Welcome, Guest. Please login or register.

October 20, 2020, 02:12:40 AM

Login with username, password and session length

Recent Posts

 

Author Topic: Peak dissipation in class B output stage  (Read 1636 times)

shasam

  • Regular
  • **
  • Posts: 12
  • Chip Points: 0
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #15 on: September 26, 2020, 10:04:27 AM »
Thanks Phatt!

How would you limit the bandwidth please? With bigger Miller capacitor? Or in feedback network? I was thinking to limit it in the preamp section.

Some of the greatest guitar sounds ever heard were produced by what would now be considered a very bad design. :lmao:

I am aware of that, but without going to digital, i prefer take a modern approach, with tone and distorsion independant of volume control (OK, speakers and ears are still going to react differently with volume change ;) )


As for design help if you can find a copy of "Art of Electronics" it's well written for those of us who are less gifted at complex maths. :duh
Thanks! I have something similar, in french (my native langage), but this one look more complete.

As noted in the ESP page you quoted, anything running below +/-35VDC is not likely to give much trouble and that covers a basic amp of 40~50Watts so 15 ~20 Watts is fairly safe.
That's the reason why I begin with this, I already have collect bigger transformers for future projects :)


I'll try and dig up some old pictures of my testing circuits to give you ideas.
I would really appreciate it, thanks a lot!

phatt

  • Legendary
  • ******
  • Posts: 2139
  • Chip Points: 261
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #16 on: September 26, 2020, 10:16:13 PM »
Hi Shasam,
                If you want the indepth analysis there are far greater minds here that can explain the finer points.
I was just a frustrated muso guy who got fed up with the crap that was being sold that never delivered so I bent my brain and read a whole lot of books and started building my own gear.
Most of my knowledge is just basic *Rule of thumb* thoughts I have gleaned from books and later  wonderful people (via the WWW) like Rod Elliot and many other great people who have shared there knowledge.

My rules of thumb;
1/ KISS (keep it simple stupid)
Don't get caught up with teck heads who want to analyze the black holes in amp design. i.e Current FB is it worth it? Well some claim a big difference others say nay,, only you can make that call  xP
My observation,,,It likely helps if the Amp is working at high volumes.
At bedroom levels I doubt if any ears could hear a dramatic difference. 
You have to use your nose (Intuition) to decide if something tecky is worth the extra effort (& often expense).

Yes all done in preamp,, I don't even use a guitar amp, lol.
It's an old SS Laney keyboard amp,, all the mojo tricks are in my pedal board so the Amp is just the power driver.
I have 3 Valve Amps but I use the SS Laney when playing live, it's just so versatile as I can play my El Acoustic, Strat or keyboards all through one Amp. 8|

I've lost count of all the circuits I've built in the last ~35 years and I've built a lot of disasters.  :-[ over that time I've designed about 4 success circuits that work really well (Some are on this site)

The Laney rebuild I'm posting for you to show the BBoard testing setup of a power amp
It's very basic and likely not an ideal bench test setup but well worth the effort.
As you can see in the pics the laney only had a small Tx and when using the Keyboards the supply could not keep up. I rebuilt the entire PSU and power amp section. The 3 Ch preamp section is basic but functional. (Hey it does have a Tank Reverb which is nice)
You can see in the pic the difference from the original back panel and the rebuild.

I've added the Schematic for you as that may help to understand the pictures.
This uses over rated devices as I've found with my basic rule of thumb knowledge. :P
As long as the transistors are way above what is needed then less chance of failure.
(i.e the 2SC5200 is a 250V-15A-150W device) They are the 3 main things to look out for.

As explained in Art of Electronics the whole idea of these DC coupled amp designs is to keep the DC Q point of *Output & the bases of Q1,Q2 at close to Zero as possible*,as I've noted on my Schem.
The Voltage and Temp measurements were taken from test circuit not a simulation.

Current limit is done with a trick little device on the output,a PTC (Poly Thermal Capacitor or just PolyFuse)
Hifi geeks might not like them but saves all the extra PCB space.
Short circuit testing was done with amp at full power on the test circuit pictured about 20 times for at least 10 Seconds.
The Poly Fuse will trip at ~110Watts, The colour will darken when they get close to trip.
Absolutely brilliant little device. :dbtu: :dbtu: :dbtu: :dbtu:

Pleae note;; This was 5 Years back I may have slightly changed something since then,, but it's all very close to how it was built.

Re bandwidth,
I used 250pF at C6 to help limit HiFreq. (Use 250V cap)
C4 sets the low Freq roll off,, smaller values can help roll off excess bass.
100uF there is likely of little use for guitar freq.
Have fun,, Phil.




phatt

  • Legendary
  • ******
  • Posts: 2139
  • Chip Points: 261
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #17 on: September 27, 2020, 06:39:26 AM »
Oppsy forgot the Test board pic. :-[
Probably not the ideal test setup but when on a budget one has to be inventive.
Most of this is recycled El stuff I've collected over time.

As I've done a few times, I did again test to see if Current FB was worth the fuss but just did not hear any magic so gave it a mis. It's those two 5watt Resistors on the side, near the Grey output jack.
Not in picture but I used a light bulb limiter for every new tweak,, It saved me a couple of times during this experiment.  ;)
Phil.



shasam

  • Regular
  • **
  • Posts: 12
  • Chip Points: 0
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #18 on: September 28, 2020, 04:19:03 AM »
Thanks a lot Phatt!

I try to une rules of thumb too, but different ones look to give really different results sometimes (for heatsinks calculation for exemple, I find results from simple to double with different rules of thumb I have found).

I won't try current feedback for this one. Like you said I try to keep it simple (and I still find it not so simple...).
Maybe for a next project.

I am not familiar with polyfuse, I need to learn more about them, but this look really interesting!  :cheesy:

I have noticed you use different heatsinks for the driver transistors. Do you find the thermal tracking to be better like this?

shasam

  • Regular
  • **
  • Posts: 12
  • Chip Points: 0
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #19 on: September 28, 2020, 08:20:37 AM »
I am going to use a 2*15V transformer, and NJW0281G (NPN) / NJW0302G (PNP) output transistors, in CFP configuration, for 8Ω output.
Datasheet can be found here : https://docs.rs-online.com/41ca/0900766b813e9eb0.pdf


For now, my calculations are :

Total DC power supply  =  2 * (Veff *√2 - 2Vdiode) = 2 * (15 * √2 – 2) = 38,5V

Output power =  (power supply - loss)² / (8*Rload) = (38,5 – 5)² / (8*8 ) = 17W (I found similar result in LTSpice)

Output peak current = (power supply - loss) / (2*Rload) = (38,5 – 5) / (2*8 ) = 2,1A

Output peak power  = (power supply - loss) * Output peak current =  (38,5 – 5) * 2,1 = 70,35W


With Rod Eliott rules-of-thumb previously quoted :
Quote
"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"
(from https://sound-au.com/soa.htm)

That give me : I = (Vrail / 2) / R = (19.25 / 2) / 8 = 1.203125A
Ppeak_resistive = Vrail / 2 * I = (19.25 / 2) * 1.203125 = 11.58...W(peak)
Ppeak_reactive = Ppeak_resistive * 2 = 23.16...W(peak)


Max dissipation for NJW0281G / NJW0302G is 150W at 25°, 0W at 150°
Derating = 150W / (150° - 25°) = 1.2W/°C

Maximum_case_temperature = (Pmax_at_25° - Ppeak_reactive) / derating + 25°
= (150 - 23.17) / 1.2 +25 = 130°C


I have use the formula given here for average worse dissipation : https://www.updatemydynaco.com/documents/Class_B_Amplifier_Dissipation_Calculations.pdf
Pdiss_worst_case =  Vps² / (19.75 * RL) = 38,5² / (19,75 * 8 ) =  9,38...W
I don't really understand the calculation here. I have use it because it give me the worse result of all the calculation I have read online. What do you think about it?

Heatsink_max_thermal_resistance = (Maximum_case_temperature - Ambiant_temperature) / Pdiss_worst_case = (130 - 50) / 9.5 = 8.42... °C/W
I don't really know what to use for ambiant temperature, I have use 50°C here. I will use bigger heatsink to limit the max temperature lower than 130°C too.


I tried to make it as clear as possible. What do you think about it, please? Could you spot some mistakes? Should I take bigger margin?

Thanks a lot!
« Last Edit: September 28, 2020, 08:54:20 AM by shasam »

Jazz P Bass

  • Elite
  • ****
  • Posts: 223
  • Chip Points: 18
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #20 on: September 28, 2020, 05:02:18 PM »
The output transistors that you picked are way overkill for the amp that you are designing.
Yeah, they will work.
But still.
Way over the top.

phatt

  • Legendary
  • ******
  • Posts: 2139
  • Chip Points: 261
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #21 on: September 29, 2020, 09:38:05 AM »
Yep Mr Jazz beat me to it,, Agree way over the top. Maybe keep those for a bigger build.

I salute you for you ability to deciyfur the maths I'm just painting by number most of the time as I'm way too old to go back to school now.  xP

Others here will correct me if I'm wrong but all that maths assumes the Transformer is an infinite current source and the tX is the size of a car battery.  :lmao:
Meantime back in the real world your 30vct Tx supply will dramatically sag long before the circuit can pull that much current.
i.e. Put your meter across the car battery while someone starts it. That 12~14 Volts will instantly drop back to 7~9volts depending on the health of the battery. Current goes through the roof and voltage sags,, same goes for Amplifiers. 8|
If you are well versed in sims you can probably setup full load conditions for a more realistic outcome.
For me understanding power supplies was likely the hardest thing to get to grips with. :duh :loco

Maybe look at what has already been built? Some cheap amps only use the case as heat sink.
Look at my Laney pic where I've shown the original back panel for comparison. Note the heat sink is only alloy angle bolted to the back panel.
That amp ran from +/-37Volt rails and rated at 40~50Watts. (more like 30ish watts clean to my ears)
Even with only that meager heat sinking it never got burn finger hot.
That design was very basic and yet performed well for 5~6 years before I rebuilt the power amp.
(just to get more clean headroom for My Synth)

Re Q on predrivers (Q5&6) on separate heat sinks.
That was just a safety backup while testing, they were getting hot until I found why.
Q4 (the Voltage stage) actually runs hotter as it's basically got the full supply across it.
In the final build Q4 has a heat tab added, hidden in pic.
Q3 (BD139) is the bias that needs to track the heat sink temperature and it can be seen in the pic.
The power devices are over speced but as I had these well why spend money.
Hope it helps, Phil.
« Last Edit: September 29, 2020, 09:39:43 AM by phatt »

shasam

  • Regular
  • **
  • Posts: 12
  • Chip Points: 0
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #22 on: September 30, 2020, 07:40:30 AM »
Thanks!

You are right, those transistors are oversized.
I have redone the calculations for BD243 / BD244.
(datasheet here : https://www.onsemi.com/pub/Collateral/BD243B-D.PDF)

I got this :

Derating = 0,52W/°C

Max dissip 25°C = 65W

Maximum_case_temperature = (Pmax_at_25° - Ppeak_reactive) / derating + 25° = (65-23,17) / 0,52 +25 = 105,45°C

Pdiss_worst_case =  Vps² / (19.75 * RL) = 38,5² / (19,75 * 8 ) =  9,38...W

Heatsink_max_thermal_resistance = (Maximum_case_temperature - Ambiant_temperature) / Pdiss_worst_case = (105,5 - 50) / 9.5 =  5,85 °C/W

Others here will correct me if I'm wrong but all that maths assumes the Transformer is an infinite current source and the tX is the size of a car battery.  :lmao:
Meantime back in the real world your 30vct Tx supply will dramatically sag long before the circuit can pull that much current.

Maybe there is something that I dont understand, but this voltage should be under load.
With no load, I have 44.9V (mesured) from Vcc to Vee.
It is true that I have negligected ripple, that would reduce the usable voltage.

phatt

  • Legendary
  • ******
  • Posts: 2139
  • Chip Points: 261
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #23 on: September 30, 2020, 08:23:51 AM »
Well we are assuming you have 30VAC ctap Tx and assuming you are using a full wave bridge then your NO LOAD Voltage is going to be 42VDC but that will vary depending on the VA rating of the Tx, the mains voltage at the time of day, the Diode drop through the Rectifier, the size of the filtercaps.
So with Amp connected and power on that 44.9VDC you read will now be under idle which is only going to *Lightly LOAD* the supply Voltage,,,,Maybe a couple of volts drop.
(the hotter you bias the more idle current will flow and voltage will drop even more)
But running at half to full power the current draw will drain the filter caps and the voltage will sag. Then your 45Volts will read more like 37~35Volts,, (Again depends on VA of Tx used and as above)
Ripple is then bigger as the Filter caps try to keep up.

Bass is the thing that drags down the supply.
Tiss why I rebuilt my laney,, sustained notes on my keyboard would really drag down the voltage and bass notes just collapsed. :grr
Tiss why guitar amps only need 40watts while bass amps need 400W. 8|
Phil. 

shasam

  • Regular
  • **
  • Posts: 12
  • Chip Points: 0
    • View Profile
Re: Peak dissipation in class B output stage
« Reply #24 on: October 12, 2020, 06:28:09 AM »
Hi! I have been away from the workbench for the last two weeks, sorry for my late answer.

I have finally run some tests  :cheesy:

Just before clipping level, with a 8 ohm load (resistive),  the power supply sag to +/- 19.8V, with 35V ptp output (approximately, I have only measured it with the scope).