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

[RobertMacy]

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.


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

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

Tim

milliseconds?! arrrgggg, need submicroseconds!

Thanks for the .gif

 

[Spehro Pefhany]

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!

Your monitor could just be a 5W zener diode across the input (or a
small zener + BJT).

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.

What the heck source are you working with, if you can talk about it?

 

[George Herold]

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.

Whoa! Hey, a big word of caution Robert. I've not built a spms in my life.. I'm taking this web course "Fundamentals of power electronics" by RobertErickson. (Colorado U.) I've done two weeks of lectures and HW. So your question sparked the theoretical part of my brain. Which is great for me. But for a practical circuit, I'd feel a lot better if one of the smps guru's signed off on it. Putting in your numbers, you're hoping for a current gain of ~16 at greater than 80% efficiency. The few curves I've looked at for voltage smps show that losses start to really bite into the efficiency with higher gains. So you'll need to put in some real numbers.
(We've only started serious loss calculations this week.. so I'll know moreafter the weekend :^)*
Maybe a combo of transformer followed by current smps would work?

Anyway good luck... and not to worry about floundering around on SED. That's what's great about it. Throw out some crazy idea and see what sticks.

George H.

*Dr. Erickson seems to know his stuff. In this weeks lecture he was modeling switching losses and at one point paused and reminisced,
(para phrasing) "We wasted several years with this model, because it lead us to try and switch current and voltage separately in time and that just didn't lead to any improvement in efficiency. Things are more complicated, but this model is a good starting point." I just love that sort talk. It shows that everyone goes down dead ends.

the current charges a cap, which then supplies large amount of

 

[Joerg]

On Wed, 16 Oct 2013 11:53:49 -0700, George Herold

.....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.

1kV doesn't have to be destructive, they make FETs for well above that.
AC presents a challenge but not an impossible one. Got to bridge-rectify
that and your circuit must withstand a "brown-out" every 8.3msec.

 

[Joerg]

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.

That's what they invented flybacks for. Huge range, single step. But you
need to employ a recuperating winding to get above your 90%. The
magnetics design also promises to be "interesting".

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.

If you need to go to 1kV compliance I would not operate at 1MHz.

 

[Joerg]

Whoa! Hey, a big word of caution Robert. I've not built a spms in
my life. ...

But now, if it don't work right, Robert could claim "But George Harrold
said it would!"



I'm taking this web course "Fundamentals of power

electronics" by Robert Erickson. (Colorado U.) I've done two weeks
of lectures and HW. So your question sparked the theoretical part of
my brain. Which is great for me. But for a practical circuit, I'd
feel a lot better if one of the smps guru's signed off on it.
Putting in your numbers, you're hoping for a current gain of ~16 at
greater than 80% efficiency. The few curves I've looked at for
voltage smps show that losses start to really bite into the
efficiency with higher gains. So you'll need to put in some real
numbers. (We've only started serious loss calculations this week.. so
I'll know more after the weekend :^)* Maybe a combo of transformer
followed by current smps would work?

Anyway good luck... and not to worry about floundering around on SED.
That's what's great about it. Throw out some crazy idea and see what
sticks.

Or get into a hot discussion about Obamacare

 

[Spehro Pefhany]

1kV doesn't have to be destructive, they make FETs for well above that.
AC presents a challenge but not an impossible one. Got to bridge-rectify
that and your circuit must withstand a "brown-out" every 8.3msec.

Did he say 60Hz? I could imagine RF here.

The diodes only have to withstand the SMPS input voltage + one diode
drop if the bridge rectifier output is clamped.

 

[Joerg]

Did he say 60Hz? I could imagine RF here.

Well, that's the difficulty here, Robert can't divulge too much so all
we can do is take potshots.

[...]

 

[RobertMacy]

Your monitor could just be a 5W zener diode across the input (or a
small zener + BJT).


What the heck source are you working with, if you can talk about it?

No, a simple shunt will NOT work. Remember, 50 mA will NOT provide the 800
mA. Only get that if the voltage coming in is allowed tor rise a bit.

Sorry, can't talk too much.

 

[RobertMacy]

Whoa! Hey, a big word of caution Robert. I've not built a spms in my
life. I'm taking this web course "Fundamentals of power electronics" by
Robert Erickson. (Colorado U.) I've done two weeks of lectures and HW.
So your question sparked the theoretical part of my brain. Which is
great for me. But for a practical circuit, I'd feel a lot better if one
of the smps guru's signed off on it. Putting in your numbers, you're
hoping for a current gain of ~16 at greater than 80% efficiency. The
few curves I've looked at for voltage smps show that losses start to
really bite into the efficiency with higher gains. So you'll need to
put in some real numbers.
(We've only started serious loss calculations this week.. so I'll know
more after the weekend :^)*
Maybe a combo of transformer followed by current smps would work?

Doesn't matter about the 'source' of concepts, what matters is that you
'triggered' inside my brain an approach that almost in a single step does
what I want. ....with a few changes.

 

[RobertMacy]

Or get into a hot discussion about Obamacare

grrrrr.

 

[RobertMacy]

Well, that's the difficulty here, Robert can't divulge too much so all
we can do is take potshots.

Actually, go ahead and try at 60Hz.

 

[George Herold]

Doesn't matter about the 'source' of concepts, what matters is that you
'triggered' inside my brain an approach that almost in a single step does
what I want. ....with a few changes.

Excellent! Carry on then (As my ret. navy friend says.)
George H.

 

[Jasen Betts]

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.

current buck-boost converter.

+--+-FET--+--|<-------
| |
=== 3||
| 3||
| |
+--+------+-----------

the more time the switch (FET in the diagram)
spends off the larger the current output

 

[John S]

current buck-boost converter.

+--+-FET--+--|<-------
| |
=== 3||
| 3||
| |
+--+------+-----------

the more time the switch (FET in the diagram)
spends off the larger the current output

I don't understand how you can get more current out of the inductor than
it had when the switch opened.

 

[Joerg]

I don't understand how you can get more current out of the inductor than
it had when the switch opened.

Ask a politician, they can do that

 

[John S]

Ask a politician, they can do that

So, I should have studied politics rather than engineering. I guess
Momma was right.

 

[Joerg]

So, I should have studied politics rather than engineering. I guess
Momma was right.

If you and I had done that we'd be rich. But probably not too happy. If
I were 18 again I'd veer towards a brewmaster's degree and keep EE as a
hobby. No kidding.

 

[John S]

If you and I had done that we'd be rich. But probably not too happy. If
I were 18 again I'd veer towards a brewmaster's degree and keep EE as a
hobby. No kidding.

It very hard to resist your true calling, I learned.

 

[Joerg]

It very hard to resist your true calling, I learned.

The problem is to figure out what your true calling is. Some people
realize that late in life. For example, a top chemical scientist in our
area who some day when he was well in his 50's quit his well-paying job
at Monsanto and started a winery. He said he'd found his true calling.
Or a sccessful surgeon who, late in his 50's, became a pilot (after an
expensive trek through commercial pilot flight schooling) and hired on
with an airline. His income probably went down by 10-15dB but he was happy.