On 03/06/2019 13:31, John Larkin wrote:
> On Mon, 3 Jun 2019 11:53:23 +1000, Chris Jones
> <lugnut808@spam.yahoo.com> wrote:
>
>> On 03/06/2019 08:22, John Larkin wrote:
>>> On Sun, 2 Jun 2019 18:07:37 -0400, bitrex <user@example.net> wrote:
>>>
>>>> On 6/2/19 6:04 PM, bitrex wrote:
>>>>> On 6/1/19 11:23 AM, John Larkin wrote:
>>>>>> 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? 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>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>> Does LTC make any class D amp modules included with LTSpice? I'm curious
>>>>> now about how using a DC servo loop into a first stage opamp driving the
>>>>> D-amp as a power buffer, instead of a cap would work out in practice.
>>>>>
>>>>> The problem with that XLR transformer driver circuit as drawn in a
>>>>> modification is that while there are two feedback loops, one local to
>>>>> the op-amps and one around the opposite side current sense resistor,
>>>>> it's assumed that the phase of the AC signals on both inputs of the op
>>>>> amps will be similar.
>>>>>
>>>>> but the phase shift produced by the class D output filter complicates
>>>>> things, where to put the sense resistor. If after the output filter need
>>>>> to compensate the phase shift somehow and if before need to filter the
>>>>> switching frequency down to DC. It may not be nearly as easily workable
>>>>> in the class D bridged power-buffer topology as with linear amplifiers
>>>>> driving the transformer, bridged.
>>>>
>>>> Putting it after the LC also means have to take the (possibly not well-
>>>> defined) R of the L into account as a component of the current sense
>>>> impedance.
>>>
>>>
>>> We plan to put the current sensor just before the customer output,
>>> which is after the filter inductors and the isolation transformer.
>>> Probably a resistive shunt, isolated by a small audio transformer.
>>
>> If you're going to use transformer isolation for the shunt, you might as
>> well use a current transformer instead.
>>That way you can have less
>> voltage per turn of the core for a given customer-visible output
>> current, so a smaller core and/or better accuracy.
I have now realised that this might not be true. There could be
arbitrarily many turns on the isolation transformer after a shunt,
though I still don't think it is a better solution.
>> Probably better
>> frequency response too. You'd also face less modifications if you ever
>> needed to switch to a hall effect sort of current sensor, if you wanted
>> to extend the frequency range downwards.
>>
>>
>>
>
> A shunt+signal transformer is a lot smaller than a CT, and generally
> has better frequency response. I could use a tiny surface-mount shunt
> and a small step-up transformer, standard parts. 50 or 100 mV drop in
> the shunt would be fine.
Although I am no longer convinced of the inferiority of your solution, I
would be surprised if there are not some small CTs available these days.
As I replied to myself above, I no longer think the CT necessarily
offers better linearity.
A possible advantage of the CT is that the manufacturer will expect it
to be used like this, whereas the audio transformer manufacturer will
not, so that the applications support and characterisation data
available might be better for the CT.