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DC to AC converter. - can this be done?

M

mook Johnson

Jan 1, 1970
0
I have a buddy that is struggling with a design. Heres what he's trying to
do.
He recieves a 45 - 200Hz 500mV p-p sine wave through a signal transformer
and would like to buffer it and drive and high current version of the signal
to an output. Output load cna be anywhere from open circuit to .25 ohms.

So far he had a 6:1 transformer primary being driven by NPN-PNP totempole
and an OPamp to elimiante the xover distortion (aka simple audio amp). It
is desired that the secondary be allowed to have about 2Vp-p of 60-120Hz
common mode voltage on it from external sources so it can't be tied to
ground but it doesn not need to truelly be isolated.

What he is attempting to do is to AC couple off both sides of the output
coil and bring that differential signal back into to the primary side to a
second opamp. The feedback signal will be compared by this opamp to the
reference signal and any load dependant droop will be corrected out. with
all the elememts in the loop (opamp 1, opamp 2+ totem, 6:1 transformer,
etc, stability is a bit shaky. He even ran into the dreaded motorboating
problem.

Basically its a DC to AC linear inverter problem but with an isolated output
and stiff voltage requirements on the output voltage.

I have some ideas about using a isolated power supply to generate the rails
for a power opamp and us it to buffer the signal transformer. This would
generate quite a bit it heat even with the +/-5V rails most power opamps
require. The low load resistance is whats challenging.

How would you analog types go about attacking this problem?
 
T

Tim Wescott

Jan 1, 1970
0
mook said:
I have a buddy that is struggling with a design. Heres what he's trying to
do.
He recieves a 45 - 200Hz 500mV p-p sine wave through a signal transformer
and would like to buffer it and drive and high current version of the signal
to an output. Output load cna be anywhere from open circuit to .25 ohms.

So far he had a 6:1 transformer primary being driven by NPN-PNP totempole
and an OPamp to elimiante the xover distortion (aka simple audio amp). It
is desired that the secondary be allowed to have about 2Vp-p of 60-120Hz
common mode voltage on it from external sources so it can't be tied to
ground but it doesn not need to truelly be isolated.

What he is attempting to do is to AC couple off both sides of the output
coil and bring that differential signal back into to the primary side to a
second opamp. The feedback signal will be compared by this opamp to the
reference signal and any load dependant droop will be corrected out. with
all the elememts in the loop (opamp 1, opamp 2+ totem, 6:1 transformer,
etc, stability is a bit shaky. He even ran into the dreaded motorboating
problem.

Basically its a DC to AC linear inverter problem but with an isolated output
and stiff voltage requirements on the output voltage.

I have some ideas about using a isolated power supply to generate the rails
for a power opamp and us it to buffer the signal transformer. This would
generate quite a bit it heat even with the +/-5V rails most power opamps
require. The low load resistance is whats challenging.

How would you analog types go about attacking this problem?
The 45Hz makes it challenging, too, if you're trying to put it through a
transformer that doesn't break your back.

One way to approach this would be to use transformers that pass well
beyond the desired frequencies, then condition the frequency response of
the circuit with the feedback components. This would help with the
instability, but it would make transformer selection difficult.

Ultimately what you want is a circuit which has a high common-mode
impedance and a low differential mode impedance. Your transformer idea
is just one way to make this happen. Another way would be to build a
pair of current drivers, with feed back coming from a differential
amplifier. This would ease your stability difficulties (and shrink your
circuit), yet it would still have enough pitfalls to make it really fun.

You could mix & match, also, by using a diff-amp feedback but a
transformer in the forward path. This would ease the need for such an
absurd transformer that I suggested in the all-transformer solution.

No matter what you do you'll have to deal with stability issues. This
all sounds very doable, but not trivial.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
M

mook Johnson

Jan 1, 1970
0
Tim Wescott said:
---snip---

No matter what you do you'll have to deal with stability issues. This all
sounds very doable, but not trivial.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Thanks. Looks like we were thinking along the same lines. Transformer has
a shield for common mode HF noise grounding. I don't thing 120Hz will jump
from coil to coil unless the coils are wound very badly.

the Diff and for feedback looks promising.

Any good reference books for stabilizing beasties like these? There is one
example in AoE but it is grounded input and ouput which makes it somewhat
easier.
 
T

Tim Wescott

Jan 1, 1970
0
mook said:
Thanks. Looks like we were thinking along the same lines. Transformer has
a shield for common mode HF noise grounding. I don't thing 120Hz will jump
from coil to coil unless the coils are wound very badly.

the Diff and for feedback looks promising.

Any good reference books for stabilizing beasties like these? There is one
example in AoE but it is grounded input and ouput which makes it somewhat
easier.
Beyond a 2nd-year networks book and a 3rd-year op-amp book -- not that I
know of. I always approach these problems with a healthy dose of first
principals leavened if necessary by the judicious use of SPICE.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
B

Ban

Jan 1, 1970
0
mook said:
I have a buddy that is struggling with a design. Heres what he's
trying to do.
He recieves a 45 - 200Hz 500mV p-p sine wave through a signal
transformer and would like to buffer it and drive and high current
version of the signal to an output. Output load cna be anywhere from
open circuit to .25 ohms.
So far he had a 6:1 transformer primary being driven by NPN-PNP
totempole and an OPamp to elimiante the xover distortion (aka simple
audio amp). It is desired that the secondary be allowed to have
about 2Vp-p of 60-120Hz common mode voltage on it from external
sources so it can't be tied to ground but it doesn not need to
truelly be isolated.
What he is attempting to do is to AC couple off both sides of the
output coil and bring that differential signal back into to the
primary side to a second opamp. The feedback signal will be compared
by this opamp to the reference signal and any load dependant droop
will be corrected out. with all the elememts in the loop (opamp 1, opamp
2+ totem, 6:1 transformer, etc, stability is a bit shaky. He
even ran into the dreaded motorboating problem.

Basically its a DC to AC linear inverter problem but with an isolated
output and stiff voltage requirements on the output voltage.

I have some ideas about using a isolated power supply to generate the
rails for a power opamp and us it to buffer the signal transformer. This
would generate quite a bit it heat even with the +/-5V rails
most power opamps require. The low load resistance is whats
challenging.
How would you analog types go about attacking this problem?

Whenever the transformer starts coming near saturation, this method will
drive it to the rail, where it hangs until the next zero-xing. Not a good
idea. Works only for very small levels. It also requires your amp to be
compensated much more, because of the additional phase shift at roll-off of
the transformer.
But you could take the feedback from the primary and implement a negative
output impedance which equals the combined ohmic losses of primary and
secondary windings. This will be stable unless shorted.
To make the negative impedance, you can put a small resistance in series
with the primary and add an appropriate level to the voltage feedback
signal.

There are more possible cures, avoiding the transformer at all and also the
floating supplies...
 
J

Joerg

Jan 1, 1970
0
mook said:
I have a buddy that is struggling with a design. Heres what he's trying to
do.
He recieves a 45 - 200Hz 500mV p-p sine wave through a signal transformer
and would like to buffer it and drive and high current version of the signal
to an output. Output load cna be anywhere from open circuit to .25 ohms.

So far he had a 6:1 transformer primary being driven by NPN-PNP totempole
and an OPamp to elimiante the xover distortion (aka simple audio amp). It
is desired that the secondary be allowed to have about 2Vp-p of 60-120Hz
common mode voltage on it from external sources so it can't be tied to
ground but it doesn not need to truelly be isolated.

What he is attempting to do is to AC couple off both sides of the output
coil and bring that differential signal back into to the primary side to a
second opamp. The feedback signal will be compared by this opamp to the
reference signal and any load dependant droop will be corrected out. with
all the elememts in the loop (opamp 1, opamp 2+ totem, 6:1 transformer,
etc, stability is a bit shaky. He even ran into the dreaded motorboating
problem.

Basically its a DC to AC linear inverter problem but with an isolated output
and stiff voltage requirements on the output voltage.

I have some ideas about using a isolated power supply to generate the rails
for a power opamp and us it to buffer the signal transformer. This would
generate quite a bit it heat even with the +/-5V rails most power opamps
require. The low load resistance is whats challenging.

How would you analog types go about attacking this problem?

Just bought running shoes from Alpine Design and this model was called
"Analog". Seriously. So I guess I qualify ;-)

If the size of the transformer is a concern one could take a different
tack: Build a switch mode power supply, basically taken from a
TI/Unitrode app note or similar. Make sure its regulation bandwidth is
well in excess of 200Hz, usually not a big deal.

Now transfer the power at over 100kHz, rectify at the output, easy on
the output cap value, load that pretty good so the cap will bleed down
fast enough for a 200Hz upper limit and provide the usual
TL431+optocoupler scheme for the feedback.

You would modulate in via the FB input of you regulator. It's not really
trivial to build a switcher that goes from truly zero to full scale but
it's been done before. SEPIC is what I usually do but that's not
isolated. Flyback or push-pull may be your ticket because IIUC the
output ground side needs to be floating to inject that other 2Vpp signal.

There is a series of really tiny lab bench supplies from Lascar
Electronics. Don't know if they can be hacked to modulate the output but
AFAIK they are fully isolated and quite cheap:
http://www.lascarelectronics.com/
 
T

Tim Wescott

Jan 1, 1970
0
Joerg said:
Just bought running shoes from Alpine Design and this model was called
"Analog". Seriously. So I guess I qualify ;-)

If the size of the transformer is a concern one could take a different
tack: Build a switch mode power supply, basically taken from a
TI/Unitrode app note or similar. Make sure its regulation bandwidth is
well in excess of 200Hz, usually not a big deal.

Now transfer the power at over 100kHz, rectify at the output, easy on
the output cap value, load that pretty good so the cap will bleed down
fast enough for a 200Hz upper limit and provide the usual
TL431+optocoupler scheme for the feedback.

You would modulate in via the FB input of you regulator. It's not really
trivial to build a switcher that goes from truly zero to full scale but
it's been done before. SEPIC is what I usually do but that's not
isolated. Flyback or push-pull may be your ticket because IIUC the
output ground side needs to be floating to inject that other 2Vpp signal.

There is a series of really tiny lab bench supplies from Lascar
Electronics. Don't know if they can be hacked to modulate the output but
AFAIK they are fully isolated and quite cheap:
http://www.lascarelectronics.com/
It sounds like the OP needs to source AC to his load; this rules out a
'regular' switcher. One could consider a switcher with synchronous
rectification, although things would get complex that way.

I wonder if one could use a class D amplifier chip intended for
subwoofers? You'd have to apply external feedback, but that should be
straight forward. You may even have to power it from an isolated
supply, but you'd still have something that was built of individually
straightforward blocks.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
T

Tony Williams

Jan 1, 1970
0
I have a buddy that is struggling with a design. Heres what he's
trying to do. He recieves a 45 - 200Hz 500mV p-p sine wave
through a signal transformer and would like to buffer it and
drive and high current version of the signal to an output.
Output load cna be anywhere from open circuit to .25 ohms.
So far he had a 6:1 transformer primary being driven by NPN-PNP
totempole and an OPamp to elimiante the xover distortion (aka
simple audio amp). It is desired that the secondary be allowed
to have about 2Vp-p of 60-120Hz common mode voltage on it from
external sources so it can't be tied to ground but it doesn not
need to truelly be isolated.

Does the value of the output load move quickly or slowly?
If slowly perhaps your colleague could keep the overall
setup above but rectify/smooth both input and output,
compare DC's, and use the average DC error to adjust AC
error. Time constants will be in the order of seconds.
 
M

Mook Johnson

Jan 1, 1970
0
output can move fairly rapidly. Not a step changes but it will not be
smooth lazily varying load.

The switching power supply and the ClassD are intereseting but switching
noise will be an issue as we are making high resolution measurements in the
vacinity.

I appreciate the input. Really gives me some ideas for tackling this
problems on similar projects.
 
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