Forums

Devil's Staircase

Started by John Larkin May 31, 2019

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?


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

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
> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Why would you add that output decoupling capacitor when it is not needed?

None of the designs I have seen has a capacitor. Dig into it and find that you are chasing ghosts and just making a bad design by add a highly unlinear device

Notice that Class D is all about removing or compensating non linearities of the output stage.

Cheers

Klaus 
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. -- 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.
You have 15mV offset on the output 

See page 8 of the datasheet:

http://www.ti.com/lit/ds/slasea8a/slasea8a.pdf

For a 4 ohm coil that is only 4mA

You have no problem, get rid of the caps

Cheers 

Klaus 
On Fri, 31 May 2019 16:26:01 -0700 (PDT), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>Why would you add that output decoupling capacitor when it is not needed?
To maybe avoid saturating the transformer. A good toroid has low DCR. I could easily circulate a few amps DC in the primary.
> >None of the designs I have seen has a capacitor. Dig into it and find that you are chasing ghosts and just making a bad design by add a highly unlinear device > >Notice that Class D is all about removing or compensating non linearities of the output stage. > >Cheers > >Klaus
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
l&oslash;rdag den 1. juni 2019 kl. 01.39.03 UTC+2 skrev Cursitor Doom:
> 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.
he wants an amplifier not a space heater
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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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.?) but some sort of neg feedback that sense's DC current? George H.
> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com