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Devil's Staircase

Started by John Larkin May 31, 2019
On Fri, 31 May 2019 16:00:33 -0700, bloggs.fredbloggs.fred wrote:

> There's nothing special about audio: > > https://www.pes-publications.ee.ethz.ch/uploads/
tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf
>
Ah, so that's how flux capacitors arose. I've always wondered. ;-) -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.
On Fri, 31 May 2019 17:33:47 -0700, John Larkin wrote:

> Isn't most audio power class D now?
I should jolly well hope not!
On Friday, May 31, 2019 at 7:46:40 PM UTC-4, John Larkin wrote:
> On Fri, 31 May 2019 16:00:33 -0700 (PDT), > bloggs.fredbloggs.fred@gmail.com wrote: > > >On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin wrote: > >> I want to make a class-D audio amp, 150 watts or so, using a TI > >> TPA3255 maybe. It's good for 600 watts mono! > >> > >> I'll use it full-bridge to drive a step-up transformer, probably a > >> custom toroid. But toroids are especially unhappy with any DC drive, > >> and the class D part will surely have some DC offset. The TI spec is > >> 60 mV max output offset, which could be a problem into a good > >> transformer. Speakers don't mind a little DC, but transformers do. DC > >> can cause stairstepped increase in circulating current, the Devil's > >> Staircase, until they saturate. > >> > >> So I'm thinking I'll add a series blocking cap so I can ignore any DC > >> problems. It will have to be big, 10s of millifarads at least. Biggest > >> thing on the board. Maybe use a low voltage electrolytic with > >> antiparallel power diodes, or a shorted bridge, to protect it from > >> accidental forward or backwards over-voltage. > >> > >> Lytics will be big, and supercaps don't seem to like ripple current. I > >> think. > >> > >> Any other ideas about driving a transformer from an audio amp? > > > >There's nothing special about audio: > > > >https://www.pes-publications.ee.ethz.ch/uploads/tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf > > > > A capacitor looks easier.
No need. Lots of work has already been done to handle this hazard. https://www.academia.edu/23741127/A_simple_control_algorithm_to_avoid_flux_density_bias_in_isolated_full-bridge_topologies "The flux density bias of the transformer in such topologies is solved by using a fully digital strategy without any auxiliary circuit, thus, core losses are decreased and the total conversion efficiency is improved. Circuit complexity and cost are reduced, enhancing at the same time the reliability of the system. The possibility of magnetic saturation due to steady-state or transients is strongly reduced, being an important performance." There are plenty of other papers on exactly this topic. And there's a small niche market of add-on protectors.
> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
John Larkin <jjlarkin@highland_snip_technology.com> wrote: 

> Isn't most audio power class D now?
My powered computer speakers JBL LSR305 Studio Monitors use class D amplifiers.
On 2019-05-31 22:08, Jan Panteltje wrote:
> On a sunny day (Fri, 31 May 2019 15:36:39 -0700) it happened John Larkin > <jjlarkin@highland_snip_technology.com> wrote in > <q4a3fe9nlmflqfmadlb1dbfrtg7slvr7f0@4ax.com>: > >> >> >> I want to make a class-D audio amp, 150 watts or so, using a TI >> TPA3255 maybe. It's good for 600 watts mono! >> >> I'll use it full-bridge to drive a step-up transformer, probably a >> custom toroid. But toroids are especially unhappy with any DC drive, >> and the class D part will surely have some DC offset. > > As somebody else already pointed out, maybe a DC control loop could zero the current.
That's what I'd do. No output cap, much smaller, much cheaper.
> Not so easy, low pass, some opamps, but not impossible. >
It is quite easy if you use that same loop for everything, DC as well as drive signal. That's how I do it with class D stuff. Mine have also always driven weird loads, not speakers. Use another opamp up front that has very low offset. It would be good to have that blessed by TI, to make sure the analog loop filter in the IC doesn't become upset.
> The idea of supercaps is nice, maybe I should order some to see how those > perform in a similar application (driving cryo cooler here). > The ebay class D unit did not like the step up transformer... and died.
Be careful not to get into a resonant mode at some frequency. That can hammer them, they see an output AC short, become instable like an opamp with a capacitive load and potentially croak.
> The normal class B audio amps with output coupling caps work fine for that. >
Not always. You were probably lucky. I had the overcurrent trip come on a regular big old stereo amp when I drove a load capacitively for aerospace testing. -- Regards, Joerg http://www.analogconsultants.com/
On Sat, 1 Jun 2019 00:42:55 -0400, bitrex <user@example.net> wrote:

>On 6/1/19 12:23 AM, bitrex wrote: >> On 5/31/19 10:55 PM, John Larkin wrote: >>> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >>> >>>> On Fri, 31 May 2019 15:36:39 -0700, John Larkin >>>> <jjlarkin@highland_snip_technology.com> wrote: >>>> >>>>> >>>>> >>>>> I want to make a class-D audio amp, 150 watts or so, using a TI >>>>> TPA3255 maybe. It's good for 600 watts mono! >>>>> >>>>> I'll use it full-bridge to drive a step-up transformer, probably a >>>>> custom toroid. But toroids are especially unhappy with any DC drive, >>>>> and the class D part will surely have some DC offset. The TI spec is >>>>> 60 mV max output offset, which could be a problem into a good >>>>> transformer. Speakers don't mind a little DC, but transformers do. DC >>>>> can cause stairstepped increase in circulating current, the Devil's >>>>> Staircase, until they saturate. >>>>> >>>>> So I'm thinking I'll add a series blocking cap so I can ignore any DC >>>>> problems. It will have to be big, 10s of millifarads at least. Biggest >>>>> thing on the board. Maybe use a low voltage electrolytic with >>>>> antiparallel power diodes, or a shorted bridge, to protect it from >>>>> accidental forward or backwards over-voltage. >>>>> >>>>> Lytics will be big, and supercaps don't seem to like ripple current. I >>>>> think. >>>>> >>>>> Any other ideas about driving a transformer from an audio amp? >>>> >>>> Feed back a (heavily) filtered signal to the input?&#2013266080; The problem is >>>> that they don't really tell you what the input of the 3255 looks like, >>>> IIRC (none do).&#2013266080; It's intended to be AC coupled. >>> >>> TI does spec 60 mV max DC offset at the output, which is pretty good, >>> but even that could push a lot of DC into my transformer. >>> >>> There is probably some way to tweak the input with a little DC. >>> Feedback loop or even a trimpot. >>> >>> >> >> You can drive the transformer bridged but using current-sense feedback >> instead of voltage feedback; put a small sense resistor in line with >> each amp output going to each end of the transformer and take off the DC >> feedback to the opposite amp from the junction. That way the amps should >> act as their own servo to keep DC out of the transformer. >> >> Lower power example like this for driving audio isolation transformer >> for XLR cable: >> >> <https://www.dropbox.com/s/8d1flmr8lko2nf1/Screenshot_2019-06-01_00-12-49.png?dl=0> >> >> > >In the case of the TPA3255 I think you would put a low-offset op amp in >front of the inputs and AC couple into that and do something similar but >the 3255 just acts as a power buffer, I don't immediately see anything >in the datasheet that says you can't intentionally apply a small DC >feedback generated offset to its single input per channel as an error >signal.
It's not stated how the normally AC coupled inputs affect the DC offset, but we have the eval board and I could have a scut bunny set it up and try it. One trimpot and 30 seconds of tech time, to turn it, is sure appealing. If it turns out we don't need it, we can leave it off the board. The input resistance of the TPA3255 is 10K, and they want 10 uF input coupling caps. Tau is 100 ms, which is 1.6 Hz corner. That suggests to me that the input caps are also used as lowpass filters for the DC feedback loop, which then suggests we could push the input pins gently to change the output offset. Datasheets tend to hide the good stuff. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Fri, 31 May 2019 22:49:02 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Friday, May 31, 2019 at 7:56:00 PM UTC-7, John Larkin wrote: >> On Fri, 31 May 2019 22:22:37 -0400, krw@notreal.com wrote: >> >> >On Fri, 31 May 2019 15:36:39 -0700, John Larkin >> ><jjlarkin@highland_snip_technology.com> wrote: >> > >> >> >> >> >> >>I want to make a class-D audio amp, 150 watts or >> TI does spec 60 mV max DC offset at the output, which is pretty good, >> but even that could push a lot of DC into my transformer. >> >> There is probably some way to tweak the input with a little DC. > >I'd just capacitor-couple the output, with a medium-size resistor parallel to the >capacitor. The digital amps I've seen want to self-bias, do NOT like >injected DC.
It would take a giant capacitor, easily the biggest thing on the board. This is a nominal 400 Hz system, but we may as well go down to 50 Hz to have more market. I can just poke in a 33,000 uF cap per amp, I guess.
> >If this is for a three-phase power drive, a small three-phase motor with no >load is a great filter/flywheel/phase balance component. So, how much ripple >current is the blocking capacitor going to pass? The voltage should be kept low by the >parallel resistor, of course, and that means a 6V capacitor is overrated for the job. >Without blocking, you'd run DC into your three-phase loads (not recommended). > >Do your AC loads burn up when given DC? Or, just magnetize and hum loudly?
We need to be frequency/voltage/phase agile, and we want to be able to program complex output impedance. The point of this box is that the customer wants to simulate alternators without spinning actual alternators, which they certainly could. Problem is, this customer likes us so they throw all the hard problems at us, and we have to hope they'll buy 300, and not 3. I guess I shouldn't complain about being furnished interesting problems. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sat, 01 Jun 2019 10:59:58 +0300, upsidedown@downunder.com wrote:

>On Fri, 31 May 2019 15:36:39 -0700, John Larkin ><jjlarkin@highland_snip_technology.com> wrote: > >>I want to make a class-D audio amp, 150 watts or so, using a TI >>TPA3255 maybe. It's good for 600 watts mono! >> >>I'll use it full-bridge to drive a step-up transformer, probably a >>custom toroid. But toroids are especially unhappy with any DC drive, >>and the class D part will surely have some DC offset. The TI spec is >>60 mV max output offset, which could be a problem into a good >>transformer. Speakers don't mind a little DC, but transformers do. DC >>can cause stairstepped increase in circulating current, the Devil's >>Staircase, until they saturate. > >Is it necessary to use a toroid, why not ordinary EI transformer with >possibly air gap ?
A toroid would be about half the footprint and half the weight of a regular transformer, and would have lower output impedance. I know a couple of guys who do nice toroids. One did this for us, lower power. https://www.dropbox.com/s/0bl6jdkw4ljx4e2/400_Hz_Toroid.JPG?dl=0
> >Or how about a high inductance but low resistance external solenoid >across toroid primary ? Apparently the frequency is in the 400 Hz >ballpark, so even less solenoid inductance should suffice, compared to >50/60 Hz.
I'm guessing that the TI amp would fight to maintain its output offset voltage. A series cap would be a lot easier. I could quit bitching and use some big caps. Probably don't need the diodes. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On a sunny day (Sat, 01 Jun 2019 07:23:04 -0700) it happened Joerg
<news@analogconsultants.com> wrote in <glfg1pFo3i8U1@mid.individual.net>:

>On 2019-05-31 22:08, Jan Panteltje wrote: >> On a sunny day (Fri, 31 May 2019 15:36:39 -0700) it happened John Larkin >> <jjlarkin@highland_snip_technology.com> wrote in >> <q4a3fe9nlmflqfmadlb1dbfrtg7slvr7f0@4ax.com>: >> >>> >>> >>> I want to make a class-D audio amp, 150 watts or so, using a TI >>> TPA3255 maybe. It's good for 600 watts mono! >>> >>> I'll use it full-bridge to drive a step-up transformer, probably a >>> custom toroid. But toroids are especially unhappy with any DC drive, >>> and the class D part will surely have some DC offset. >> >> As somebody else already pointed out, maybe a DC control loop could zero the current. > > >That's what I'd do. No output cap, much smaller, much cheaper. > > >> Not so easy, low pass, some opamps, but not impossible. >> > >It is quite easy if you use that same loop for everything, DC as well as >drive signal. That's how I do it with class D stuff. Mine have also >always driven weird loads, not speakers. Use another opamp up front that >has very low offset. > >It would be good to have that blessed by TI, to make sure the analog >loop filter in the IC doesn't become upset. > > >> The idea of supercaps is nice, maybe I should order some to see how those >> perform in a similar application (driving cryo cooler here). >> The ebay class D unit did not like the step up transformer... and died. > > >Be careful not to get into a resonant mode at some frequency. That can >hammer them, they see an output AC short, become instable like an opamp >with a capacitive load and potentially croak.
No worry, I am making 60 Hz (almost) http://panteltje.com/pub/cryocooler_vibration_damper_side_view_img_2583.jpg for the cry-cooler, that is the mechanical resonance of the plunger in it. Moves like this: https://www.google.nl/search?dcr=0&q=FreePistonsterling.jpg.&tbm=isch&source=hp&sa=X&ved=2ahUKEwjv5dDm0cjiAhXGPFAKHT9FDpIQsAR6BAgAEAE&biw=1670&bih=854#imgrc=JzxBQ21xfCh2JM:&spf=1559404338866 Could be on the mains with a variac in the US (soem did that) but in Europe we have 50 Hz. Using a 50 Hz power transformer in reverse on the audio amp output.
>> The normal class B audio amps with output coupling caps work fine for that. >> > >Not always. You were probably lucky. I had the overcurrent trip come on >a regular big old stereo amp when I drove a load capacitively for >aerospace testing. >
This one never failed me in all those years : http://panteltje.com/panteltje/amplifier/index.html from year 2000! I have some bigger stuff too... It is not so critical, I have the original switch mode board too, and it works, but is difficult to control.
On Sat, 1 Jun 2019 09:34:19 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Fri, 31 May 2019 16:00:33 -0700, bloggs.fredbloggs.fred wrote: > >> There's nothing special about audio: >> >> https://www.pes-publications.ee.ethz.ch/uploads/ >tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf >> > >Ah, so that's how flux capacitors arose. I've always wondered. ;-)
The problem is that the DeLoreans are getting hard to find. ebay has an '82 for $40K. -- John Larkin Highland Technology, Inc lunatic fringe electronics