IS Heat Sink Required?

[Nick]

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

 

[Anthony Fremont]

Why do you jump to such a conclusion ?

Sounded like a practical, interesting question to me.

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?

 

[Phil Allison]

"Anthony Fremont"

= a criminal, autistic pile of Yank sub human shit.

** Well, on a good day - that is.

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?

** Anthony Fremont is a incorrigible usenet PUBLIC MENACE.

Narcissistic, autistic cretins like him are a pox on the face of earth.

I bet everyone here knows at least a few obnoxious idiots like Fremont.

And avoid them like the plague.

Unfortunately, millions of the fucking morons are breeding like sewer rats
and getting themselves "gmail" accounts.

A new version if the bubonic bacterium is very much needed.




......... Phil

 

[Winfield Hill]

Anthony Fremont wrote...

... 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.

No, most of us use very small thermocouples whose wires carry
an insignificant amount of heat, so that's not an issue. A
tougher issue is it's not practical to press the thermocouple
to the junction, more likely one is constrained to the case.
But if the heat input is known, the junction temperature can
be calculated, assuming the datasheet thermal-resistance value
is accurate (the effect of this assumption can be reduced by
minimizing heat removal from the case, allowing it to reach a
higher temperature). However, we must also assume the system
is in steady-state equilibrium, and that can be a tough issue,
especially if airflow is a significant part of heat removal.

What we can do, and most of us has done, is estimate the Tj.

 

[Pooh Bear]

Anthony Fremont wrote:

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?

The way I measure Tcase is to attach a thermocouple as closely as possible
to the mounting flange. The type I use don't conduct much heat away ( bead
types ) and shouldn't disturb things too much.

To get Tj - you'll need to back-calculate from Rtheta j-c

I recall seeing an app note once that showed a modified heat sink with a
'probe hole' for a thermocouple that was intended to measure Tc as closely
as possible to the die itself.

Graham

 

[Phil Allison]

"Spehro Pefhany" >

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

** 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

 

[Anthony Fremont]

Anthony Fremont wrote...

No, most of us use very small thermocouples whose wires carry
an insignificant amount of heat, so that's not an issue. A
tougher issue is it's not practical to press the thermocouple
to the junction, more likely one is constrained to the case.
But if the heat input is known, the junction temperature can
be calculated, assuming the datasheet thermal-resistance value
is accurate (the effect of this assumption can be reduced by
minimizing heat removal from the case, allowing it to reach a
higher temperature). However, we must also assume the system
is in steady-state equilibrium, and that can be a tough issue,
especially if airflow is a significant part of heat removal.

What we can do, and most of us has done, is estimate the Tj.

Ok, thanks that really makes allot of sense. I guess I wasn't too far
off on my concerns.

 

[Spehro Pefhany]

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?

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

 

[Phil Allison]

"Anthony Fremont"

Ok, thanks that really makes allot of sense.

** Like HELL you got one word of it.

I guess I wasn't too far off on my concerns.

** Fremont is a lying, criminal asshole.


PISS THE CRETIN OFF NOW !!!!




......... Phil

 

[John Devereux]

"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.

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

 

[Jim Thompson]

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?

Two ways Tj is measured accurately:

(1) Run device at operating conditions, momentarily reduce current to
where series resistance doesn't alter vBE, and measure the vBE.

(2) On ASIC's I often include a separate junction just for measuring
chip temperature.

...Jim Thompson

 

[Pooh Bear]

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?

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

 

[Spehro Pefhany]

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

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

 

[Anthony Fremont]

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.

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. ;-)

 

[Paul Burridge]

"Paul Burridge"


** Drop dead - fuckhead.

OTOH, if you *did* get around to the course, I suggest you demand your
money back.

 

[Phil Allison]

"Anthony Fremont" = criminal LIAR

Cool! That's what I like, real world information.

** It is the WRONG test .

You absolute dickhead.




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

 

[Phil Allison]

"Spehro Pefhany"

I tried shorted output (a realistic kind of test for overload) and got
peak junction temperatures in excess of 200°C.

** That is * NOT* a test of when the internal thermal shutdown first
alters the ability of the device to hold spec.

You must do a test where the device is in normal operation ( including being
fed from a DC supply that has ripple) and then begins to drop output voltage
( and or output hum) as the temp rises due to self heating or applied heat.

Being an un-speced parameter - I expect it will vary with different
makers, samples and voltages.




......... Phil

 

[Ken Smith]

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

Yes, the higher the input to output voltage, the lower the shutdown
temperature. I think it must be the SOA protection circuit that does it.

IME:
If the LM7805 is run from a soft input supply, you can get a low frequency
oscillation of the output voltage. With LM7812s it happens even with a
stiff input.

 

[Phil Allison]

"Ken Smith"

Phil Allison
[...]

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

Yes, the higher the input to output voltage, the lower the shutdown
temperature. I think it must be the SOA protection circuit that does it.

** Yep - seen this happen on the bench.

Wind back the old variac, reduce the voltage across the LM7815 and
INSTANTLY the device comes out of shutdown - far to quick for it to be
thermal.

IME:
If the LM7805 is run from a soft input supply, you can get a low frequency
oscillation of the output voltage. With LM7812s it happens even with a
stiff input.

** I'll run some 7815s up a bit later - with a load and my thermocouple
attached.




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

 

[Phil Allison]

"Phil Allison"

"Ken Smith"

** I'll run some 7815s up a bit later - with a load and my thermocouple
attached.

** 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