Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Vbe Thermal Coupling  Issue
Vbe Thermal Coupling  Issue
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th January 2005, 10:44 PM   #1
sam9 is offline sam9  United States
diyAudio Member
 
sam9's Avatar
 
Join Date: Jun 2002
Location: Left Coast
Default Vbe Thermal Coupling Issue

I am trying to use D.Selfs suggested method of thermal coupling the Vbe Multiplier by attaching it directly to the top of a TO-247 output (his example is TO-3) transistor. It is an EF output topology. Setting quiescent current is cery "fiddly" it is very hard to find a stable point. Any distrubance sends the measured current (volts across the RE resistors, actually) off in one direct or the other. It is kind of like sitting in the center of the saddle surface that is often used to describe chaos theory!

Setting quiescent current is not new to me, but this behavior is. The heatsink does not seem to be an issue as the situation persists even with a "monster" sink. However, in the past I've always bolted the Vbe multiplier to the main heatsink. Is it possible that the Vbe multiplier to to sensative and over-compensates in one direction or the other? Is TO-247 just not ameanable to this - my temperature probe says the heatsink gets hotter than the top of the package so that is another suspiicion I have.

I'm going to experiment with a thermal bar and with going back to main heatsink attachment. But while I'm doing the metalwork, I would be interested if anyone has experienced something similar.
  Reply With Quote
Old 5th January 2005, 11:05 PM   #2
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
This fact is something I have already commented altough nobody seemed to trust me. I also did the same experiment in the past and got the same frustrating results as you

The worst place to mount a Vbe multiplier is over the plastic case of a power device. This plastic resin usually has very high thermal resistance and just does not follow the die temperature at all, actually it's allways sustantially cooler than the heatsink and much cooler than the die itself. The temperature of the plastic surface is mostly dependent on ambient temperature only
  Reply With Quote
Old 5th January 2005, 11:10 PM   #3
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Vbe Thermal Coupling  Issue
May I add that you are compensating all the devices, not just one. I always mount the vbe multiplier to the heatsink to average out the differences.

I listened and am agreeing with you Eva.
-Chris
  Reply With Quote
Old 5th January 2005, 11:38 PM   #4
sam9 is offline sam9  United States
diyAudio Member
 
sam9's Avatar
 
Join Date: Jun 2002
Location: Left Coast
Eva,

Thank you for your prompt reply. I made a some searches on the forum but didn't come up with your post. Anyway, it is reassuring to find confirmation of my results.
----------
Anatech,

I had considered a thermal bar to address the multiple device issue, but Eva's comment makes that moot.
----------------
All,

I noticed some time back that ESP's P3a doesn't couple the Vbe multiplier but doesn't suffer for it. I'm wondering if the thermal coupling business isn't as critical as claimed.
  Reply With Quote
Old 6th January 2005, 02:51 AM   #5
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Vbe Thermal Coupling  Issue
Hi sam9,
All bipolar stages need thermal compensation of some sort unless they are not biased on. Whether they need a vbe multiplier or not is another question. Your thermal bar would be the heatsink in this case.

I am just prototyping an amplifier right now that does not appear to need a vbe multiplier. I designed for it, but left it unstuffed on the board. Bias seems stable right now. However, the circuit has built in compensation, the vbe multiplier was / is in case I need it at higher power levels and temperatures. I won't know until I beat it up. That is some time away yet.

So depending on your circuit and thermal layout, you may or may not require the vbe multiplier (bias control circuit).

-Chris
  Reply With Quote
Old 6th January 2005, 03:24 AM   #6
sam9 is offline sam9  United States
diyAudio Member
 
sam9's Avatar
 
Join Date: Jun 2002
Location: Left Coast
I'm quite sure mine does need it.

I've seen cicuits by Hagerman and Self which are claimed improve the tracking and stability but sill require some kind of thermal coupling.

Just as interesting was a comment that the need for thermal tracking could be avoided by using output triples. The example cited was the Quad 303. I look at the schematic and it is quasi-complementary. I think the reason you can avoid the coupling requirement is that it's the pre-drivers which which determine the bias of the rest And they can be kept at a low stable temperature if the current demants are small enough. I assume thiwould not apply to an EF tripple but would to a full CFB with predrivers.

But these are all ideas for the future. Right now I have to go drill and tap a heatsink!
  Reply With Quote
Old 6th January 2005, 03:38 AM   #7
quasi is offline quasi  Australia
diyAudio Member
 
quasi's Avatar
 
Join Date: Sep 2004
Location: Adelaide, Australia
Default My explosive experience;

I had an experience once where my Vbe transistor was mounted directly to a very large heatsink.

The FETS ran away thermally for no apparent reason very quickly after the bias seemed to settle . This happend in a few seconds and resulted in destroyed FETS. The FETS were warm and the heatsink was cold.

My diagnosis was the heatsink was too big (wierd huh?) and / or the sensor location on the heatsink was thermally too far from the output devices.

I fixed this by attaching the output devices and Vbe transistor to a seperate mounting bar and then attaching this to the large heatsink. The Vbe now senses the temperature of the mounting bar rather than the heatsink and this improved the tracking no end.

This solution was specific to that amp. On smaller heatsinks I mount the Vbe directly on the heatsink within 1-2 centimetres of the output devices. This is helps track the output devices closely. For large heatsinks I always try to use a seperate mounting bar. I never mount the Vbe directly to an output device. Even with matched devices one could cruising while the one next to it is screaming, but the Vbe multiplier thinks everything is fine beacuse it is mounted on the cruiser.

A friend of mine has (and continues to) mount the Vbe on the FET directly. He has not had any failures but his bias does swing a lot more than mine. I think he is lucky so far.

My $0.02 worth.


Cheers
  Reply With Quote
Old 6th January 2005, 07:05 AM   #8
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
In my experience not all bipolar devices show the same thermal behavior in all circumstances :

- Devices with lower Vce rating [<=100V] start turning on at a lower Vbe [0.5V] and show a much sharper knee of current versus Vbe turn on characteristic. These devices are harder to thermally stabilise and his Vbe-on drops quicker as temperature increases. Devices with higher Vce rating [>=150V] show higher Vbe-on and more progressive turn-on, being easier to thermally stabilise and showing also less impact of temperature over Vbe

- Vbe-on and Vbe temperature coefficients are not constant. They depend mostly on Vce and Ic. For the same Ic : The higher the Vce the lower the Vbe-on and the higher the temperature coefficient of Vbe, so the same transistor model is much harder to thermally stabilise at higher Vce than at lower Vce

- In the low Ic range [<500mA approx.] and given fixed Vbe and Vce values : The higher the Ic, the quicker Ic increases as temperature is increased. This means that the same amplifier may be thermally stable with 50mA bias but may suffer thermal runaway if bias is increased to 100mA


To understand all this you may try to thermally stabilise a pair of BD911/BD912 or TIP35C/TIP36C devices [rated at 100V Vce] with +-45V rails and 50mA bias. Then try to stabilise a pair of Sanken 2SA1295/2SC3264 devices [rated at 230V Vce] with the same +-45V rails and 25mA bias. You may also repeat the experiment with +-15V rails for the 100V Vce devices and +-85V rails for the 230V Vce devices

Huge differences in bipolar transistor behavior will show during these experiments

Now I wonder why Rod Elliot suggests 250V and 350V Vce devices for +-35V supply rails. This is an overkill, there are better things to do with these devices, I think

Also note that the only way I've found to thermally stabilise 100V Vce devices with up to 100mA bias when used near its Vce and dissipation limits is to use a buffered Vbe-multiplier with a low-vbe transistor as the sense element [PN2222A], operated at an Ic around 100uA for maximum temperature coefficient [these small signal devices show their higher temperature coefficients at very small Ic]

Actually the temperature coefficient of any Vbe multiplier may be adjusted by increasing or decreasing sense transistor Ic to match the output devices behavior, and also to compensate for the temperature difference between heatsink and transistor dies, whose value is allways proportional to heatsink temperature
  Reply With Quote
Old 6th January 2005, 07:23 AM   #9
richie00boy is offline richie00boy  United Kingdom
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
Vbe Thermal Coupling  Issue
Quote:
Originally posted by sam9
I noticed some time back that ESP's P3a doesn't couple the Vbe multiplier but doesn't suffer for it. I'm wondering if the thermal coupling business isn't as critical as claimed.
That's because the P3a output stage is Complementary Feedback Pair where the Vbe of the output devices (and hence their thermal instability) is servoed out by the drivers. Thus you only need to sense the driver thermal situation. As the drivers run cool/don't see much thermal cycling, it's safe to run the Vbe multiplier in free air.
  Reply With Quote
Old 6th January 2005, 02:43 PM   #10
sam9 is offline sam9  United States
diyAudio Member
 
sam9's Avatar
 
Join Date: Jun 2002
Location: Left Coast
Oh. I hadn't actually looked at the circuit for sometime. I should have been able to answet that one myself, if I had looked. Thanks
  Reply With Quote

Reply


Vbe Thermal Coupling  IssueHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Biasing/thermal compensation of Thermal Trak transistors Bob Cordell Solid State 163 Yesterday 10:53 PM
Floor coupling issue (I supose...) andruca Multi-Way 0 11th January 2007 06:58 AM
ALEPH 3 HEATSINK - thermal coupling between single transistors lokuspokus Pass Labs 2 7th February 2004 11:23 PM
thermal coupling in aleph hifi Pass Labs 2 14th March 2001 09:45 PM


New To Site? Need Help?

All times are GMT. The time now is 01:24 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki