Just how hot can things get?

[itsmoked]

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

 

[itsmoked]

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

 

[BobbyNewmark]

"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.

 

[cbarn24050]

her's another way,

http://www.irf.com/technical-info/designnote/dn503.pdf

 

[itsmoked]

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