Temperature compensation for output stages - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 6th June 2002, 07:30 AM   #11
BrianL is offline BrianL  United States
diyAudio Member
 
Join Date: Mar 2002
Location: USA
Doug Self discusses the topic of bias control and temp
compensation extensively in his EW/WW articles and/or
his book(s). Additionally Malcolm Hawksford had a JAES
paper on the same topic.
__________________
bel
  Reply With Quote
Old 6th June 2002, 03:09 PM   #12
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
I read this thread at work and as I had been agonising over how to do the bias on my Circlotron doomsday amplifier, I put my brain into high gear to try and think of a solution. I think I have something workable.

First off, the temp sensing fet MUST have the same threshold as BOTH the MATCHED output fets. I measure threshold voltage by joining gate to drain and then run a 100 ohm resistor to +12v, source to neg, and measure drain source voltage. Usually you can find a pair that will match within 10mV. Rude and crude maybe, but it's simple and works well.

For the test I got a matched pair of N-channel IXYS IXFH32N50Q 500v 32 amp 375 watt fets and wired one of them in threshold measurement config as described above. The second fet had it's source and gate to the first one's source and gate. It's drain went to a 40 vdc supply via a 10 ohm current sense resistor. The threshold voltage was 4.06 v.

With the heastink at 20 deg C the second fet pulled a constant 108.6 mA. At 70deg C heatsink temp when heated via a soldering iron the second fet pulled 112.0 mA. Is that reasonable enough?

For a complementary symmetry setup the matching still applies but the temp sense fet MUST have equal resistors on the gate to source and gate to drain so it makes EXACTLY double the drop it would otherwise. It is probably the best idea if you use the same kind of fet for sensing as you do for the main output.

GP.
  Reply With Quote
Old 6th June 2002, 08:16 PM   #13
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
I have been think since I started this thread. I haven't got much input from you (advanced subject!) but I got a crazy and extreme idea:

PIC or AVR processor with ADC and EEPROM, placed between the upper and lower cascode transistors. My circuit replaces the conventional transistor+2 resistor thing. Working voltage 2.5 - 8V and current consumption max 5-8 mA

Processor in sleep mode most of the time and internal RC-oscillator or 32kHz crystal.

Tempsensor made of transistor + 2 resistors to create enough voltage for the ADC.

ADC for sensing voltage across the gates of the output FET's.

R-2R ladder to create a DAC, directly from the processor or via a shift register (depending on current consumption and emission).

My idea is to calibrate the amp and then store the values in EE-memory.

Calibration like this:

Enter "learn"-mode

1 Set desired bias with cold heat sinks

2 Store temp and voltage

3 Increase current in order to get the heat sinks really hot (60-80 deg C).

4 Store temp and voltage

5 The processor calculates the temp coeff and store it. Since I have a processor it's possible to let the temp curve be unlinear.

Sounds my idea like an overkill? I have to think a litte bit more about the DAC-thing. My goal is to make it easy and "quite".
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 7th June 2002, 08:39 AM   #14
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
OK. this is an all-stops out effort to measure the variation in mosfet quiescent current with change in temperature. That is to say, any variation using the compensation method I am suggesting. The test I did last night was pretty much a seat of the pants effort at home. Today at work I used several meters and a thermocouple temp sensor that all have traceable calibration, and besides, one of my job functions is to actually perform temp measurements on power electronic stuff, so no shortcuts were taken. These figures are accurate.

Actuall, afterI began testing I realised the test setup is actually a current mirror of sorts. But I can't stress enough "all the devices MUST have the same Vgs threshold voltage".

The test setup consisted of both TO-247 fets bolted opposite sides of a 20mm square aluminium bar 150mm long. A small hole was drilled between the two fets and filled with heatsink paste. The thermocouple was inserted into this hole so it would be shielded from air currents and get the best possible thermal coupling. The thermal compound used on the main fet was Thermstrate TC, (amazing stuff, that) and the bias/sense fet had a silpad under it. The aluminium bar also had a 25 watt metal clad resistor attached so I could adjust the aluminium bar to whatever temp I wanted. The thermocouple meter was a fluke 52.

The main fet had a constant 50 vdc across it, and every 5 degrees I recorded it's current, the bias voltage, the bias fet current and the temp. It's all here on a spreadsheet with graphs. This was a BIG effort. I'm certainly going to make use of what I found. I hope others like it too.

P.S. The normal way of setting bias with a transistor and a pot is all wrong I think because when you set it at a particular temp what you are doing is changing the slope, or tempco, of the bias so that it only works at THAT temp. At other temps it will either dry up or dig in too hard. The only way to do it is to have the temp sense fet gain fixed at x2 with equal value resistors and use a fet that has the same characteristics as the output devices. That way the nonlinearites in the tempco will track the whole way. But remember, make sure the sense fet has intimate thermal contact with the output fets, e.g. opposite side of the heatsink, and... *match the 3 device thresholds* Comments invited please.

GP.
Attached Files
File Type: zip mosfet bias vs temp measurements.zip (24.8 KB, 223 views)
  Reply With Quote
Old 8th June 2002, 11:46 AM   #15
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
This approach looks quite simple. Essentially 2 Vbe multiplier ccts in series between the output device gates. Only *one* is thermally coupled to the output fets. Both are adjustable, one for offset (the initial or cold bias voltage) and the other for slope (rate of bias voltage decrease per deg C increase).
See page 15.
http://www.ee.mtu.edu/faculty/goel/E...put-Stages.pdf

GP.
  Reply With Quote
Old 9th June 2002, 01:17 PM   #16
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
I just can't seem to stay away from this thread...
LT have a chip specifically for this biasing bizzo. LT1166. In their words:
The LT®1166 is a bias generating system for controlling class AB output current in high powered amplifiers. When connected with external transistors, the circuit becomes a unity-gain voltage follower. The LT1166 is ideally suited for driving power MOSFET devices because it eliminates all quiescent current adjustments and critical transistor matching. Multiple output stages using the LT1166 can be paralleled to obtain higher output current. Thermal runaway of the quiescent point is eliminated because the bias system senses the current in each power transistor by using a small external sense resistor. A high speed regulator loop controls the amount of drive applied to each power device. The LT1166 can be biased from a pair of resistors or current sources and because it operates on the drive voltage to the output transistors, it operates on any supply voltage.
http://www.linear.com/pdf/lt1166.pdf
  Reply With Quote
Old 9th June 2002, 09:53 PM   #17
BrianL is offline BrianL  United States
diyAudio Member
 
Join Date: Mar 2002
Location: USA
Default GO READ!

There's no agonizing to do. Some reading and thinking, yes,
but no agonizing. And surely you don't need microprocessors
to do this job.

Go read Doug Self's book, or his articles in EW/WW. I don't
know if the bias stuff is covered on his web site so you just
may need to spend $35 and buy the book.

Self covers it all in painstaking detail.
__________________
bel
  Reply With Quote
Old 11th June 2002, 09:27 AM   #18
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
The LT1166 seems to be able to do the job but I'm not very keen on using unusual chips with no second source. There seems to be a very little problem with this chip and this is speed limitation. Since the chip creates a control loop it seems that extremely low distortion isn't possible.

My microcontroller idea are interesting and also rather compact and not very expensive. My brain is at the moment puzzled by creating analogue signal in a cheap simple way.

Microcontrollers are very rare in hifi but it's nowadays a component as the transistors. Nothing wrong in using those parts but they need to be carefully designed into the whole amp because they can hang up and also interfere the audio signal with clock signals.

My experience with PIC and AVR is very good. They very seldom creates problems in our products (at work) nor in real life.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 13th June 2002, 02:56 PM   #19
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
This is interesting, using a photovoltaic coupler to supply the floating (and digitally controllable) bias voltage. Full article here http://www.elecdesign.com/1998/sept1498/ifd/1IFD.pdf

GP.
  Reply With Quote
Old 15th June 2002, 01:09 PM   #20
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
When you are sensing the temperature of your output stage, all you can really measure is the heatsink temp. When the amp is working hard the junction may be quite a few degrees above the heatsink. Is there a good way to make a more reliable estimate of junction temp?

GP.
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Cascode Output Stages pietjers Solid State 118 20th April 2008 05:56 PM
Temperature compensation in circuit? stef1777 Parts 13 16th July 2007 05:21 AM
cheap output stages hacknet Solid State 0 8th December 2002 01:55 PM
Using The AD-744 Compensation Pin as an Output ppl Everything Else 0 27th May 2001 11:51 AM


New To Site? Need Help?

All times are GMT. The time now is 09:29 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2