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Author Topic: Peak dissipation in class B output stage  (Read 819 times)

shasam

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Peak dissipation in class B output stage
« on: September 04, 2020, 06:27:15 AM »
Hi!


I'm trying to understand how to design a class B power amp, and I am a little confused with the method to define the peak of power dissipated by the output transistors.
All the mathematical methods I have seen are based on sinewaves.
But it look to me that with an assymetrical heavilly clipped signal, like you could have from a fuzz, things could be really worse. With a big phase shift, you could have all the voltage (Vcc - Vee) accross a transistor in the same it would pass full curent. So Pd(peak) could be I(peak) * (Vcc - Vee). Is this really pessimistic?


Here, https://sound-au.com/soa.htm, the result look the same, but the method look strange to me :

"Having discounted the idea of any 'rules-of-thumb', I'm going to give you one anyway . Let's assume that you want to deliver 100W into 8 ohms, so you need a power supply with ±42V rails (I'm going to ignore losses here). The amp must also be able to drive nominal 4 ohm loads, so expect the minimum impedance to be 3 ohms. Worst case (resistive load) dissipation is therefore ...
I = V / 2 / R = 21 / 3 = 7 Amps
P = V / 2 * I = 21 * 8 = 168 Watts (peak)
This accounts for the resistive part of the load, and as we saw above, the reactive part of the load causes dissipation to double. Just like second breakdown, we aren't interested in the average dissipation - this influences the size of heatsink needed, but not the transistor's safe area. Therefore, Ppeak will be ...
Ppeak = P * 2 = 168 * 2 = 336 Watts"

How could the transistor pass full current with the output being at Vcc/2 with a resistive load?


What would you assumed to be the whorse impedance and phase shift from a 8ohms guitar speaker please?
I am right assuming at the lower impedance, the load would look only resistive, so there won't be phase shift, so the the worse cases couldn't come hand in hand? I have not found anything about this yet.


Thanks a lot for your help!
Please apologise my poor English.

Loudthud

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Re: Peak dissipation in class B output stage
« Reply #1 on: September 07, 2020, 09:27:25 AM »
Check this thread over at MEF.

Link: https://music-electronics-forum.com/forum/fun-with-computers/36422-testing-zip-files?t=35493

If you are not a member there, I think you will have to join to see the attachments. What is shown is an X--Y movie clip of current and Voltage in a speaker when connected to a solid state amp banging rail to rail.

When the trace is above mid screen, current is positive flowing through the transistor connected to the + rail. See how some of the time it stays above the mid screen line all the way from the right side of the screen to the left side ? The inductance of the speaker keeps the + side transistor on even when the Voltage goes negative.
« Last Edit: September 08, 2020, 11:57:33 AM by Loudthud »

shasam

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Re: Peak dissipation in class B output stage
« Reply #2 on: September 07, 2020, 11:09:56 AM »
Hi Loudthud!

Thanks for your answer!

I have join MEF, but I am having hard time to fine the thread you are talking about. Have you an idea of what keyword should I search, please?

Loudthud

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Re: Peak dissipation in class B output stage
« Reply #3 on: September 08, 2020, 12:01:25 PM »
Sorry, I thought I had included the link. I edited the post above but here it is again

Link: https://music-electronics-forum.com/forum/fun-with-computers/36422-testing-zip-files?t=35493

shasam

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Re: Peak dissipation in class B output stage
« Reply #4 on: September 09, 2020, 05:25:37 AM »
Thanks Loudthud!

It is nice to see real measurement of what append with a scope!  :) Is there something else than phase shift between voltage and current due to speaker reactance that I should see here?

Loudthud

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Re: Peak dissipation in class B output stage
« Reply #5 on: September 09, 2020, 10:08:46 PM »
To really study this subject, you would need a lab full of equipment. You would need to simultaneously monitor Voltage and current in a transistor, then multiply the two to get power. In a practical sense you would need to look at the instantaneous power over a time scale and compare that to the SOA limits from a transistor data sheet. I say practical because you don't want a 100 W power amp with 20 output transistors.

Attached is the SOA graph from an On Semi power transistor. If you look at the 1 Second line, it stays at 250W out to about 50V. I have extended the 250W line to show the Second Breakdown region in red. The 1 Second line is the one bordering the bottom of the red zone. As the Voltage increases beyond 50V, the allowable power is reduced. The 50V limit for full power is common to many transistors. Some have a lower limit, a few higher up to 80V. Let me know if you find any bypolar transistors that are higher than 80V.

To keep your output transistors out of the red zone, you need to limit their Voltage and that means stacking them in series. Otherwise you need about twice as many transistors to be safe.

MOSFETs don't have Second Breakdown so that is a big incentive to use them.

Edit: Changed pic slightly to make it more clear.
« Last Edit: September 09, 2020, 11:11:14 PM by Loudthud »

shasam

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Re: Peak dissipation in class B output stage
« Reply #6 on: September 10, 2020, 05:30:09 AM »
Thanks again Loudthud!

"To really study this subject, you would need a lab full of equipment. You would need to simultaneously monitor Voltage and current in a transistor, then multiply the two to get power. In a practical sense you would need to look at the instantaneous power over a time scale and compare that to the SOA limits from a transistor data sheet. I say practical because you don't want a 100 W power amp with 20 output transistors."

I was trying to have a theorical approch to avoid this, but as you say, this lead to lot of output transistors.
Do you think Spice simulations would differ a lot from reality?
What I find annoying, is what is a practical signal for testing a guitar amp? An hifi amp won't have to produce continuous square waves. But for a guitar amp, for exemple, a larsen with a super fuzz type pedal is a possible situation, and look really more painfull for the amp.
Should I think of it as "How could I suppidly distort the signal before the VI limiter began to conduct?"
Furthermore, what would you use for practical load? Results would vary a lot in fonction of the load used for measurments, with different impedance dips and phase shifts between real speakers.


"Some have a lower limit, a few higher up to 80V. Let me know if you find any bypolar transistors that are higher than 80V."

On SemiNJW21194G / NJW21193G seem to have 100V / 110V limit. (SOA graph in attachment)
https://docs.rs-online.com/420b/0900766b8126cc12.pdf

edvard

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Re: Peak dissipation in class B output stage
« Reply #7 on: September 13, 2020, 01:24:45 AM »
...
Do you think Spice simulations would differ a lot from reality?
What I find annoying, is what is a practical signal for testing a guitar amp? An hifi amp won't have to produce continuous square waves. But for a guitar amp, for exemple, a larsen with a super fuzz type pedal is a possible situation, and look really more painfull for the amp.
...

The biggest drawback to using simulators is that they will consistently give you an ideal result; that is, any given part will have real-world variations due to many factors, but the simulation will run at exact specs every time.  That can lead to disappointing results with real parts, but as long as you're aware of that, it's not so bad. 

In LTSpice (and probably others), you can use a sound file as input to your simulation, so I sometimes record a 1 or 2 second clip of a strummed open E, or plucking of a high note as "real-world" input (the clips are short because the simulation with a sound file takes quite a bit more time to process, as you can imagine).  Your guitar and pre-amp will NEVER produce a perfect sine wave, so though a sine wave can be used as a useful baseline to see if the circuit is doing what you think it is supposed to do, it won't show you how the circuit will react to an actual guitar signal.

If you are doing your testing on a "real-world" amp, I would suggest investing in a looper pedal.  That way you can strum a few chords into it, start it looping, then jack it directly into the amp and it will repeat until you stop it while you take measurements or gawk at an oscilloscope trace.  You could even run it through a pedal of your choice.

Quote
Furthermore, what would you use for practical load? Results would vary a lot in fonction of the load used for measurments, with different impedance dips and phase shifts between real speakers.

There are circuits that simulate a speaker load with varying impedances, etc. that are very close to how a real speaker will affect the operation of the circuit it's connected to.  Try this one:
https://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator
Whether you build a real one, or run it through a simulation, I think something like this will work fine.  Tonal differences between speakers are often down to the material and construction of the speaker cone itself; the engineering of the power amp is only concerned with the magnet/coil part.

shasam

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Re: Peak dissipation in class B output stage
« Reply #8 on: September 16, 2020, 08:41:51 AM »
Hi Edvard! Thanks!

What annoy me is, with real world measurments, some real speakers could be worse than the ones I have under my hand now, and input signal could came in wicker shapes that the ones I generate for testing. We can simulate the worse theorical case but this could be really worse than real practical situation so being useless. So I don't know what to really use...


"Tonal differences between speakers are often down to the material and construction of the speaker cone itself; the engineering of the power amp is only concerned with the magnet/coil part."

Could you please elaborate a little more on this? A bass reflex cab would generate other phase shift, why shouldnt we care about this?


I have seen on some websites that a phase shift of 45° it the worse case for output stage (ignoring 90°), as this exemple:
"As discussed in the Audioholics article, the key phase angle (denoted as Greek phi Φ) to keep in mind is +/-45°. This is the point where the amplifier must dissipate double the power compared to just a purely resistive load while the speaker receives only half the power. On either side of that key phase angle, the amplifier demand is reduced. Knowing this allows the amplifier designers to ensure that they can deliver adequate power and maintain the temperature of the devices at safe levels. I've seen my own speakers vary between +/-70° at the extremes with +/-60° being more common."

Douglas Self stated : "Average device dissipation also increases as the load angle increases. A 45 load increases average dissipation by 1.4 times, and a 60 load by 1.8 times.
Peak device dissipation increases more rapidly than average dissipation as the load angle increases.
A 45 load increases power peaks by 2.7 times, and a 60 load by 3.4 times."

From my simulation too (screenshot in attachement), 60° is worse than 45°. Is the first quote false, or am I missing the point?
« Last Edit: September 16, 2020, 12:29:09 PM by shasam »

phatt

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Re: Peak dissipation in class B output stage
« Reply #9 on: September 18, 2020, 10:19:23 AM »
Likely over my head but I'll ask.
                              What is it you are trying to design?

Re the last part of the link to SOA by Rod Elliot, (Section5  Protection Schemes)
To my mind that is how you cover the extremes.
I doubt any circuit is totally bullet proof but if it's just a 100Watt guitar amp then considering how many millions of 100watt amps get flogged and survive for years ,,, maybe you are over thinking the design. :-\
Or have I completely missed the point? xP
Phil.

shasam

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Re: Peak dissipation in class B output stage
« Reply #10 on: September 21, 2020, 06:13:15 AM »
Hi Phatt!

Likely over my head but I'll ask.
                              What is it you are trying to design?

I am trying to design a discret class B power amp, with CFP output. Something in the 15 -20 Watts range for beginning.


Re the last part of the link to SOA by Rod Elliot, (Section5  Protection Schemes)
To my mind that is how you cover the extremes.

So, I should place the number of output devices to endure most real life situations, and put I-V limiter for protection against anything worse?

I doubt any circuit is totally bullet proof but if it's just a 100Watt guitar amp then considering how many millions of 100watt amps get flogged and survive for years ,,, maybe you are over thinking the design. :-\
Or have I completely missed the point? xP

I am probably over thinking it  ;)

phatt

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Re: Peak dissipation in class B output stage
« Reply #11 on: September 22, 2020, 08:14:14 AM »
Well an LM1875 can easy produce 15~20Watts.
Just google it, there are tons of circuits out there using that chip, a few to be found here on SSguitar

The LM1875 delivers 20watts into a 4Ω or 8Ω load
from ±25Vsupplies.
Used within specs they are perfectly happy.

As long as using ONLY 8Ω load you can run them from ±30V for even more watts. ~30W would be the limit.

For such a small amp there would be little benefit from designing discrete stuff. Unless of course you just want the experience or learn how to blow stuff up. ;)
I sent many perfectly good transistors to magic smoke heaven learning how to build amps.  :-X
Phil.

Jazz P Bass

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Re: Peak dissipation in class B output stage
« Reply #12 on: September 22, 2020, 11:43:35 AM »
I would build a tried & true amplifier.
Attached is a simple low power amplifier.
Tube Works TD 742.
Build it.
Study it.
Blow it up.  :lmao:

shasam

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Re: Peak dissipation in class B output stage
« Reply #13 on: Today at 12:43:56 AM »
Thanks!
I have already built little amps, chipamp and class B amp drived by an ampli op (but simplier than this TD 742).
My real goal is to learn how to design a discret one, not just having a new amp.
I know I could buy PCB with TDAxxxx, or used ss guitar amp  for cheap. I could etch Randy Slone PCB too, and follow his schematic.
I just want to learn how to design it myself.  ;)