Forums

Devil's Staircase

Started by John Larkin May 31, 2019
On 31 May 2019 18:06:55 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>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.
I could have my transformer guy make the primary DCR a bit higher than normal. That would soften up the source a little, but I am simulating an alternator, which is highish impedance already. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Fri, 31 May 2019 17:56:27 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>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, :^)
TCA0372 is an amazing amp at a shocking price. They seem to be pretty tough. Silk and solder mask are often misaligned. Which is why I don't do "mask defined" pads. -- John Larkin Highland Technology, Inc lunatic fringe electronics
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? 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.
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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Fri, 31 May 2019 16:41:05 -0700 (PDT), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>You have 15mV offset on the output > >See page 8 of the datasheet: > >http://www.ti.com/lit/ds/slasea8a/slasea8a.pdf
60 mV max.
> >For a 4 ohm coil that is only 4mA
My transformer primary will be maybe 20 milliohms DCR. At the max 60 mV, that's 3 amps DC.
> >You have no problem, get rid of the caps > >Cheers > >Klaus
Why does TI use such goofy file names? -- John Larkin Highland Technology, Inc lunatic fringe electronics
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? 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. > > 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>
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?&nbsp; The problem is >>> that they don't really tell you what the input of the 3255 looks like, >>> IIRC (none do).&nbsp; 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.
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. Not so easy, low pass, some opamps, but not impossible. 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. The normal class B audio amps with output coupling caps work fine for that.
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. 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?
John Larkin wrote:
> > > > 60 mV max. > > > > >For a 4 ohm coil that is only 4mA > > My transformer primary will be maybe 20 milliohms DCR. At the max 60 > mV, that's 3 amps DC. >
** Hmmm - so a 4 ohm impedance in the working range with 20mohms of resistance. That's a ratio of 200:1. The transformer will have a regulation factor of 1%, or better. 150VA trannies normally have regulation factors of 8 to 10% and there is little difference between E-core and toroidal types other than size and weight. .... Phil ..... Phil
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 ? 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.