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Topic: Sizing transformer for linear DC supply (Read 7794 times) previous topic - next topic

dc42

What size heatsink do you have on the regulator? With a 2A load it will only dissipate 5W, and the TO220 version should be able to handle that easily, with a modest heatsink. The worst case thermal resistance die-to-case is listed on the OnSemi datasheet as 2.3C/W and the maximum operating temperature is 125C. So you only need to keep the case temperature below about 100C for the regulator to function properly.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

justjed


... you won't buy anything by 'tuning' the regulator with the input filter value.. When it dries out prematurely due to high ripple currents it won't have far to go to fail, leave it where it is and either put your money in an efficient power supply or into getting rid or your waste energy as heat in an efficient heat waster.. Sinkr.

... your 1.5 A (Read the Specs carefully the LM350T is a 1.5A part it's the K version that is rated @ 3A) linear regulator and it would require a minimal heatsinking


The TI datasheet I have makes no distinction between the T and the K. Here's what seems relevant.
Quote
These specifications are applicable for power dissipations up to 30W for the TO-3 (NDS) package and 25W for the TO-220 (NDE) package. Power dissipation is ensured at these values up to 15V input-output differential. Above 15V differential, power dissipation will be limited by internal protection circuitry.

Well, maybe it isn't the right datasheet, but I hope I can trust Digikey to provide such. Going straight to TI.COM, I find the same datasheet, and it specifies 3A.

Now I do realize that there's an unspoken 'given sufficient heat sinking'.


What size heatsink do you have on the regulator? With a 2A load it will only dissipate 5W, and the TO220 version should be able to handle that easily, with a modest heatsink.


Looking again at the Digikey page, I think too small, but not by much. I will see what I can do about that. Yet another case of being in a hurry ending up costing time in the end. ;)
... it is poor civic hygiene to install technologies that could someday
facilitate a police state. -- Bruce Schneier

CrossRoads

What current out did you need?
Got e-mail today about these being on sale
http://www.kitsusa.net/phpstore/html/PS-28-SWITCHABLE-REGULATED-DC-POWER-SUPPLY-3-12V-2A-1260.html
http://www.kitsusa.net/phpstore/html/OTE-HY-152A-VARABLE-DC-POWER-SUPPLY-0-15V--2AMPS-1229.html

I thought  the thread started with P-something as a model number.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

dc42



What size heatsink do you have on the regulator? With a 2A load it will only dissipate 5W, and the TO220 version should be able to handle that easily, with a modest heatsink.


Looking again at the Digikey page, I think too small, but not by much. I will see what I can do about that. Yet another case of being in a hurry ending up costing time in the end. ;)


That heatsink is specified as 16.7 degC/W heatsink-to-air. You can add about 1degC/W for the thermal resistance case-to-heatsink assuming you used thermal compound, plus 2.3C/W junction-to-case (from the LM350T datasheet). Total 20 degC/W. Therefore, with 5W dissipation (@ 2A load) and 25C ambient temperature, the junction will be at about 125C, which is right on the limit for normal operation. I suggest you look for a heatsink with a thermal resistance of not more than 8 degC/W.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Docedison

The 'newer data sheets make only one 'veiled' reference... my information comes from some engineering I did with the LM350T in the 1990's and from the original National data sheets... You are operating a 'thermally' challenged part because of the TO-220 case... The TO-220 case has nearly twice the thermal resistance junction to ambient of the LM350K and as dc42 points out 125C (257F) is right on the edge of a "safe" ?? operating temperature... I wouldn't want to have to deal with the rise in ambient temperature that wasting that amount of power as heat and it's effects on other components. The LM350K is a valid choice the "T" package simply is a poor choice.. I realize that you need to use the LM350T's because that's all you have BUT it's Exceptionally POOR ENGINEERING as many have tried to point out. The alternatives are a much better deal.
I fully understand that you don't want to use the efficient method because it involves parts and experience that you simply don't have or wish to acquire but in this case operating a thermally challenged part as a preregulator for a battery charger... simply isn't a great idea. I would STRONGLY advise you to make the change to the LM338K if you must build a small space heater and camouflage it as a piece of electronics equipment, If you cannot... Well...
The reason why I stuck my $0.02 in is that I've been reading this thread since you started it a week or more back and I remember a device I designed... just like you did and the thinking was the same... LM350 3A 'oh yeah', thats the one... but the device WILL shut down if the current is too high and extended to over temp can make the shutdown problematical. Especially in the TO-220 case...
When you consider the power out VS heat rise against the markedly increased J to A thermal resistance.. and this..
quote from the later data sheet, I've taken the liberty to point out your difference in judgment..
Quote
Thermal Resistance, Junction K Package             1.2 1.5 §C/W
                                   to Case T Package 3 4       3    4 §C/W
{From the attached data sheet, Page 3)

4/1.5 = 2.66 X the thermal resistance
Mine failed with a 50 mm X 75 mm X 25 mm heatsink... in about 3 weeks but there is a difference here that might work, You are charging a battery and the load is heavy for about 45 - 50% of the initial time and tapers off, My device powered a 450 MHz 10W transmitter load current was 2 A and 100% duty cycle. I replaced the 350T with a K device and ultimately used a 338K (it worked) I later changed to a small buck mode switcher and never had a spec of trouble. Safe engineering margins are 50% or more. Not an (interpolated) 33% that really isn't there because of the packaging thermal restrictions. I've said all I'm going to say.

Doc
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dc42


Thermal Resistance, Junction K Package             1.2 1.5 §C/W
                                  to Case T Package 3 4       3     4 §C/W
{From the attached data sheet, Page 3)


Looks like it also depends on the manufacturer of the regulator. The datasheet I quoted (from OnSemi) is http://www.onsemi.com/pub_link/Collateral/LM350-D.PDF quotes lower values than that for thermal resistance junction-to-case.

I still think the TO220 version is usable for the OP's application, if the maximum load current is 2A and the voltage drop is only 2.5V or less. 5W is not a lot of heat to get rid of from a TO220 package. Unless the device will be used in hot climates or with poor ventilation, then I think a 75C rise in junction temperature can be accepted, which at 5W dissipation means total thermal resistance of not more than 15 degC/W. The 8 degC/W max thermal resistance heatsink I suggested can meet this requirement, even if the thermal resistance junction-to-case is 4 degC/W. But of course, the TO-3 version would be able to cope with higher load current and/or more voltage drop.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

sonnyyu


What current out did you need?
...


very tough question;-

The Pila IBC charger is lithium ion battery charger.

Data sheet of LG Lithium Ion ICR18650 C1 2800mAh


  • Capacity  2800mAh/2700mAh Nominal/Minimum

  • Standard Charge Constant current: 0.5C (1350mA)

  • Standard Charge Constant voltage: 4.35v

  • Standard Charge End current(Cut off): 50mA





3 stages of charge


  • Constant current

  • Constant voltage

  • Idle (Cut off)



Current dependent on


  • Battery capacity

  • Stages of charge

  • How many batteries

  • The Charge mode, Fast/Standard/Slow




sonnyyu

#22
Jun 18, 2013, 03:27 pm Last Edit: Jun 18, 2013, 03:29 pm by sonnyyu Reason: 1
@OP You might be able to find out what is IC which Pila IBC charger use.

LTC4002, Standalone Charger IC;-
Wide Input Supply Range: 4.7V to 22V
in this case we have free run.

http://cds.linear.com/docs/en/datasheet/4002f.pdf

billhowl


justjed

Thanks all for the continued advice. In re. switching mode power supplies, sure I'd like to learn about them. For various reasons, at the outset this didn't seem like the time/place for such. Well, no matter, I've learned a few things, and that has value to me, by itself.

The Pila IBC charger is embossed with the legend "6V / 2-3A". This wasn't something I looked at initially. The supplied wall-wart switching supply is labeled 6V 2A. I simply assumed that Pila would supply a wall wart adequate to the task, and it never occurred to me to measure it. I have noticed that it does continue to charge when the voltage sags to 5.5V. What circuitry is in there besides the actual charge controller I don't know, so I'm not going to mess with supplying higher voltage. (Yes, I looked at that thread on Candlepower Forums, and it looks there's not much else, but I'll stay conservative here.) Every Li-Ion charger I've seen will accept a range of cells, so trying to infer current requirements from the spec for one model of 18650 isn't informative. The Pila states that it charges at 600mA. I have cheap-o Ultrafire charger which states that it charges at 750mA. But the cells get warmer in the Pila -- explained, maybe by the presence of heatsinks right below the battery bay. Well, that's pretty much a tangent.

I'm going to go with more heat sinking, and see what happens. Grabbing some smaller caps too, just for grins.
... it is poor civic hygiene to install technologies that could someday
facilitate a police state. -- Bruce Schneier

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