mikeks said:
Hello Mikeks,
A question I was going to ask you as we are talking about protection circuits now. Is why Douglas Self in his designs and other amps that use miller comp. do not include a current limiter over the base of the VAS to offer protection on the negative swing on a output short circuit.
The CCS for the VAS offers protection on a output short circuit on the positive going swing.
Or is this to do with the current limiter interfering with the miller compensation and causing instability or something like that ?
Not sure on that one.
Kevin
G.Kleinschmidt said:
Hi Glen,
Nice little gem!
I understand it assumes an inductive load? Any way to use neg phase shift as in cap load?
jan Didden
Hi Janneman,
here's how Bensen did it for a FET. http://www.diyaudio.com/forums/attachment.php?s=&postid=644996&stamp=1116447049
If you want to see a BJT version with second breakdown then Email me.
Hey,
you stole my word "gobbledigook..."
I charge a licence fee on it's usage😉
here's how Bensen did it for a FET. http://www.diyaudio.com/forums/attachment.php?s=&postid=644996&stamp=1116447049
If you want to see a BJT version with second breakdown then Email me.
Hey,
you stole my word "gobbledigook..."
I charge a licence fee on it's usage😉
AndrewT said:Hi Janneman,
here's how Bensen did it for a FET. http://www.diyaudio.com/forums/attachment.php?s=&postid=644996&stamp=1116447049
If you want to see a BJT version with second breakdown then Email me.
Hey,
you stole my word "gobbledigook..."
Yes! That did it, it dawned on me when I saw how he combined those graphs. Now I can even combine graphs from different Exel files!
Thanks,
Jan Didden
AndrewT said:[snip]Hey,
you stole my word "gobbledigook..."
How about 'pupucaca', that still free?😀
Jan
AndrewT said:
I've never seen the full leach schematics you have have referred to, including the full compensation etc but if I recall from memory he uses a fully complimentary input stage and different VAS arrangement.
Also we do not know what happens when C.dom becomes current starved by the Input stage when the current limiter activates. Slew rate would definately be affected.
Frankly I don't have a clue, but there seems to be alot of designers of miller amps that exclude protection of the VAS for some reason.
Maybe Mikeks could enlighten us.....
janneman said:
G'day Jan.
The I phase shift can be either leading or lagging. It doesn't make any difference to the plotted reactive load lines.
Cheers,
Glen
janneman said:
Unfortunately, I do not agree. It simply isn't possible to approximate with a decent degree of accuracy all possible reactive load lines with a single straight line, least of all an "envelope" to safely inclose them.
Having a play with that spreadsheet Andrew mentioned or my program will give you a good idea of just how dramatically the shape of the load lines change when the load becomes reactive.
Cheers,
Glen
Edit:
On second thoughts, the method you describe would probably be an adequate compromise for simple, fixed I-limiting output device protection.
I’ve spent many happy? hours calculating and manipulating Mikeks’s (fig 27) method for dual slope protection in order to utilize standard resistor values where possible. Eventually I attempted an Excel spreadsheet model (my first!) based on Mike’s article to ease this task.
I then applied the Bensen arrangement (an invaluable source of inspiration) alongside this to display the SOA and load lines, in linear form to fit in with the protection slopes. Working with log-log’s isn’t intuitive, to me.
By inputting the required data, it may be useful as a general model.
It is with much trepidation that I attach this, since I’m no mathematician, much less a macro-modeller, so if I could impose upon those who are to corroborate, or demolish (and correct?), I would be grateful.
Best regards,
Brian.
I then applied the Bensen arrangement (an invaluable source of inspiration) alongside this to display the SOA and load lines, in linear form to fit in with the protection slopes. Working with log-log’s isn’t intuitive, to me.
By inputting the required data, it may be useful as a general model.
It is with much trepidation that I attach this, since I’m no mathematician, much less a macro-modeller, so if I could impose upon those who are to corroborate, or demolish (and correct?), I would be grateful.
Best regards,
Brian.
G.Kleinschmidt said:
Hi Glen,
I was not really happy with Doug Self's recommendation either; he didn't elaborate on it. I have since found out how I can include a reactive load line, calculated separately, in the graph of the SOA and protection loci.
I am using the spreadsheet to automagically calculate the protection circuit component values interactively by setting the breakpoints on the graph to follow any required shape with two breakpoints.
Jan Didden
Pingrs said:
Brian, looks good!
I do something similar but with a different circuit. I never felt quite comfortable with Mike's circuit, too many variables to juggle!
I have two zeners in my ciruit to set the breakpoints, but I can use the Exel solver to manipulate one or more values to get to (at least one) "nice" zener value.
Jan Didden
Brian, Glen,
Please se the attached.
There is a Icprot protection locus (plus the Spice results; not quite the same, but I'm working on it😉 ). But that's not my point right now.
You also see that the reactive (8 ohms @ -45 deg) load line is inside the 100mS SOA but outside the DC SOA.
My question is: would you accept this SOA situation, because since the reactive load only comes into play at AC signals, the lowest audio is 20Hz or more, and the reactive line is within the 100mS (10Hz equivalent) SOA?
Jan Didden
Please se the attached.
There is a Icprot protection locus (plus the Spice results; not quite the same, but I'm working on it😉 ). But that's not my point right now.
You also see that the reactive (8 ohms @ -45 deg) load line is inside the 100mS SOA but outside the DC SOA.
My question is: would you accept this SOA situation, because since the reactive load only comes into play at AC signals, the lowest audio is 20Hz or more, and the reactive line is within the 100mS (10Hz equivalent) SOA?
Jan Didden
janneman said:
A 100mS pulse represents a frequency of roughly 5Hz (unity mark-space ratio assumed).
Thus we can reasonably assume that these pulses are outside the audio band.
Solution:
Design for your DC SOA and then apply single or double pole filter attenuating these pulses by say 0.5 at ten times 5Hz.
In other words you're right.
mikeks said:
Mike, thank you. Most helpful.
But if I attenuate this lf signals it will destroy my freq response, or did I misunderstand you?
I need to somehow protect the amp against short circuit and that is the DC SOA at the supply voltage. I cannot at the same time do a short circuit protection AND a, say 100mS SOA protection against signals above a few 10's of Herz.
mikeks said:
I think I will frame this and hang it on my lab wall 😀
Jan Didden
Hi,
filter the protection signal sent to the base of the trigger transistor.
Then you can set up the DC protection to match the DC SOAR and use the filter to allow transient peaks to be attenuated before the trigger activates.
A very short transient could reach 2 to 3 times the DC current and a longer duration transient maybe just 150% of the permitted DC current. These variable attenuation values follow roughly what a low pass filter lets through to the base and match up with the transient SOAs that are published.
Can some one tell me how to test/measure that the trigger is operating at the predicted levels on both DC and longer term transients?
A word of caution.
The manufacturers state consistently that the transient values apply to single shot currents only. A long term note could pass many hundred of cycles and the one shot limits must surely be reduced for repeated cycles.
I suggest that a 1mS current limit repeated 10 times should remain under the 100mS SOA, similarly 100uS transient under the 1mS SOA. What do you think?
A mid or treble frequency tends to be quite short duration, in terms of number of cycles that are repeated, whereas a bass note may repeat for considerably more cycles.
I think that bass note overload should be the criterion to design for and that 100mS SOA is inappropriate if the one decade reduction rule is applied. Keep in mind that the filter will still attenuate and thus let through short term peak and thus delay triggering on single event overloads.
Pingrs,
I note that you have de-rated Vce for operational Tc values. I think you should be de-rating Ic. The effect is not the same. Have a look at your log/log graph and compare it to the data sheet graph. The permissible maximum Vce is not reduced, it is the permissible current that is reduced. Similarly the max current value should be reduced for increased Tc. The graphical effect is to move the whole graph downwards rather than sideways. Mathematically both reductions are easily achieved. Any further thoughts?
filter the protection signal sent to the base of the trigger transistor.
Then you can set up the DC protection to match the DC SOAR and use the filter to allow transient peaks to be attenuated before the trigger activates.
A very short transient could reach 2 to 3 times the DC current and a longer duration transient maybe just 150% of the permitted DC current. These variable attenuation values follow roughly what a low pass filter lets through to the base and match up with the transient SOAs that are published.
Can some one tell me how to test/measure that the trigger is operating at the predicted levels on both DC and longer term transients?
A word of caution.
The manufacturers state consistently that the transient values apply to single shot currents only. A long term note could pass many hundred of cycles and the one shot limits must surely be reduced for repeated cycles.
I suggest that a 1mS current limit repeated 10 times should remain under the 100mS SOA, similarly 100uS transient under the 1mS SOA. What do you think?
A mid or treble frequency tends to be quite short duration, in terms of number of cycles that are repeated, whereas a bass note may repeat for considerably more cycles.
I think that bass note overload should be the criterion to design for and that 100mS SOA is inappropriate if the one decade reduction rule is applied. Keep in mind that the filter will still attenuate and thus let through short term peak and thus delay triggering on single event overloads.
Pingrs,
I note that you have de-rated Vce for operational Tc values. I think you should be de-rating Ic. The effect is not the same. Have a look at your log/log graph and compare it to the data sheet graph. The permissible maximum Vce is not reduced, it is the permissible current that is reduced. Similarly the max current value should be reduced for increased Tc. The graphical effect is to move the whole graph downwards rather than sideways. Mathematically both reductions are easily achieved. Any further thoughts?
AndrewT said:
... but of course, I didn't read that in it. In fact, I have it in my circuit.
AndrewT said:
I am using a circuit developed by Peter Baxandall of Quad. It involves loading the amp with 40uF in series with 1 ohms. Then drive it with a 20Hz sinewave, superimposed on which are short (50uS) pulses with a 500Hz rate. The sine does exercise the Vce (without actually loading the amp because of the 20Hz/40uF), but the (asynchronous) pulses cause high current load pulses at varying Vce's. So the protection circuit moves through the 'landscape' of all possible Vce/Ic combinations.
If you now make an x-y display of Vce and Ic you will see that envelope of the allowed combinations of Vce and Ic.
I did get it to work but not really as well as Baxandall described in his article; it's still a work in progress.
Jan Didden
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