# Need the topology for a current to current SMPS, not voltage to voltage SMPS

Started by October 16, 2013
```John Larkin wrote:
> On Wed, 16 Oct 2013 09:35:10 -0700, RobertMacy
> <robert.a.macy@gmail.com> wrote:
>
>> Need a current to current SMPS, with greater than 95% efficiency. and NOT
>> a transformer! because has to be variable.
>>
>> The output will be a 'defined' current-time profile. Output Power is low,
>> less than 5-10W delivered. Voltage compliance is low, less than 10 Vdc.
>> I'm having difficulty wrapping my brain around potential topologies, need
>> some help here.
>>
>> Anybody ever do this, or have some URL's with topologies?
>>
>
>
> A synchronous buck running at fixed duty cycle is the electronic
> equivalent of a pair of gears, or an ideal autotransformer. Current
> out is current in times 1/n where n is duty cycle.
>

But you need to take into account losses and its own power consumption
because Robert likely has to feed it from the input current source as
well. In forced non-burst mode a sync buck can burn off an incredible
amount of the input energy, easily 5% or more just by idling. Like a car
with the A/C running.

--
Regards, Joerg

http://www.analogconsultants.com/
```
```"George Herold" <gherold@teachspin.com> wrote in message
> On Wednesday, October 16, 2013 2:04:35 PM UTC-4, Tim Williams wrote:
>> In the current domain, the conventional topologies (boost, buck, etc.)
>>
>> work just fine with capacitors.  Waveforms are an exercise for the
>> student
>>
>> :)
>>
> I scribbled this at lunch... which I think works as a current buck.
> https://www.dropbox.com/s/h2a8dyz0vzoovev/DSCF0031.JPG
>
> (Fun question)
> George H.

Yup :)

I think one of the strangest V-I transformations is the current mode
push-pull inverter.  The first thing that should be apparent is, it looks
like a half bridge (switches go from side-by-side to in series), but what
the CT transformer winding does is tricky to synthesize.

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

```
```On Wednesday, October 16, 2013 6:34:41 PM UTC-4, Tim Williams wrote:
> "George Herold" <gherold@teachspin.com> wrote in message
>
>
> > On Wednesday, October 16, 2013 2:04:35 PM UTC-4, Tim Williams wrote:
> >> In the current domain, the conventional topologies (boost, buck, etc.)
> >>
> >> work just fine with capacitors.  Waveforms are an exercise for the
> >> student
> >>
>
> >> :)
>
> >>
>
> > I scribbled this at lunch... which I think works as a current buck.
> > https://www.dropbox.com/s/h2a8dyz0vzoovev/DSCF0031.JPG
> > (Fun question)
>
> > George H.
>
> Yup :)
>
> I think one of the strangest V-I transformations is the current mode
> push-pull inverter.  The first thing that should be apparent is, it looks
> like a half bridge (switches go from side-by-side to in series), but what
> the CT transformer winding does is tricky to synthesize.
>
By push-pull do you mean the buck-boost?
I had to draw all three out after work today.
The buck-boost took the longest, (I had the current going the wrong way.)

I ended up with this,

+---+----+---+--LLL--+
|   |    |   |       |
^  FET   |   _       R
I   |    C   ^       R
|   |    |   |       |
+---+----+---+-------+

^
where I is the current source.

George H.
>
>
> Tim
>
>
>
> --
>
> Seven Transistor Labs
>
> Electrical Engineering Consultation
>
> Website: http://seventransistorlabs.com
```
```On Wednesday, October 16, 2013 6:57:52 PM UTC-4, George Herold wrote:
> On Wednesday, October 16, 2013 6:34:41 PM UTC-4, Tim Williams wrote:
>
> > "George Herold" <gherold@teachspin.com> wrote in message
>
> >
>
>
> >
>
> > > On Wednesday, October 16, 2013 2:04:35 PM UTC-4, Tim Williams wrote:
>
> > >> In the current domain, the conventional topologies (boost, buck, etc.)
>
> > >>
>
> > >> work just fine with capacitors.  Waveforms are an exercise for the
>
> > >> student
>
> > >>
>
> >
>
> > >> :)
>
> >
>
> > >>
>
> >
>
> > > I scribbled this at lunch... which I think works as a current buck.
>
> > > https://www.dropbox.com/s/h2a8dyz0vzoovev/DSCF0031.JPG
>
> > > (Fun question)
>
> >
>
> > > George H.
>
> >
>
> > Yup :)
>
> >
>
> > I think one of the strangest V-I transformations is the current mode
>
> > push-pull inverter.  The first thing that should be apparent is, it looks
>
> > like a half bridge (switches go from side-by-side to in series), but what
>
> > the CT transformer winding does is tricky to synthesize.
>
> >
>
> By push-pull do you mean the buck-boost?
>
> I had to draw all three out after work today.
>
> The buck-boost took the longest, (I had the current going the wrong way.)
>
<Snip>

No! I think it needs another diode... maybe there is a better way?

+---+->|-+--+-LLL--+
|   |    |  |      |
^  FET   C  _      R
I   |    C  ^      R
|   |    |  |      |
+---+----+--+------+

George H.
>
> > Tim
>
> >
>
> >
>
> >
>
> > --
>
> >
>
> > Seven Transistor Labs
>
> >
>
> > Electrical Engineering Consultation
>
> >
>
> > Website: http://seventransistorlabs.com

```
```"George Herold" <gherold@teachspin.com> wrote in message
> By push-pull do you mean the buck-boost?

Nah, PP, as typically used in low voltage inverters (DC input, balanced AC
output), or tube amplifiers.  Should really be called "pull-pull" (often
was, back in the day), but that doesn't roll off the tongue so well.

> I had to draw all three out after work today.
> The buck-boost took the longest, (I had the current going the wrong
> way.)
>
> I ended up with this,
>
> +---+----+---+--LLL--+
> |   |    |   |       |
> ^  FET   |   _       R
> I   |    C   ^       R
> |   |    |   |       |
> +---+----+---+-------+

Hmm, buck-boost is an inverting topology, needs to produce negative
current (L and R should be "below" the ground wire :) ).

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

```
```On Wed, 16 Oct 2013 09:43:03 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:

>> ...snip...
> So SMPS SHUNT regulator ?:-}
>
>                                         ...Jim Thompson

If the shunt is linear, it will eat power; but if it is a switched
shunt..., got to think about that.

However, I forgot to mention something really important here. The input is
AC, which [I think] I do NOT want to allow to go maximum compliance.
ARRRGG! Usually right after I post a question the answer is obvious, but
not this time. I must not even have a handle on the problem yet. My
question is coming out like drivel. It's just I have this sense that the
solution is an 'invrsion' of a normal solution, and I must turn my
thinking 'wrong-side' out.

I could allow full compliance and then tap off what's necessary, but
allowing the full compliance voltage and only tapping off to what is
needed raises the spectre of what-ifs. One should ALWAYS allow for
potential operation ANYWHERE within a supply loop. I remember the days
when power supply designers neglected to consider 'brown outs'.

```
```On Wed, 16 Oct 2013 11:53:49 -0700, George Herold <gherold@teachspin.com>
wrote:

.....snip...
> I scribbled this at lunch... which I think works as a current buck.
> https://www.dropbox.com/s/h2a8dyz0vzoovev/DSCF0031.JPG
>
> (Fun question)
> George H.

Thanks for the schematic, I see the philosophy in it.

I neglected to mention the MOST important aspect, the input is AC, not DC.
There is a potential full compliance could 'pop' to over 1kV, which is
obviously destructive. But, with a dead short voltage is not much, because
current is not much. Thus, my question here.
```
```On Wed, 16 Oct 2013 12:17:01 -0700, Vladimir Vassilevsky
<nospam@nowhere.com> wrote:

> On 10/16/2013 11:35 AM, RobertMacy wrote:
>> ...snip...
>> Anybody ever do this, or have some URL's with topologies?
>
> Done that. Stability of the control loop could be tricky.
>
>> ...snip...
>
> It could be actually simpler to take classic voltage controlled buck or
> boost as a base for design; and implement whatever current control loop
> around it using MCU. This would be easy to do using standard SMPS ICs.
>
> DSP and Mixed Signal Designs
> www.abvolt.com
>

I neglected to mention that the input is AC! Sorry. I can't take full
copliance voltage, so a dead short protects all the ocmponents.

Why would stability of the control loop be a problem?  Let me reword that.
Why would trying to do this type of control loop be more difficult?

I'm not sure how to do what you suggest.

Picture 1kV AC in and I either short it out or use it, that way the
voltages on all the components keeps low.

```
```"RobertMacy" <robert.a.macy@gmail.com> wrote in message
news:op.w420jrjk2cx0wh@ajm...
> Picture 1kV AC in and I either short it out or use it, that way the
> voltages on all the components keeps low.

What kind of source?  Huge assed resistor?  Analogous to series street
lighting (massive AC CCS transformers, the kind with the secondary on a
hinge and counterbalance!), etc.??  Any idea of parasitic capacitance or

AC input isn't *quite* a big deal, because you can use a synchronous
switching design, where MOSFET and diode are both replaced with antiseries
MOSFETs.  (A bidirectional synchronous buck, at fixed duty cycle, is
pretty cool -- it's a VARIAC without the iron core!)

Also... EMI concerns?  Can the line be switched directly, or does it need
filtering?  (You might recall from the current-to-current buck circuit,
the line will see full switching voltage -- just as a conventional
voltage-to-voltage circuit sees full switching current.  Both kinds are
nasty to nearby radios without bypass and EMI filtering.)

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

```
```On Thursday, October 17, 2013 12:28:51 AM UTC-4, Robert Macy wrote:
> On Wed, 16 Oct 2013 11:53:49 -0700, George Herold <gherold@teachspin.com>
>
> wrote:
>
> .....snip...
> > I scribbled this at lunch... which I think works as a current buck.
> > https://www.dropbox.com/s/h2a8dyz0vzoovev/DSCF0031.JPG
> >
> > (Fun question)
> > George H.
>
> Thanks for the schematic, I see the philosophy in it.
>
> I neglected to mention the MOST important aspect, the input is AC, not DC.
> There is a potential full compliance could 'pop' to over 1kV, which is
> obviously destructive. But, with a dead short voltage is not much, because
> current is not much. Thus, my question here.

OK... We haven't done many AC circuit yet.. but I'm guessing the same sort of thing can be done.
For completeness here's the way a current source buck-boost should look.

https://www.dropbox.com/s/eey79dkci1sqpp0/DSCF0034.JPG
(I figured out in the shower that the capacitor was in wrong.)

George H.
```