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.

Dual Slope Protection Circuit Values...
Dual Slope Protection Circuit Values...
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
Thread Tools Search this Thread
Old 16th November 2008, 12:44 PM   #1
SpeakerScott is offline SpeakerScott  United States
diyAudio Member
Join Date: Jul 2008
Send a message via AIM to SpeakerScott Send a message via MSN to SpeakerScott Send a message via Yahoo to SpeakerScott
Default Dual Slope Protection Circuit Values...

Many individuals have published output stage protection circuits. However, they are usually just one example circuit for a particular transistor. More importantly, they are for a particular value of emitter degeneration resistor. The value of the resistor is important to the protection circuit, changing Re from .22 ohms to .1 ohms means the protection circuit won't protect! The circuit below is a standard dual slope protection circuit, published by Sloane, Self, Duncan and others.

Click the image to open in full size.

Click the image to open in full size.

The response above shows the load lines for a 8 ohm and 4 ohm reactive load of 45 degrees, and a 4 ohm resistive load. For the first plot I’ve included both the 3281 and 21193 transistor SOA. Notice that the 21193 is a little bit better than the 3281 transistor (and their associated complementary pair…)

For this circuit R1 = 4.99k, R2 = 4.22k, R3 = 2.21k, R4 = 182, R5 = 665, Re = .22 Ohms…

If you want to go with the 21193 you can move the protection circuit response so that it does not impede as much of the 4 ohm reactive load line.

Click the image to open in full size.

For this response R1 = 4.99k, R2 = 4.22k, R3 = 1.00k, R4 = 182, R5 = 665, Re = .22 Ohms…

If you want to use the 21193 transistor with an Re of .1 ohms….you can get the following:

Click the image to open in full size.

For this response R1 = 4.02k, R2 = 2.68k, R3 = 698k, R4 = 182, R5 = 201, Re = .1 Ohms…

I was also curious to see how fast the load line would move outside of the SOA for a single transistor. Below is the diagram with the same MJL21193 transistor SOA and the protection circuit from the example just before this. Instead of plotting 8 ohms, 45 degrees I plotted 4 ohms 45 degrees at just before clipping with an amp with 40V rails, and one with 50V rails. Both load lines (elipse) are CFP amps, with a single pair of output transistors. Notice that with 50V rails a single transistor will NOT be sufficient. In fact for transistor power dissipation, the 40V rails present enough of a challenge…continuous operation into difficult loads with a single pair of transistors requires temperature protection, not just SOA protection.

Click the image to open in full size.

Remember, the SOA curve is rated for a peak junction temperature. The junctions heat up quickly in a power transistor, there is quite a bit of thermal lag from transistor to case…where you can measure temperature. Also remember that in real life, voltage rails sag somewhat protecting the amplifier. The dynamic nature of music means that the continuous stress on the transistor is considerably less than using tones to test, not enough (in my humble opinion) to get away without SOA protection…but enough to rarely activate the circuit in real use.

I’ve also been asked before if it’s worth-while to worry about load impedance phase angle with amplifiers…and the answer is emphatically yes…

The Avalon Indra, and Sonus Fabor Cremona are just two examples.


I know this list of circuits isn't exhaustive (I've left out Fairchild power BJTs and any type of MOSFET...I'll get around to plotting those one of these days as well.) But I hope this helps.
https://www.facebook.com/DIYRM/ See FaceBook page for >100 pages of design guides and articles.....
  Reply With Quote
Old 16th November 2008, 09:29 PM   #2
megajocke is offline megajocke  Sweden
diyAudio Member
megajocke's Avatar
Join Date: Jan 2003
Sure, the junctions heat up pretty fast - but it's not as fast as one might think at first. But of course - using more pairs will give better performance due to less beta droop and such things.

I've played a bit with thermal models because I couldn't understand how commercial equipment gets away with about half the number of output devices many here at diyAudio say are absolute minimum. The thermal models show that the thermal capacity of the transistors is enough to make the short peaks from reactive loads a pretty small problem even at the lowest of frequencies.

For transistors with crappy second breakdown performance like 2N3055, a lot of darlingtons, TIP35 and older transistors you probably do need to stay inside the DC SOA though but for transistors like MJ(L)2119x, MJL3281, 2SC3281, 2SC5200, 2SC3264 and so on it's not a big problem at this low voltage.

The 150 degree max is really set by the packaging material and as this has pretty slow temperature transfer from the chip designing for an average temperature of 150 degrees (using averag power dissipation) during worst-worst-case seems to often be done. This practive seems sensible as long as peak temperature is sensible, say below 200 degrees or so (which it will be). I'd probably derate the calculated thermal resistance by about 1.5 - that would keep almost all peaks below 150 degrees according to models of the transistor time constants.

An example is the QSC RMX1450 PA amp - it has ~75V rails and gives 700W for 2 ohm loads. 4 pairs of 2SC5200/2SA1493. Doesn't have thermal pads for the transistors though so can push them a bit further than otherwise.

But still, nothing wrong with using more pairs - you will often get lower distortion!
  Reply With Quote


Dual Slope Protection Circuit Values...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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Please teach me - triple slope SOA protection R.G. Solid State 41 28th February 2013 03:05 PM
looking for a dual pot, with concentric shaft with values of 100K and 50K jealousblues Parts 4 21st July 2006 06:02 PM
Trouble with speaker protection circuit (Randy Slone's circuit) whalefat Solid State 3 13th April 2005 10:13 AM

New To Site? Need Help?

All times are GMT. The time now is 07:30 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio