Thought this was puzzling...
 

On a sunny day (26 Sep 2006 05:33:33 -0700) it happened "Telstar Electronics"
wrote in
.com:

LeIand C. Scot wrote:
Tacking the diodes on to the case of the transistor works better than
attaching them to the heat sink due to the relatively slow thermal response,
the heat sink changes temperature slower than the cap on the transistor.

That's simply not true... while the heat sinks response might be a bit
slower... it's a much better indicator of the temp of the transistor
die. The ceramic cap on those transistors will overshoot the die temp
in both directions

This is not correct. there is no 'overshoot' in temp possible.
Unless you know how to heat solder to 250C with a 200C soldering iron?
An other Telstar miracle ;-)?
The thermal response will be faster, resulting in better protection.
A slower response could result in reducing current too late.

Thought this was puzzling...
 

Jan Panteltje wrote:
This is not correct. there is no 'overshoot' in temp possible.
Unless you know how to heat solder to 250C with a 200C soldering iron?
An other Telstar miracle ;-)?
The thermal response will be faster, resulting in better protection.
A slower response could result in reducing current too late.

You're right... overshoot was the wrong word. I think you knew what I
meant... but if you didn't... I'll clarify. There is no friggin' way...
having a sensing device on the ceramic cap of those transistors tracks
the die temp as good as having that same tracking device in close
proximity on the geat sink.

www.telstar-electronics.com

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"james" wrote in message
...
On Mon, 25 Sep 2006 19:06:49 -0400, "Leland C. Scott"
wrote:

+++Believe it or not I've seen many of the old Motorola RF devices use
+++internal emitter resistors. Those took the form of many small tungsten
+++bonding wires from different areas of the emitter structure to the
emitter
+++terminal. The main idea there was the many wires, resistors, in
parallel
+++resulted in a very small overall emitter resistor. Also they found that
a
+++problem called "second break down" would occur if they didn't do this.
+++What it amounted too was local hot spots, thermal runaway, in tiny
areas
+++of the transistor's emitter structure. I think the term they used for
RF
+++devices built this way was "emitter ballasting".
*********

I have never seen tungstun bonding wire. All I have ever seen is gold.

Correct. That's a mistake on my part.

You need a soft malible metal to bond to the die pads on any
semicondcutor. The bond wire is sonic heated to the aluminum metal die
pad. This forms the nice ball on the die pad that is a weld of the
aluminum and gold. Tungstun is far to hard a metal for bonding.

I did read in the Motorola manual they did, or tried, to use it in a stack
up of metals since they saw a problem with electromigration of the gold used
in the bonding wires with the silicon base material.


Emmitter ballasting is done on the die within the emmitter matrix.

Correct. The two methods I did read about are polysicicon resistors or
Nichrome resistors made on the chip.

There are several metods of fabricating an RF transistor. Major
factors are power, frequency and device operating point. For most
transistors operating below 50 MHz use an interdigitated emmitter
geometry. Incorparated within are current balancing resistors in the
emmitter matrix. This does increase die size and reduces gain. It does
spread heat and current more evenly through the die. Interdigitated
emmitters will have multiple bond wires.

For those really interested in the details I found the PDF document below
that goes in to it a bit more.

rfwireless.rell.com/pdfs/AN_IRFPT.pdf


--
Regards,
Leland C. Scott
KC8LDO

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"Telstar Electronics" wrote in message
oups.com...
Like I said before... it's not quite that simple just tacking a couple
diodes on top of the devices... LOL

http://perso.orange.fr/f6itv/p2032001.htm (look at location of diodes in
photo at right side)


--
Regards,
Leland C. Scott
KC8LDO

Thought this was puzzling...
 

LeIand C. Scot wrote:
http://perso.orange.fr/f6itv/p2032001.htm (look at location of diodes in
photo at right side)

Yes, this method is similar to a few "reference" designs shown in the
Motorola RF Data Manual . This is a poor method for two resaons. The
first is what I explained before... trying to get two diodes in
parallel to turn on together is very difficult... if not impossible on
a repeatable basis. Second, the emitters you are feeding with the DC
bias are very low impedance. This creates its own problem when
attempting to control the base current. This biasing scheme with temp
compensation is a "brute force" method that dissipates large amounts of
power... and plain doesn't work well. It's not the first time that
circuits shown in a reference manual are not production ready. Motorola
also shows a much better method of temp comp bias in that same data
manual. It uses an op amp and sink mounted thermistor. I'm also using
an "active" approach to the temp comp bias in my amplifier... but a
totally different circuit using a bipolar transistor as the sense and
gain mechanism. I can hold the 500mA bias to 10% from -30 to +85C.

www.telstar-electronics.com

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On 25 Sep 2006 16:27:30 -0700, "Telstar Electronics"
wrote in
. com:

Leland C. Scott wrote:
Exactly. That's why those diodes are place on the ceramic cap of the
device and not on the heat sink.

Having the sensing on top of the transistors is a poor location.


It's better than the heat sink.


The internal die is in intimate contact with the heat sink... not the
top!


Wrong. The ceramic package isn't hollow; on the contrary, it contacts
more of the junction's surface area than the heat sink flange (which,
BTW, doesn't make "intimate contact" with any part of the junction
because it is insulated from the die by the Be Oxide substrate). The
fact is that a transient pulse can heat and blow the junction before
it can dissipate into the -anything-, which it's more likely to happen
when the transistor is already hot from normal operation.


The heat sink... preferably near the device is the proper location

for any tracking device.


What part of "heat sink" don't you understand?

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Frank Gilliland wrote:
Wrong. The ceramic package isn't hollow; on the contrary, it contacts
more of the junction's surface area than the heat sink flange (which,
BTW, doesn't make "intimate contact" with any part of the junction
because it is insulated from the die by the Be Oxide substrate).


If the ceramic cap is tied so well to the heat source internal to the
transistor... then why don't you just attach the heat sink to the
ceramic caps on your new design?... LOL

www.telstar-electronics.com

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On 26 Sep 2006 17:15:35 -0700, "Telstar Electronics"
wrote in
. com:

Frank Gilliland wrote:
Wrong. The ceramic package isn't hollow; on the contrary, it contacts
more of the junction's surface area than the heat sink flange (which,
BTW, doesn't make "intimate contact" with any part of the junction
because it is insulated from the die by the Be Oxide substrate).


If the ceramic cap is tied so well to the heat source internal to the
transistor... then why don't you just attach the heat sink to the
ceramic caps on your new design?... LOL


Actually, that's not such a bad idea. Computer processors and other
chips are encased in ceramic and the heat sinks are typically mounted
on top. Having an additional heat sink on the top of the transistor
certainly can't hurt. I'll check into this and see if it's do-able.

Thanks, Brian!

Thought this was puzzling...
 

Frank Gilliland wrote:
Actually, that's not such a bad idea. Computer processors and other
chips are encased in ceramic and the heat sinks are typically mounted
on top. Having an additional heat sink on the top of the transistor
certainly can't hurt. I'll check into this and see if it's do-able.

I hate to tell you that processor chips (and some others) are specially
designed to attach the sink on the top... the RF transistors we're
talking about are not.

www.telstar-electronics.com

Thought this was puzzling...
 

On 26 Sep 2006 17:29:43 -0700, "Telstar Electronics"
wrote in
.com:

Frank Gilliland wrote:
Actually, that's not such a bad idea. Computer processors and other
chips are encased in ceramic and the heat sinks are typically mounted
on top. Having an additional heat sink on the top of the transistor
certainly can't hurt. I'll check into this and see if it's do-able.

I hate to tell you that processor chips (and some others) are specially
designed to attach the sink on the top... the RF transistors we're
talking about are not.


And Viagra was designed to be a heart medication. What's your point?