Thought this was puzzling...
 

I came across this site... and picture. Are these diodes an attempt at
thermal tracking for the bias?
http://www.worldwidedx.com/gallery/a...pad_and_diodes

www.telstar-electronics.com

Thought this was puzzling...
 

"Telstar Electronics" wrote in message
oups.com...
I came across this site... and picture. Are these diodes an attempt at
thermal tracking for the bias?
http://www.worldwidedx.com/gallery/a...pad_and_diodes

www.telstar-electronics.com

No, someone has just put them in as an attempt to protect against reverse
polarity!
Slightly mad.

Thought this was puzzling...
 

Paul wrote:
No, someone has just put them in as an attempt to protect against reverse
polarity!
Slightly mad.

Is that what you think that is???? Why have the diodes right on top of
the transistors? No, I think that was an effort to temp track the bias.
Well, at least they were trying. Unfortunately, don't think that's
going to work that way... LOL

www.telstar-electronics.com

Thought this was puzzling...
 

On 24 Sep 2006 08:53:06 -0700, "Telstar Electronics"
wrote:

I came across this site... and picture. Are these diodes an attempt at
thermal tracking for the bias?
http://www.worldwidedx.com/gallery/a...pad_and_diodes

www.telstar-electronics.com

It provides negative thermal feedback.

The voltage across the transistor will decrease as the transistor
temperature increases. This prevents thermal runaway.

Thought this was puzzling...
 

wrote:
It provides negative thermal feedback.

The voltage across the transistor will decrease as the transistor
temperature increases. This prevents thermal runaway.

That's what it's supposed to do... I claim it can't work as shown. It's
not quite that simple as tacking a few diodes on the bias circuit and
laying them on top of the transistors... LOL

www.telstar-electronics.com

Thought this was puzzling...
 

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 that the cap on the transistor. If
you want as fast a response as possible then the diodes are placed about as
well as you can get. I've also seen small diodes clipped on to the flange of
TO-3 style cases for the same reason. I've seen some high power transistors
used for motor drive and high frequency inverter use that has the
temperature sensing diode, or thermistor, built inside of the package.

http://pdf1.alldatasheet.co.kr/datas...50HM75STG.html

As the temperature of the base-emitter junction goes up the required forward
voltage for a given base current decreases. If nothing was done, the
base-emitter bias voltage held fixed, the base current for increases and
thus the collector current too. Of course increased collector current
results in more power dissipation, thus more heat and higher resulting
temperature, i.e. thermal runaway.

The diode used for bias temperature compensation has to very closely match
the Ib-Vbe curve of the transistor to work effectively . You can't use just
any diode and expect it to work very well. The problem with mounting the
diode on the heat sink is the large thermal mass. If the transistor starts
to go in to thermal runaway the heat sink will take a significant time to
heat up, thus reducing the bias to the base-emitter junction, long after the
runaway has started. You want to get the compensation right as soon as
possible, reduce the base-emitter bias, before the junction gets too hot.
The small cap on the transistor will heat up much faster that anything else
giving the designer a chance to design a fast responding bias circuit which
could avoid transistor destruction.

Diode bias circuits are on their way out in favor of more advance bias
circuits. The chief problem with diode bias compensation is either too much,
the transistor gets starved for needed bias current, or not enough, tendency
to go in to thermal runaway.

The reason the transistor can get starved for bias current is simple. The
Beta, current gain, of a bipolar transistor is not constant over the
operating range, it varies based on the instantaneous operating point. At
some point the gain is so low that the current flowing through the bias
compensation diode may drop to a low value, or go to zero, limiting the bias
current forcing the operation to go non-linear over part of the cycle.
That's why some of the simple diode bias circuits draw so much standing
current. It has to supply the worst case bias current. Newer active bias
circuits use a voltage source type design. The bias voltage tracks the
transistor temperature and since it is a "voltage source" there is no
practical limitation on the bias current. The transistor will draw what it
needs without being limited by the current limiting resistors in the diode
bias type of circuit.

I'm sure somebody is going to nit-pick this post. They're welcomed.

--
Regards,
Leland C. Scott
KC8LDO

"Telstar Electronics" wrote in message
ups.com...
wrote:
It provides negative thermal feedback.

The voltage across the transistor will decrease as the transistor
temperature increases. This prevents thermal runaway.

That's what it's supposed to do... I claim it can't work as shown. It's
not quite that simple as tacking a few diodes on the bias circuit and
laying them on top of the transistors... LOL

www.telstar-electronics.com

Thought this was puzzling...
 

On Sun, 24 Sep 2006 23:30:24 -0400, "LeIand C. Scot"
wrote in :

snip
I'm sure somebody is going to nit-pick this post. They're welcomed.


Not picking nits, just taking a different approach.....

Thermal bias compensation works to a point but neglects one important
issue: it takes time for heat to travel from the junction to the
outside of the package, and thermal runaway can happen faster than a
-thermal- compensation circuit can respond to it. Since the heat
buildup is due to excessive EC current, it makes more sense to control
the EC current directly.

There are two solutions that use this approach. One is to include a
resistor on the emitter with a TC opposite of the transistor. Not
precision but much faster response. The other is to monitor the EC
voltage and the base current; send both measurements to a differential
OP amp and use the output as feedback for the bias regulator.

I tend to favor the first choice because it has the added benefit of
improving linearity. I would only use the second choice in a high
profit, 'stick-it-to-the-consumer' product.

Thought this was puzzling...
 

On 24 Sep 2006 17:17:29 -0700, "Telstar Electronics"
wrote:

wrote:
It provides negative thermal feedback.

The voltage across the transistor will decrease as the transistor
temperature increases. This prevents thermal runaway.

That's what it's supposed to do... I claim it can't work as shown. It's
not quite that simple as tacking a few diodes on the bias circuit and
laying them on top of the transistors... LOL

Why does every post griffey does end with LOL????


************************************************** **
There are a good many who will be surprised by this
seasons NY Knicks. I think larry made a mistake and
how many would be surprised if the Knicks make the
playoffs and go to the second round?
************************************************** **

Thought this was puzzling...
 

On a sunny day (Sun, 24 Sep 2006 23:30:24 -0400) it happened "LeIand C. Scot"
wrote in :


I'm sure somebody is going to nit-pick this post. They're welcomed.

No it is a nice post, I appreciate it, we should have more like this.

Thought this was puzzling...
 

Bill Gates wrote:
Why does every post griffey does end with LOL????'

Because I can't help but laugh at some of the stupid comments I read
out here. Bottom line... that circuit with those two little doides
isn't doing any temperature tracking for the bases of those
transistors. Several things are wrong:

1.) The bias that feeds those bases needs to be very low impedance...
on the order of around 5 Ohms. There is no freaking way that those
little diodes could carry enough bleeder current to feed those bases.
2.) And if that's not enough... there is another fatal flaw with that
scheme. It's effectively paralleling diodes... the small diodes hanging
on top... and the diodes in the base-emitter junction of the power
transistors. Electronics 101 says that it's a bad plan to parallel
diodes (why you never just parallel bipolar transistors!). The
reason... because one will always hog the current... and starve the
other. One will be on... the other is cut off. You will never get them
both on at one time. Soooooo... when this happens... you can't get any
tracking... because only one diode is on at a time.
3.) Having the sensing on top of the transistors is a poor location.
The internal die is in intimate contact with the heat sink... not the
top! The het sink... preferably near the device is the proper location
for any tracking device.

Like I said before... it's not quite that simple just tacking a couple
diodes on top of the devices... LOL

www.telstar-electronics.com