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

[RobertMacy]

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?

Problem hint: voltage to voltage means unused input power is OPEN, but
current to current means unused power is SHORTED. See the difference?

 

[George Herold]

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?



Problem hint: voltage to voltage means unused input power is OPEN, but

current to current means unused power is SHORTED. See the difference?

So you've got a current source driving it?
Do you need more current/ less current or both?
(I'm taking a smps on the web... which means I know enough to say somethingstupid.) My first thought was to switch around the L and C and then change shunt and series... so a shunt cap on the input side and a series L on the output. (I'd have to play with it some though...)

George H.

 

[Joerg]

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?

Problem hint: voltage to voltage means unused input power is OPEN, but
current to current means unused power is SHORTED. See the difference?

You didn't give much data here. Assuming that measuring the load current
is iffy (because it usually it) one could instead measure the waste
current. This has to include the current used by the converter itself.
You could have a sense resistor and run a sync buck into that, which
except for this sense resistor would be a dead short situation. The
ground of the converter would also connect to the top of that current
sense resistor so it includes its own consumption. Now set the loop so
it sinks away the portion of the input current you don't want to reach
the output into ground. The load would hang on it's input and would see
the balance of the current.

Use a converter that will not go into foldback mode or goose it into
regular mode, for example by fooling the voltage control (which is
otherwise disabled).

 

[Tim Williams]

In the current domain, the conventional topologies (boost, buck, etc.)
work just fine with capacitors. Waveforms are an exercise for the student


I'm not so sure about isolation (flyback, forward), that might look like a
signal coupled with two capacitors, one for power and one for ground
return.

How ideal is the source it's attached to? If it's got a honking big
bypass cap, it's not very constant for SMPS purposes. If it's got a
massive inductor (or an open collector or something like that), chances
are it's a pretty good current source.

Tim

 

[George Herold]

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.

 

[Joerg]

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.

 

[Tim Williams]

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

 

[George Herold]

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.

 

[George Herold]

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 Williams]

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

 

[RobertMacy]

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'.

 

[RobertMacy]

......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.

 

[RobertMacy]

Done that. Stability of the control loop could be tricky.


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.

Vladimir Vassilevsky
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.

 

[Tim Williams]

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
inductance near the load?

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

 

[George Herold]

On Wed, 16 Oct 2013 11:53:49 -0700, George Herold <[email protected]>

wrote:

.....snip...

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.

 

[RobertMacy]

...snip question on source...
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

Is that component, antiseries MOSFETs, commercially avaialable, or does
one have to always 'roll their own'

What's the schematic for it?

RFI/EMI is always important, but in this case it's make it work, then
worry.

 

[RobertMacy]

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.

Thanks again for the images, surprisingly this time dropbox let me view
them - usually doesn't.

IN: AC with source current maybe 30-50mA, compliance to 1kV
OUT: approx 5Vdc, with variable current, adjusted with time envelope from
0.8A down to 50uA.

with source current that low, can't just feed it straight thru.

assuming NO losses...
with input compliance at 80V and using all 50mA, supply the 5V at 0.8A
You see why I first said current to current conversion?

After major floundering around [and rather embarrassingly] in front of all
of you; I am starting to perceive the final topology as a multi-step: AC
to DC at power required, then DC to DC at current required.
But even 90% at each step yields a terrible 80% over all, so I was trying
for a single step: AC to DC at current required.

I can always flip the diode to make it a positive supply. That does look
like a 'single' step.

Tim's right about paying attention to the RFI/EMI kicking back into that
source. Any parasitics and !!! It's going to be fun trying to filter out
that 50V ON/OFF appearing across the MOSFET terminals let out into the
world through that inductor at 1MHz.

 

[RobertMacy]

So SMPS SHUNT regulator ?:-}

...Jim Thompson

Alas, the compliance current is around 50mA with a compliance voltage as
high as 1kV, which is destructive. A 'straight' shunt regulator either
shorts out the incoming 50 mA or let's it pass thru, but in this case I
sometimes need 800mA at a small 5Vdc. So, need some 'multiplication' of
the current - a capacitor. There is sufficient power if the system
operates with the compliance at around 80V at 50mA just have to somehow
convert that to 5Vdc at 800mA. But, in this case it will be super
necessary to monitor compliance voltage so it won't EVER go above 100V in
order to NOT kill parts. In this supply that protection circuit, or
'voltage limit', is the corollary of the standard 'current limit' in a
voltage supply!

So far it appears a variation of George Herold's approach may get me
there. the current charges a cap, which then supplies large amount of
power. Aain, corollary, using cap instead of inductor for the energy
storage/conversion.

 

[RobertMacy]

I'm not quite understanding your requirement. You can send me details
in private if you like.

...Jim Thompson

Thank you for your kind offer. I am being vague for two reasons:
1. purposely obfuscating the actual application/techniques under cloak of
NDA.
2. inadvertantly floundering around, albeit embarrassingly, but in my
defence, this is NEW territory here.

 

[Tim Williams]

Is that component, antiseries MOSFETs, commercially avaialable, or does
one have to always 'roll their own'

Yes... and yes.

Commonly available with a photovoltaic gate "drive" as the smaller AC/DC
SSRs. They switch in miliseconds.

You'll need an isolated gate driver for each switch, plus whatever
coupling (optos?) and control (PWM something or other?) to make it go.

What's the schematic for it?

http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Antiseries_MOSFET_Switch.gif

Obviously, substitute 9V and switch with whatever driver is suitable

Tim