IS Heat Sink Required?

** Win - if you supplied a context and purpose for your Q you might get a reply.

Cos to me it looks just like another of your TROLLS.

........ Phil

Why do you jump to such a conclusion ?

Sounded like a practical, interesting question to me.

Graham

Hi,

I am mounting the Regulator Straight (Perpendicular to the board). By the way the answer for the calculation is 0.27 W which is well below the safe limit "0.6W". I am using the pcb in a hot enviornment almost

50 degrees max., will that matter?

Thanks

might get a

It was, the problem is that he called phil's bluff about measuring the shutdown temps. Now phil must direct his aberrant behavior towards Mr. Hill (or anyone else that speaks up) in order to save face. I wouldn't count on seeing any numbers posted soon.

At any rate, I'm not sure how phil could have measured Tj with a thermocouple. Tcase maybe, but even that would be tough IMO. It would seem to me that anything touching the device (as in a thermocouple) could act as a sink and give erroneously low readings or act as a barrier and give high readings. At the risk of being branded a troll, how do these problems get solved, or aren't they problems at all?

"Spehro Pefhany" >

** Much lower on the 7815s and 7915s I tested

Lucky for any to make it over 140 C.

I think the in-out differential has an effect on the actual shutdown temp too.

Must be some *mighty dubious* reasons why it is non speced parameter.

The ONLY 100% safe bet is that it will not happen if the chip temp is <

125C.

........ Phil

A fairly fine thermocouple such as AWG30 or finer type T soldered to the package right under the die would be close enough on a non-heatsinked TO-220. Especially in an enclosed insulated space- probably within a fraction of a degree under those conditions.

I would be interested to know the actual cutout temperature- I suspect it's something like 160 or 170°C typically on a 7805.

Best regards, Spehro Pefhany

--
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The ones I use do shut down at >40V input. (Not permanently, it appears to be a controlled shutdown!).

--

John Devereux

Since you have the info on how to do the calculations now, why not check the thermal resistances and run the sums ?

It's wise to be aware of the local ambient temp. You might also want to consider if it rises much when the equipment is left powered on for prolonged periods. The only practical way to do this is with a thermocouple of course. They aren't exepensive and many cheap DMMs have a thermocouple input too.

Graham

I tried shorted output (a realistic kind of test for overload) and got peak junction temperatures in excess of 200°C. They seemed to stabilize with 130mA or so output current into the short with 12V in, which represented about 170°C Tj, give or take a degree or three. My test results exhibited an anomaly at the point there the K thermocouple got unsoldered by the high temperatures. ;-)

That's with a Motorola 7805 ACT 1994 date code.

Best regards, Spehro Pefhany

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Cool! That's what I like, real world information. ;-) It's so much better than speculation or outright misinformation. I wonder if newer devices can take more abuse or if they are "better engineered". By better engineered, I mean would they shutdown at very close to a Tj of

125C. As opposed to the, apparently, over-built device that you tested. ;-)

PS: I changed the subject since the thread seems to be moving towards electronics. ;-)

"Anthony Fremont" = criminal LIAR

** It is the WRONG test .

You absolute d*****ad.

........... Phil

"Phil Allison"

** The test was like this.

Input: 26 volts DC average with 2.2 volts p-p ripple at 50 Hz.

Load: 68 ohm, 10 watt WW resistor.

Device dissipation: 2.3 watts.

Thermometer: K-type bead, coated in thermal grease, wedged into device mounting hole.

CRO and DMM monitoring output voltage.

Device sitting on bench in free air.

Sample #1: Teledyne LM7815 CSP ( 8446 )

Sample #2. ST LM7815 CV ( 99244 )

Thermal shutdown ( TS ) indicated by sudden rise in output ripple and DC voltage drop > 0.2 volt.

RESULTS:

Sample #1:

TS at 145 C in 90 seconds, voltage then dropped to 6.7 volts.

Sample #2:

TS at 151 C in 100 seconds, voltage then dropped to 0.33 volts.

........ Phil

I get 2.43 watts.

If we take the thermal resistance to be 4C/W, then the shutdown Tj was about 10 degrees higher, or 155 and 161C resp. We can assume a bit of thermal lag between the case temp and your TC reading, a few more degrees may be added, yielding an estimated Tj = 160 and 165C for these two samples. Nice. This confirms Spehro's guess of "160 to 170°C" three days ago. It also means one shouldn't use a Tj = 150C maximum for designs, as Anthony Fremont was asserting, because that's much too close to the unspecified shutdown temp. The 78xx datasheet's 125C maximum "operating" temperature spec includes a safety margin against shutdown, along with prolonging chip life.

Ah, Phil, old chap, while you're at it, could you measure up a few more parts to build up some statistics for us? :-)

You could also lower the input voltage, along with changing the load resistor to maintain the same dissipation, to test your suggestion that shutdown Tj changes with input-output voltage difference. A glance at NSC's LM7815 schematic shows that the shutdown Tj should increase for input-output voltages below 6.5V, when Q11 to D1 becomes saturated. Perhaps to allow more dissipation in the safer regions of the pass-transistor's SOA.

In fact, it appears for some low value of input-output voltage, perhaps below 3.5V, there's effectively no overtemp shutdown!, and an NSC LM7815 is only protected by its current limit. Even though this drops as Q14's temperature increases, it may still be over 1.5A, according to the datasheet curves, which *cough* would be over 5 watts for 3.5V, which means (I'm speculating) that using a ripple-free 18.5V source, a 10-ohm load, and *no* heatsink, you could force an LM7815 die into excessively-high temperatures, certainly over 200°C, and eventually destroy it.

NSC's LM340, and most other manufacturer's '7815's, including ST's L7800, TI's uA7815, On Semi's MC7815, LinFinity's SG7815, JRC's NJM7815, etc., use a different overtemp shutdown scheme, which is not affected by the input-output voltage differential. Ditto for the original Fairchild's uA7815 (1978 datasheet). (I couldn't find a copy of Teledyne's '7815 datasheet here.)

Hey, it's even possible NSC no longer ships the LM7815 circuit we see in their datasheet. :-) I note their combined LM340- LM78xx datasheet shows the alternate scheme, like everyone else.

These could be rather interesting tests.

--
 Thanks,
    - Win

"Winfield Hill"

** The other *inescapable* conclusion is that you cannot expect a LM7815 etc to dissipate anything like 2 watts with no heatsink or fan cooling in a REAL application.

Free air conditions on the bench do not typify actual use conditions, let alone any sort of worst case scenario where other heat sources are contributing to the local ambient, convection is restricted and heat sensitive parts like electro caps are sited adjacent to the regulator IC.

If one also allows for a high room ambient ( ie 40C ) conditions and the possibility if the unit being in direct summer sunlight - then even the suggested figure of 0.6 watts may be too high.

......... Phil

Exactly, it's just a rule of thumb. If it looks like it might be close, then a more detailed evaluation may be in order.

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Whoa there, let's hang on a second here. I never meant that value should be used as a design guideline, I just said that's what the data sheet had as a max Tj. Even I wouldn't attempt to operate parts at their absolute maximum ratings. :-) If I was trying to design something for production, I'd probably assume that "ambient" meant the trunk of a car in houston in August. When I'm tinkering "ambient" is 21C with a good breeze. ;-)