Overload / Short Circuit Protection, Yet Again...
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 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
 3rd February 2011, 09:11 PM #1 Buckeye diyAudio Member   Join Date: Jan 2011 Overload / Short Circuit Protection, Yet Again... I'm working on a real basic BJT amp design, but want to incorporate short-circuit protection. So I've been doing some reading. In his article "Testing Amplifiers To Their Limits", Phil Allison gives an outstanding overview of VI limiting and some if its limitations (pun intended). (VI Limiters in Amplifiers) The main problem is that to protect against shorts, you have to set the VI limit too low. Reason is from the SOA graph (see File:BDV66C limits.png - Wikipedia, the free encyclopedia) and the low current allowed at high Vce voltages. Note that a nominal Ic and a huge Vce is exactly what could happen during a short. And when you have a normal speaker load, high currents are allowed because Vce is small. So to be effective under short-circuit conditions, the conventional VI limiter limits current even when it shouldn't. It's a compromise solution, and maybe not a very good one. I was searching the web and found this 1982 patent by Kaplan, Power Protection Circuit For Transistors. I think he's on the right track. His invention measures Vce to limit the current. It offers a better approximation to the SOA in the secondary breakdown region. But not many implement his approach because it's complex and costly. So here's an idea... how's about we measure the Vce indirectly by measuring Vout and using that to gate the conventional VI limiter? I call this the Output-Gated VI Limiter (OGVIL). Here's the drawing of just the positive half of the typical BJT AB amp: GatedLimiter.jpg So you can see from the above, when a short is placed between output and ground, Q4 is off and the conventional VI limiter operates as usual. We are now free to set the current limit as low as needed to protect the transistor from its secondary breakdown. However if no short exists, Vout rises with the output signal, and Q4 turns on effectively reducing the gain of the limiting transistor Q3. Some additional notes on the above circuit... Depending on your drive circuit, a diode may be needed in series with the collector of Q4 to prevent forward bias of its collector-base junction. Most likely the value of R4 will be zero. I've done a bit of searching and haven't seen anyone try this approach for short-circuit protection. (There may be good reason for that .) Renardson did something like this in his post here: Overload and Short Circuit Protection.. He used a resistor (R1) from base of the limiter transistor, Q1, to ground. Q1 in his circuit is Q3 in my circuit. By using a transistor to ground, we're able to totally disable current limiting when Vce is low, i.e., Vout is high. We can even go wild and implement a log function in the current limiter to more closely match the SOA curve near the secondary breakdown region. What do you think?
 4th February 2011, 02:49 AM #2 audiohead   diyAudio Member   Join Date: Mar 2005 Location: Los Angeles, California Diode D1 in your circuit is drawn backward? Art
 4th February 2011, 02:54 AM #3 Buckeye diyAudio Member   Join Date: Jan 2011 oops, yes it is. thanks
 4th February 2011, 08:46 AM #4 ontoaba   diyAudio Member     Join Date: Nov 2009 Location: Kudus, Malang, Dieng Good idea.. Add capacitor too. With slow charge and fast discharge. Then thermal sense, like NTC or transistors. Actually I never use any SOA for myy amplifiers
AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by ontoaba Actually I never use any SOA for myy amplifiers
your amplifiers should be using the SOA all the time they are operating.
It's when a device strays outside the SOA that you should be concerned about.

We don't care if you ignore SOA. WE know that SOA matters to the devices. They have to survive what we ask them to do.
__________________
regards Andrew T.

Buckeye
diyAudio Member

Join Date: Jan 2011
Quote:
 Originally Posted by ontoaba Actually I never use any SOA for myy amplifiers

If the amp is used for a powered speaker or subwoofer, speaker wiring is permanent and no protection is needed. But if there's a user involved in connecting the speakers...

The intended purpose of my amps is live sound and perfect reliability is more important then perfect fidelity. Users are not always (or more accurately are rarely) careful in making their speaker connections. Without SOA protection, the amp becomes and anchor or a door-stop.

What I'm suggesting here could possibly help fidelity by eliminating the limiter when Vout is high and the speaker load is normal. During normal operation, the power supply can limit the amp to its rated power. That's probably what's keeping your amps' operation inside the SOA. Just don't short those speaker wires!

 4th February 2011, 12:05 PM #7 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders Buckeye, you are on the right lines. I have never seen q4 used like that. Combinations of Zeners are more often used to obtain multiple slopes into the IV locus. My version of a good IV limiter is that it must pass all valid audio signals to all valid audio loads. If it does this it can never have any effect on sound quality. If abused the IV limiter should interfere with the signal and thus protect the devices from damage. Capacitors have been mentioned. I don't see any mechanism to allow high transient currents to pass uninterrupted while detecting and protecting from lower levels of excessive long term currents. __________________ regards Andrew T.
 4th February 2011, 12:05 PM #8 MJL21193   Account disabled at member's request     Join Date: Mar 2007 IMO, the best way to limit current to a short circuited output is via the power supply. Low side (0 volts) sensing and cut both rails when it is exceeded. Leave the limiter out of the amp itself, where it may have an impact on performance.
Buckeye
diyAudio Member

Join Date: Jan 2011
Quote:
 Originally Posted by AndrewT Capacitors have been mentioned. I don't see any mechanism to allow high transient currents to pass uninterrupted while detecting and protecting from lower levels of excessive long term currents.
Agreed. Shorts in stage speaker wiring can happen any time and protection must be instantaneous. There doesn't seem to be any reason to keep it active longer than necessary.

Quote:
 Originally Posted by MJL21193 IMO, the best way to limit current to a short circuited output is via the power supply. Low side (0 volts) sensing and cut both rails when it is exceeded. Leave the limiter out of the amp itself, where it may have an impact on performance.
That's a great approach. Are there manufacturers doing this? Seems like cost and complexity would suffer. I have some questions like: How is the output short sensed? Seems like you would have to cycle the power to see if it's still there. How is the rail power cut -- a couple big transistors along with their heat sinks? I haven't seen any cheap and simple circuits that do this can you point me at some?

Building you your idea... You know, it would be easier and cheaper to only cut the power to the input stage. Then you get the same benefit of nothing in the audio path, but protection by eliminating the drive signal.

That still needs a good method of sensing a short. You got anything?

 4th February 2011, 01:04 PM #10 MJL21193   Account disabled at member's request     Join Date: Mar 2007 I've seen a few that are high side sensing, but nothing that is low side. Low side is where I'd be doing it as it covers both rails simultaneously. I used low side sensing in my lab power supply HERE with outstanding results. Cutting the rails would be the best way to go. This could be done a number of ways, but I think I would use power bjts, and incorporate a basic regulator in as well. Since the voltage drop across the devices is very low, power dissipation would be minimal.

 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 Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Holton Precision Audio         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post roofingboom Solid State 116 24th February 2017 11:55 PM black Solid State 2 30th October 2010 08:09 PM space2000 Solid State 17 5th March 2009 10:26 PM Fritzell Solid State 16 19th September 2005 02:19 PM JBL Solid State 20 5th August 2002 07:27 AM

 New To Site? Need Help?

All times are GMT. The time now is 09:04 PM.