Forums

Devil's Staircase

Started by John Larkin May 31, 2019
lørdag den 1. juni 2019 kl. 01.55.43 UTC+2 skrev John Larkin:
> On Fri, 31 May 2019 23:38:58 -0000 (UTC), Cursitor Doom > <curd@notformail.com> wrote: > > >On Fri, 31 May 2019 15:36:39 -0700, 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! > > > >Why class D? Doesn't seem like an obvious first choice for audio unless > >it's for a PA system or something of that sort. > > We plan to do three channels, 150 watts each, in a 2U rackmount > chassis. > > It will simulate a 3-phase PM alternator hung on the gearbox of a jet > engine. We'll be able to program frequency, voltage, and complex > output impedance. >
why do you need a transformer? and why not some COTS amplifier, you can a 4 channel several 100watt amplifier in 2U for a few hundred $
On Fri, 31 May 2019 17:15:14 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>l&#2013266168;rdag den 1. juni 2019 kl. 01.55.43 UTC+2 skrev John Larkin: >> On Fri, 31 May 2019 23:38:58 -0000 (UTC), Cursitor Doom >> <curd@notformail.com> wrote: >> >> >On Fri, 31 May 2019 15:36:39 -0700, 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! >> > >> >Why class D? Doesn't seem like an obvious first choice for audio unless >> >it's for a PA system or something of that sort. >> >> We plan to do three channels, 150 watts each, in a 2U rackmount >> chassis. >> >> It will simulate a 3-phase PM alternator hung on the gearbox of a jet >> engine. We'll be able to program frequency, voltage, and complex >> output impedance. >> > >why do you need a transformer?
For isolation, to make 3-phase and other configurations, and to get various output voltages. Some of the downstream voltage regulators are weird. People like to short out PM alternators for regulation. Our box will have to tolerate that.
> >and why not some COTS amplifier, you can a 4 channel several 100watt amplifier in 2U for a few hundred $
We'll probably do our own class-D modules. We could use one of the TI eval boards, but they are big and have a zillion jumpers and goofy connectors. We can get a 48V, 600 watt MeanWell power supply for $75! We were thinking about putting the TPA3255 on the bottom of the board and heat sinking it to the bottom of the box, which avoids a fan. It has a huge, grounded power pad on the top of the chip. Isn't most audio power class D now? -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Fri, 31 May 2019 17:01:21 -0700 (PDT), George Herold
<gherold@teachspin.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? >Huh, I have more questions than ideas. (How can you use a polarized >cap.?)
Most polarized caps don't mind a bit of back bias. Polymers don't mind a lot. A couple of power diodes would guarantee that a cap would never see more than +-1 volt. I should run a lot of AC current through some supercaps and see what happens.
> >but some sort of neg feedback that sense's DC current? >George H.
Sure, but a cap is a lot less thinking. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
bloggs.fredbloggs.fred@gmail.com 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
That looks rather special! -- Thanks, - Win
On Friday, May 31, 2019 at 8:38:34 PM UTC-4, John Larkin wrote:
> On Fri, 31 May 2019 17:01:21 -0700 (PDT), George Herold > <gherold@teachspin.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? > >Huh, I have more questions than ideas. (How can you use a polarized > >cap.?) > > Most polarized caps don't mind a bit of back bias. Polymers don't mind > a lot. A couple of power diodes would guarantee that a cap would never > see more than +-1 volt. > > I should run a lot of AC current through some supercaps and see what > happens. > > > > >but some sort of neg feedback that sense's DC current? > >George H. > > Sure, but a cap is a lot less thinking.
OK, just my only 'other' idea. I was going to report an almost first time pcb spin today. (Silk screen was a little off in places.*) I then I blew up the TCA3072... I forgot bypass caps, and had some hanging things... that shorted or something. Otherwise it all looked good. George H. * I also forgot the pads for the output connector. I did say almost, :^)
> > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
John Larkin wrote...
> > To maybe avoid saturating the transformer. A good toroid has > low DCR. I could easily circulate a few amps DC in the primary.
Doesn't sound so bad. Add a little series R. -- Thanks, - Win
l&oslash;rdag den 1. juni 2019 kl. 03.07.13 UTC+2 skrev Winfield Hill:
> John Larkin wrote... > > > > To maybe avoid saturating the transformer. A good toroid has > > low DCR. I could easily circulate a few amps DC in the primary. > > Doesn't sound so bad. Add a little series R. >
if it needs to support shunt regulation it would probably need some to measure current any way
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? The problem is that they don't really tell you what the input of the 3255 looks like, IIRC (none do). It's intended to be AC coupled.
On Fri, 31 May 2019 17:33:47 -0700, John Larkin
<jjlarkin@highland_snip_technology.com> wrote:

>On Fri, 31 May 2019 17:15:14 -0700 (PDT), Lasse Langwadt Christensen ><langwadt@fonz.dk> wrote: > >>l&#2013266168;rdag den 1. juni 2019 kl. 01.55.43 UTC+2 skrev John Larkin: >>> On Fri, 31 May 2019 23:38:58 -0000 (UTC), Cursitor Doom >>> <curd@notformail.com> wrote: >>> >>> >On Fri, 31 May 2019 15:36:39 -0700, 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! >>> > >>> >Why class D? Doesn't seem like an obvious first choice for audio unless >>> >it's for a PA system or something of that sort. >>> >>> We plan to do three channels, 150 watts each, in a 2U rackmount >>> chassis. >>> >>> It will simulate a 3-phase PM alternator hung on the gearbox of a jet >>> engine. We'll be able to program frequency, voltage, and complex >>> output impedance. >>> >> >>why do you need a transformer? > >For isolation, to make 3-phase and other configurations, and to get >various output voltages. > >Some of the downstream voltage regulators are weird. People like to >short out PM alternators for regulation. Our box will have to tolerate >that. > >> >>and why not some COTS amplifier, you can a 4 channel several 100watt amplifier in 2U for a few hundred $ > >We'll probably do our own class-D modules. We could use one of the TI >eval boards, but they are big and have a zillion jumpers and goofy >connectors. > >We can get a 48V, 600 watt MeanWell power supply for $75! > >We were thinking about putting the TPA3255 on the bottom of the board >and heat sinking it to the bottom of the box, which avoids a fan. It >has a huge, grounded power pad on the top of the chip. > >Isn't most audio power class D now?
Most, except in automotive (where the TPA3255 is intended to play). Automotive is still mostly class-AB, or some "high efficiency" variation of AB (i.e. 'SB', 'KB', or 'TB'). It's changing though.
On Fri, 31 May 2019 17:38:07 -0700, John Larkin
<jjlarkin@highland_snip_technology.com> wrote:

>On Fri, 31 May 2019 17:01:21 -0700 (PDT), George Herold ><gherold@teachspin.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? >>Huh, I have more questions than ideas. (How can you use a polarized >>cap.?) > >Most polarized caps don't mind a bit of back bias. Polymers don't mind >a lot. A couple of power diodes would guarantee that a cap would never >see more than +-1 volt. > >I should run a lot of AC current through some supercaps and see what >happens.
What's their ESR? High temperature is death to capacitors.
>>but some sort of neg feedback that sense's DC current? >>George H. > >Sure, but a cap is a lot less thinking.
But I thought electronics was fun?!