  Don't know how closely "tied" to the product you are and what sort of liability you want to assume.  Even if the client REALLY wanted to pinch pennies, I'd sure make the "well the cost of repair" arguement to them.  For me, I'd be irritated and somewhat nervoue with anything really greater than 40C.

Hope this helps!

  Cheers,

    Rich S.

RE: Just how hot can things get?

What logbook says is gospel.

However "case temperature" is a slippery concept with surface mount semiconductors, where each terminal has a different thermal resistance and heat sink.  Either be very conservative, or use the junction itself as a thermometer (which requires a calibration effort that is likely to outweigh the benefits for a small volume design).  Unless the design is very price sensitive, I'd just throw big strong devices at the problem.

Electrical and chemical degradation rates tend to about double for every 10 °C increase in temperature, and Tj,max is a guarantee of functionality not reliability.  Staying toward the cool end of that exponential curve does wonderful things for reliability.

If battery overcharge is expensive and/or hazardous, I'd strongly recommend an overvoltage crowbar on the output.  Just give it good RF filters so it won't get set off accidentally.

You'll probably also want a fuse or mechanical circuit breaker too, to protect the charger from the battery and vice versa.

A rectifier on the output might be a good idea too, if anybody might ever plug the battery in backwards.

RE: Just how hot can things get?

Yep, logbook is right, you have to _compute_ the junction temperature, and design accordingly.

One other thing about the land of high currents; you can't use the usual vapor- deposited foil on your circuit boards.  You must use heavy, heavy foil, and wide traces.  At 10A, every wire and every trace is a resistor.



Mike Halloran
Pembroke Pines, FL, USA

RE: Just how hot can things get?

(OP)
Well thanks for the good input everyone!

This is an SMD design. All the parts are SMT mounted onto relatively large copper planes.

These parts are all D2PAK or TO-236(same thing) essentially a TO-220 with chopped, formed leads for SMD.

A large effort was made to maximize the thermal planes, with dozens of large vias connecting generally the bottom board plane and an inner plane to the top component mounting plane.

VE1BLL: Yes! A serious concern. That 80C is based on a lab temperature of 21C so we are talking a 59C rise. A 50C ambient equates to 50+59 = 109C (Tab temp)

Mike: I haven't seen that but I have often seen brown ditches under resistors that have run hot and long. Occasional charring too.
I wonder if there are heat-sinks you can mount between an SMD and the board to augment the thermal aspects... Never seen one - not sure how the other two pins would like the interjected angle a heat-sink (there) would cause.

Scotty: Already thought of that!  It's a dual 10A Shottky to start with.  I am seriously considering a TO-220 on a heat-sink.

logbook: (theta j-c) = 2.2C/W worst case as this is for one diode and there are two in parallel in this package.
http://www.st.com/stonline/books/pdf/docs/8001.pdf
P = 0.5V x 10.1A
P = 5.05W
So presumably we have Junc-temp = 5.05W x 2.2C/W + 80C(measured case)
JunctionTemp = 11.11C + 80C = 91.1C

This is 40C below 150C (150C being somewhere we don't want to ever visit..)  Wherein lies my question; What's reasonable for not turning boards funny colors and having scary smells occur, and having the system be thermally 'tender'?


OperaH: Classic industrial enclosure. Sadly at the top. Think Hoffman box - no ventilation. CPU controlled fan is being considered.  NO GROANING!!! STOP THAT!! This isn't that bad an app for a fan since it would only be used as needed.  Remember this is a charger that may not run at all for 95% of the time and when it is running may be dishing out only an amp or two to top up the batteries occasionally, and then a larger load long enough to replace an occasional engine startup debt.

I ran an hour test yesterday.  Board horizontal took all temps after S-S reached.  Then I stood the board on edge in final mounting profile expecting much better cooling due to both side's convection improvement.  I saw about a 2C drop in most locations but surprisingly I saw 2C increases in two places (one being the dang blocking diode!).  So I am seeing exactly what you're mentioning.  The output cap bank is probably blocking the diode's wind.

Dan:  They're already parallel diodes in one case. Which probably isn't what you're thinking.rainbow
They do mention that these two diodes can experience thermal runaway.  I don't recall, do diodes have a negative tempco on their forward voltage?? That might mean I don't actually have two diodes only one with a fellow passive observer.

Hi Rich: I was having those qualms but you are in the same field as I and most our designs have no detectable heat rises anywhere so having a board covered with parts that are actually 'radiating' makes me queasy - but think about it.  Your 40C is only a cold hot tub! I could spend a lot of money  on hundreds of boards to battle for that.  If it really isn't required what is  required? That! Is the question..

Bobby: Welcome to Eng-Tips.
Yep I hear you on the 10C double the reaction rates my Dad impressed that on me long ago.  I have fusing and multiple feedback paths including fold-back protection so I'm not concerned with blitzing the batteries - especially since they are large diesel starting batteries with the ability to actually dissipate 150W all day long in some twisted error mode.  The blocking diode I'm referring to is the diode to prevent back-feeding the charger or reversing the battery leads.

The job is somewhat price sensitive.

Mike: See above logbook. This is a 2oz layout and most traces are over 1/2" wide usually on both sides.

I'm hoping someone who makes switching supplies or? to say, "we run our 100,000hr MTBF switcher diodes at xxC all the time".  That would give us a serious clue as to what the reliability, board neighbor components, and board material can tolerate.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Just how hot can things get?

Keith,

Often, the copper within the board is used as heat spreader, particularly with lots of thermal vias.

The reliability question is, of course, compounded by the fact that most people don't run their systems at max ambient at all times.  Ideally, you need to come up with an operational profile for percentage of time at max ambient and calculate according.  However, without something like Relex, accuracy is dubious, although it's dubious even with Relex.  MIL-HDBK-217, is extremely conservative for complex circuits, but might be adequate for discrete devices

TTFN

Eng-Tips Policies FAQ731-376


RE: Just how hot can things get?

Keith, you can RF your initial post and ask to have the title edited.  I've done it many times, and not always just for my own posts.  Site management seems happy to do the editing.

RE: Just how hot can things get?

Wow!  Surface mount!  I wouldn't have imagined it possible.

The last SMD part with which I became familiar was a pass transistor for a servoamp in a printer carriage drive.  I became familiar because I was using the printer heavily, and every once in a while, the solder under the transistor would crack, and the transistor would fall off.  It was running at 1.25 A.

That was >10 years ago.  Maybe SMD is better now.

Mike Halloran
Pembroke Pines, FL, USA

RE: Just how hot can things get?

You might consider running one, or a small batch, through some burn-in tests in an oven set to maybe +40C. If it survives, repeat at +50C. To find the weak spot, ramp it up slowly from there until something happens.

It's not an unresonable test since many places really are +40C in the shade.

RE: Just how hot can things get?

+40ºC would be a DREAM environment.  Gad, you guys are SO LUCKY!!!  We run our reliability predictions for a constant 35ºC ambient.  Our latest specs call for +60ºC ambient, which is at the outside of the enclosure, putting the interior ambient at a minimum of +70ºC.

TTFN

Eng-Tips Policies FAQ731-376


RE: Just how hot can things get?

Itsmoked,

since you are using Schottkys you should be aware of thermal runaway due to reverse current:

a Schottky diode operated close to it's maximum rated juction temperature might have a reverse current of several 10 mA, so blocking losses beeing really signicficant

In this case your design will not only have 29 K higher junction temperature when moved from 21°C ambient to 50 °C ambient, it might exceed maximum juction temperature within a few minutues or exibit an increase in junction temperature significantly higher than the increase in ambient temperature.

RE: Just how hot can things get?

Well you have 10A output from 10A diodes. Not good. I know you have two in parallel but they don't parallel perfectly. You are running them far too hard, regardless of the temperature. It is the forward voltage that is too high. If you use 20A diodes everything should calm down. The power dissipated will be less because the volt drop will be less.

Have you worked out the peak current in the pair due to the duty cycle? I would rate each diode above this peak to make a robust design. ( I have several thousand switchers out in the field to my design.)

RE: Just how hot can things get?

(OP)
Thanks log and eletric.

I've had problems with Schottkys used for protection on analog INs leaking like a sieve back into the 'protected' inputs.  These big diodes do in fact leak horribly at elevated temps, (130mA), I see what you mean about extra heating due to that alone.

I'll check out some other diode options.

<later>
After a couple hours.. I can't even find any non-Schottkys.  Sheesh.  I'll continue.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Just how hot can things get?

When forward biased, heating and positive tempco effects can cause current bunching, accelerating failure.  High temps/currents over a period of years will affect the impurity diffusion in the device structure and destroy the diode's junction properties.

You may be hard-pressed to find 20A SMD diodes, though without looking myself I probably shouldn't say that.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Just how hot can things get?

A couple of things;

You can get a Wakefield 217 series surface mount heatsink that goes over top of the D2PAK type of devices. You have a copper pad under the device and you just put solder strips on each side for the heatsink to mount to. Not as effecient as direct connected thru-hole I'm sure but should lower the temp.

Are you using thermal via's with the "fingers" connecting your plane to the hole? I don't know the technical name but thermal vias are done to keep soldering heat from being transfered to the planes. If so, try going to straight holes without the "fingers".

Have you looked at International Rectifier?

RE: Just how hot can things get?

Thermal relief pads ("fingers") should not be used for high-current applications in the first place, for obvious reasons.  They're typically used when the components will be hand-soldered, so for production line reflow/IR soldered there's no need for them anyway.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Just how hot can things get?

You could try using Silicon Carbide Schottky diodes. They are more efficient, faster, and handle higher heat than standard silicon. But, they are expensive.

Following link provides some data. This company appears to manufacture SiC diodes mostly for aerospace applications. There are other companies making them.

http://www.sensitron.com/Hi_brochure/SiCSelector.pdf

RE: Just how hot can things get?

Isn't there a way to "wire" a FET (e.g. MOSFET) with the gate tied to the high side, and have it act as a diode?  Seems like you can get very low Vf that way, but I'm probably missing something important.  What is the drawback to that configuration, is there some circumstance where it smokes and stops acting like a diode?  Is there a way to add some circuitry to avoid smoking the FET?

The wikipedia article for Schottky diodes mentions the reverse leakage going high as junction temp. increases being the main drawback to them, and then suggests an "ideal diode" (FET with control circuitry) as being a possible fix.  The circuit they mention doesn't come anywhere close to smoked's requirements, so was wondering how you'd build something similar.

RE: Just how hot can things get?

"Isn't there a way to "wire" a FET (e.g. MOSFET) with the gate tied to the high side, and have it act as a diode?"

Probably, but it does not quite work.  The rectifier's forward voltage would be about equal to the MOSFETs threshold voltage of 2 to 5 volts.  Power dissipation would be very high.

What you can do is get a switching controller chip that can directly drive the MOSFET's gate at the appropriate time, turning it into a synchronous rectifier.  This gives very low losses in the forward direction, and low leakage in the reverse direction.  (Many designs also put a modest Schottky diode in parallel with the MOSFET.  The diode reduces the voltage swing on the MOSFET's large capacitance, which saves a little power.  It also is much gentler to the MOSFET's body diode.)

RE: Just how hot can things get?

For silicon based parts and good reliability, I'd recommend a maximum junction temperature of 100C under normal conditions, and 20K below the rated temperature for fault conditions.

RE: Just how hot can things get?

(OP)
Yep Dan; you can find 2x20A diodes in SMD I will probably try that.  I just figured out that as the I increases Vf increases which would force sharing by the two diodes in a package. BUT!! Simultaneously as the diode temp goes up the Vf plunges which has a grossly larger effect.. Meaning I have a 10A diode sucking down 10.1A while it's same die neighbor watches with amusement.. Damn!

Mr. Hutz; Thanks so much for that heat sink tip I shall look at it forthwith!
Yes I am not using "thermal reliefs" (the name you're looking for) for those mondo heat transfer vias.  Haven't looked at I.R. parts directly - I will give that a go too!

Dan; Ah, I see you beat me to the "name". Good on you.

Comcokid: Interesting.. I have seen many diodes that purport to being higher rated but the prices jumped mysteriously from the $1 range to the 15-25+ dollar range(half my product budget) so I never pursued them.  I will check, I bet that's the deal.  They look really spectacular which means I bet their price fits that same description. LOL

btrue: This is an area I need to learn more about as I have five FETs on the board now running those currents that have no heat issues at all.  One more, works well, in the quantity realm of manufacturing too.  Thanks.

Bobby; Dang.. To reiterate: This diode is only to prevent a large battery from back feeding a broken charger...  All my other FET's are high speed switching and have the parallel diodes specifically to deal with the slow FET's diode.  I have a micro with about 30 unused PIO pins that could run an output FET diod replacement but fear a crashed CPU "allowing the beast to get in".  Perhaps a High Side driver somehow controlled by a smaller voltage drop(much smaller) would work....  I already have a highside driver that works great doing a standard task on the board, another would fit easily.

I also have a non committed single sided OPAMP doing nothing in that same corner. (LM2903) Thoughts anyone?

David; thanks for the Topic fix!! (If I didn't thank you previously.) I shall remember that.

jimkirk: I need to see what those SiCarbide puppies cost. I hear you're admonition. Or were you speaking to std Si diodes which I have problems finding period, in that format. Or even the heatsinkable TO-220 flavor, probably owing to the nearly double voltage drop and greater yet heating..  Thanks.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Just how hot can things get?

International Rectifier 30LJQ045
30A schottky at 45V.
(Use a 40V if you can to improve the volt drop).
I think a single schottky is going to be better than a double paralleled for your application.

The one above is probabaly too expensive being a HiRel part, but it shows what is possible. Clearly the hard part is fitting the larger die into the package.

RE: Just how hot can things get?

General Semiconductor MBRB2545CT
A new part. 2x30A at 45V in D2PAC.
That should do you.
Found on the RS website
rswww.com
part number 215-7760
£2.19 each and in stock!

RE: Just how hot can things get?

Hi Keith,

I was referring to standard Si semiconductors.  SiC can generally handle much higher temperatures, but keep in mind that it's often the epoxy plastic outgassing within the package that can cause the reliability hit.  

If you have to live with a hot junction, consider a metal or ceramic package.

Also, be sure to use a circuit board with high enough glass transition temperature or do something to mitigate that.


RE: Just how hot can things get?

I have used reverse-biased FETs in the past for power backup solutions (sounds like what you're doing here).  Linear has an app note using a PFET, an NFET, and one of their voltage monitors to switch between "line" voltage and a battery backup when the main line drops too low... I'll see if I can dig it up.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Just how hot can things get?

Curse you Macgyver!
Your computer must be faster than mine.

RE: Just how hot can things get?

HA!  Thanks Madcow and mac.  Silly mechanical type (me) saw an "ORing controller", and skipped it (wondering why anybody would need to control a circular rubber seal, they do just fine by themselves :).

RE: Just how hot can things get?

(OP)
Ooooookay. I've ordered a potpourri of heat sinks partly to see how they'd fit this layout and partly to see how SMD heat-sinks perform.  Pretty cool - you just pick-an-place the parts then P&P the heat-sink over the part and re-flow.  {Should be fun to hand solder!!!}

I also bagged some TO-220s to keep the show on-the-road if the SMD sinks don't quite cut-it.

I also found a diode that is a "super fast" diode that has zilch reverse leakage - even when cooking.  It has higher Vf but a more linear Vf-I that I can take advantage of because it's a 30A single diode in a package.

ORing or replacing the diode with a non-micro analog controlled FET needs more thought.  I'd need to consider this carefully due to the complex power sources, initial conditions, blah, blahblah.  The waste heat that would be involved would be pretty much null and void.. So it's pretty tempting. If a board rev crawls out of darkness I will think a lot about ditching the diode for a FET.

btrueblood; You think ORings take care of themselves?!?! You should see my Mom's Moen shower fixture.  It has about 10 ORings in it and about every 6 months one of the ORings goes walk-about never to be seen again. Dribbledribble.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Just how hot can things get?

This has been a far more interesting discussion than most posts here.  A low cost heat sink that can work in some situations are 1/4 inch male spade connections that solder to the board (bare terminal without plastic spacer).  These can be obtained which are 0.6 inches tall.  Adding any hardware to a board is a pain in the assembly process.  Just throwing that out as an option someone might use.

RE: Just how hot can things get?

110 degree junction temp. is the old military standard.
As far as the board, they are rated for different temps.
We moved to 130 degree boards for the surface mount.
SMT power resistors get real hot. Board temps also
become an agency issue if your planing UL/TUV/CE/ect..
Some time I just glue a hunk of heatsink to the part
with some thermally conductive glue.

 The SiC diodes maybe worse for you. They are mostly
high voltage. The forward drop is close to 2 volts. A
6 amp 600vdc is running about 5$ in 1k pricing.They
are great for pfc/boost applications. I've gone to
400khz on a hard switched pfc. Prices may drop soon,
with the galium-nitrate diodes coming out.

 Nasa thought those Orings took care of themselves too.

RE: Just how hot can things get?

Good Day Keith, just slightly off topic, but I wonder if you could tell us what charging regime you are using, ie. voltage, current, time, etc.
Back to topic, also, how does your charger respond to a battery with, say, one cell that has a partial short and will not come up to full charge?
I could envisage a prolonged bout of high current supplied in this situation.

Regards, Ray.

RE: Just how hot can things get?

MIL-STD-883 parts were rated for -55º to +125ºC.

However, if you're looking for high temperature parts, you might consider looking for automotive rated parts, which are often specified to operate at 125ºC ambients.

TTFN

Eng-Tips Policies FAQ731-376: Eng-Tips.com Forum Policies


RE: Just how hot can things get?

(OP)
Nice tip Opera, never thought of that but cross sections and turbulent flow aspects would probably make spades work pretty well if not unorthodox. I shudder to think about the description to a Board House. LOL  

Madcow; That is a nice number to know; 110C junction temp as a Mil Std!  That's kinda what I was after. SiC is way too expensive for me.  I also DO NOT want to go to the esoteric board materials that they command/allow.
Very true O-Ring comment too.

Hello rtronics; I haven't specifically decided on those values as they are dependant upon this discussion and test results.  I can say the target is 10A for 12V LA.  This is at ~14V depending on chemistry.  This charger also can charge 24V systems and NiCd if desired.  The charger will handle the details but it's actual battery nominal voltage, and chemistry to be charged is sent to it via serial comm.  It also provides; present current, voltage, charge mode;(initial, bulk, cutoff, float, etc.), ambient, and charger board temperatures back over the comm.

It's micro controlled and will time the high current charge period.  If the cell voltage fails to increase over time then a fault will be declared.  That's how you sniff out shorted cells, as the battery voltage will rise but plateau somewhere wrong as the battery turns into a boiling vaporizer.  Something I expect to detect.

One note though is that a 10A charger really isn't a big problem with respect to a shorted cell.  Not like an engine driven (serious)alternator.  My Suburban's 120A alternator violently boiled its battery when a jaunt thru Death Valley shorted a cell.  While a 10A charger would warm up a shorted cell battery, it would likely not cause much damage before a timeout/fault could be declared.

Irstuff; Thanks! Another good number/point.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Just how hot can things get?

"The charger will handle the details but it's actual battery nominal voltage, and chemistry to be charged is sent to it via serial comm."

Does the serial protocol use an error detecting code?  The CCITT-16 CRC algorithm is reasonably good at detecting errors, and a table-driven implementation will fit in many microcontrollers.

RE: Just how hot can things get?

(OP)
Bobby: I haven't really gone there yet.  I'm a big fan of KISS so I usually just use a simple checksum followed by frequent checking.  If I want to go hardcore, the PIC I'm using has hardware CRC if I enable it.  It's probably massive overkill though. Ever try debugging something you're developing with a spare laptop providing the interface and the device wants to talk CRC or something else complex?  Brrrrr.


Thanks cbarn24050;  Looks like that might work.  I need to look at the data sheet more to see if their definitions translate to my situation.  Dropping that 4-5W diode waste down to some subwatt value is certainly looking better and better...  Hopefully this weekend I can test the new diodes that showed up today and make enough headway to stick with the present layout.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

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