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:
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?
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:
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?
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.
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!
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.
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.
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.
MJL21193 said:
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?
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.
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.
By low-side sensing do you mean your Q6 network? That's a bit complicated and I don't understand how that would fit into an amp design. Mostly what I don't understand is how current limiting the PS can distinguish between normal (permitted) high current draws and short-circuit current draws. To the PS, they are the same.
The circuit evolved over the course of the thread, with some component name changes, so I'm not sure which schematic you are referring to. The current sense portion of the circuit is at the bottom of the schematic.
Actually, any VI limiter on the amp doesn't differentiate the load any differently than the power supply. Power is power, current is current.
The calculations for setting the current limit are not complex. Determine the anticipated maximum current for the minimum impedance load and then set your current limit slightly more than that. It has the dual purpose of protecting against extra low impedance loads.
Again, this is how I'd do it. That doesn't mean it's the best way or that I saying you have to do it this way. I do know that I'll be looking at this method in a future amp build and will further develop the idea there.
Right, I was looking at the latest schematic so had the right part picked out.
This is exactly the point of my output-gated VI limiter. The current permitted for shorts is a different value from the current permitted for normal loads. Selecting a fixed current limit is a compromise. It either does not protect the amp from shorts or limits the max current allowed for large signal levels. Have another look at that article I linked at start of thread. It goes into some detail about this. (VI Limiters in Amplifiers)
What class do you use? Will that really protect the amp from short-circuited outputs?
No it will not.
Assume a +-50Vdc smps with a current limit set to 10Apk.
A fault in the amplifier or it's load would be required to dissipate 1kW of energy, continuously.
How hot could that take a combustible item too?
No fuse, no switch, no protection!
Not a model for anyone one to copy.
If there were a bank of smoothing/decoupling capacitors to meet the short term speaker demands then the heat problem is even worse, but, the caps will stop supplying extra energy when they have depleted to the SMPS overloaded voltage.
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I am using tracking rails, with limited current for low voltage and regulator is automatically current limited by its topology for higher tracking voltage.
I ever post it here:
http://www.diyaudio.com/forums/soli...-hybrid-linear-switching-power-amplifier.html
Hi, Andrew, most SMPS will switch off while overloaded and turn into safe pulsed system. Of course some of them are poorly designed, and just choosing to blow up when overloaded, some other has big electrolyt and slow turn off response, that turned off after amplifier transistors blown.
I think one must define under what conditions the short circuit would apply. Furthermore is the short circuit a stub of zero ohm wire at the amplifier output or at the end of the speaker cable.
Finally is the amplifier running at full bore when the idiot causes the short circuit. If an amp is not playing anything, thus idling, you can short the speaker terminals indefinitely.
Also if the transistor happens to be fully turned on at the moment of the short, Vce is maybe a volt or so and the bulk of the power is dissipated by the poor 3 watt emitter resistor, the resistor would surely blow.
One should also fuse the power supply to the amplifier board (after the reservoir capacitors) correctly rated for the continuous current that the amp is expected to produce.
In my 45 years of messing around with audio, I have never blown any output devices, for that reason I don't bother with protection. The amount of money that I have saved not putting protection circuits in my amplifiers I could easily afford a few new amplifiers should they be destroyed for any reason.
Finally is the amplifier running at full bore when the idiot causes the short circuit. If an amp is not playing anything, thus idling, you can short the speaker terminals indefinitely.
Also if the transistor happens to be fully turned on at the moment of the short, Vce is maybe a volt or so and the bulk of the power is dissipated by the poor 3 watt emitter resistor, the resistor would surely blow.
One should also fuse the power supply to the amplifier board (after the reservoir capacitors) correctly rated for the continuous current that the amp is expected to produce.
In my 45 years of messing around with audio, I have never blown any output devices, for that reason I don't bother with protection. The amount of money that I have saved not putting protection circuits in my amplifiers I could easily afford a few new amplifiers should they be destroyed for any reason.
In my opinion, if one wants to protect an amplifier/speaker/ear chain, a simple method by just comparing the input with NFB using an op-amp is ultimate.
In other words if the input/output differs by some decided margin, whether this is due to transistors blowing, capacitors failing, speaker impedance out of range, shorts, clipping or any condition that exists at the output and is not present at the input that you consider damaging, then simply shut the system down.
Your decision making can be accomplished either in the analog, or if you want to be smart in a digital domain using a small microprocessor, it is irrelevant.
I have done this for a couple of commercial designs were it was insisted that protection of the chain is imperative because of the commercial standing of the brand name.
In other words if the input/output differs by some decided margin, whether this is due to transistors blowing, capacitors failing, speaker impedance out of range, shorts, clipping or any condition that exists at the output and is not present at the input that you consider damaging, then simply shut the system down.
Your decision making can be accomplished either in the analog, or if you want to be smart in a digital domain using a small microprocessor, it is irrelevant.
I have done this for a couple of commercial designs were it was insisted that protection of the chain is imperative because of the commercial standing of the brand name.
I had the same idea HERE using Rod Elliott's SIM.
I never did implement it though.
One problem is that it will not detect an existing short on start up, and a major drawback for most amps is that clipping will trip the protection. I try to make amps that cannot clip, as they are gain limited (according to input sensitivity) to operate well within the power supply limits. In other words, the supply voltage is about 20% more than what would normally be required for the same output power - built in headroom.
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I read that article quite a while ago and considered using it in one of my first amp builds. I didn't follow through though and like Nico, don't have any short circuit protection in place. I'll have a closer look.
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