20-60A adjustable ~200vdc current mode buck design

[John Barrett]

Ermmmmm...... Current sense transformers, something like

http://www.cd4power.com/data/magnetics/kmp_5600.pdf

Each PFC section needs to control its own inductor current otherwise they
won't share the total load. It's tied up with the volt-second balance,
inductors integrate the voltage placed accross them as the current through
them. If there is a mismatch in the drive to the inductors then one of
them will hog the current.

The local current error amplifiers force them to share it but you need
separate sense signals for each stage so one honking big current sensing
thing on the input won't do. You also need a reasonable frequency response
from the devices if you want to compensate the current loops correctly.


Ahaaaa, LT1248, shows you how crap I am for not checking if they did one
or if LTSpice had the model. I knew they did but there is something wrong
with my head. I suppose I'm fixated on TI because I grew up with Unitrode.
It's all much of a muchness and once you get happy with one you can see
that a lot of the internals are the same in all the others.

hehehe not suprising -- now if I could get the reference design from the
datasheet to model -- I'd be in hog heaven

If you really really need to ground the output of your beast....... Uhm,
right, your side of the pond has strange electricity with something like
110V balanced either side of ground. My head hurts but I think you might
only get away with it if you use half wave rectification which is going to
be dirty and waste half of the capacity of your supply.

our 220v is really a pair of 120v lines 180 out of phase, when you plug into
a 120 socket, the "nuetral" side is pretty much power company ground

so sticking a full wave bridge on it directly gets you half wave pulses
-- you need both sides to get full wave rectified output

I dont REALLY need earth ground referenced, but it would make some other
issues I have to deal with one HECK of a lot easier

Since you seem to enjoy pain you might as well go the whole banana and
dangle a transformer isolated coverter of the end of your PFC stages.
Something like a half or full bridge. That will make any (well most)
concerns about earth disappear.

Trying to avoid iron as much as possible, and I see what you mean about pain


I've been trying off and on for 24+ hours now to get a pos->pos boost to
work side by side with a neg->pos boost with a shared ground tied to power
company ground (modeled as 2 120v sources, 180 phased, with neg sides tied
to ground, pos sides feeding a full bridge and splitting the pos and neg to
the 2 boost modules.

I'm getting close -- not using full PFC yet -- just a fixed gate pulse to
see where things are going.. and I'm getting fairly stable output, but have
one major weird thing going on.... HV spikes showing up on the rectified
input to the positive boost that need to go away ... they start on the
falling edge of the input voltage pulse, continue through zero and into the
begining of the next pulse, with a particularly hard spike as the input
voltage bottoms out and starts back up. its got to be the inductor causing
it somehow since it is being switched on the other side by the mosfet to
ground, so maybe the PFC will make it go away. If you dont think that will
help, I'd love to hear what it might be -- Spice simalation available on
request

Other than that -- its behaving as well as can be expected since I havent
fine tuned any of the component values yet, and am still using a spice
switch instead of a FET or IGBT.... once all that is done, I'll pack it up
in a subckt and start on the PFC controller and stuffing enough of these in
parallel to handle my load requirements

 

[Genome]

hehehe not suprising -- now if I could get the reference design from the
datasheet to model -- I'd be in hog heaven



our 220v is really a pair of 120v lines 180 out of phase, when you plug
into a 120 socket, the "nuetral" side is pretty much power company ground

That's what I meant..

so sticking a full wave bridge on it directly gets you half wave pulses
-- you need both sides to get full wave rectified output

That's sort of what I thought would happen if you wanted to maintain a
ground/neutral connection.

I dont REALLY need earth ground referenced, but it would make some other
issues I have to deal with one HECK of a lot easier

May as well do it so you can.

Trying to avoid iron as much as possible, and I see what you mean about
pain

Embrace the 'iron'. You can use the full/half bridge directly to control
output current and provide your isolation. There is no need to generate an
intermediary DC supply and then add a buck converter you can do it all in
one go.

I've had a fiddle with a half bridge using average current mode control and
it's looking funky.

I've been trying off and on for 24+ hours now to get a pos->pos boost to
work side by side with a neg->pos boost with a shared ground tied to power
company ground (modeled as 2 120v sources, 180 phased, with neg sides tied
to ground, pos sides feeding a full bridge and splitting the pos and neg
to the 2 boost modules.

I'm getting close -- not using full PFC yet -- just a fixed gate pulse to
see where things are going.. and I'm getting fairly stable output, but
have one major weird thing going on.... HV spikes showing up on the
rectified input to the positive boost that need to go away ... they start
on the falling edge of the input voltage pulse, continue through zero and
into the begining of the next pulse, with a particularly hard spike as the
input voltage bottoms out and starts back up. its got to be the inductor
causing it somehow since it is being switched on the other side by the
mosfet to ground, so maybe the PFC will make it go away. If you dont think
that will help, I'd love to hear what it might be -- Spice simalation
available on request

It might get messy. I haven't got my brane screwed in yet so I can't draw
what you've got.... However boosting neg to pos is going to place extra
voltage stress on that part of your converter and its control electronics
are going to be referenced to a different point which may well be going up
and down.

Spikes are common in simulation. Sometimes it's because you've done
something stupid. Sometimes it's because there are two answers, the one you
want and the one that has a big spike. Some series and or parallel
resistance in inductors can help. However, in this case, it is likely
that......

Yes, the node you mention can and will cause stress. Try adding a capacitor
from that end of the boost inductor to circuit ground. You'll need one in
real life as well. By all means, send me a copy of what you have got. There
is an e-mail icon on the front page of my webshite....

http://www.genomerics.org

Other than that -- its behaving as well as can be expected since I havent
fine tuned any of the component values yet, and am still using a spice
switch instead of a FET or IGBT.... once all that is done, I'll pack it up
in a subckt and start on the PFC controller and stuffing enough of these
in parallel to handle my load requirements

Ideal switches are nice..... (ideal everything) stick with them until it's
working the way you expect. Then you might start adding 'real' stuff.

Later

DNA

 

[John Barrett]

----- Original Message -----

It might get messy. I haven't got my brane screwed in yet so I can't draw
what you've got.... However boosting neg to pos is going to place extra
voltage stress on that part of your converter and its control electronics
are going to be referenced to a different point which may well be going up
and down.

Voltage stress ?? try CURRENT stress -- the neg boost is drawing twice the
current for the same voltage out

probably because of the non optimal component values I'm testing with --
gonna have to plug all the math into a spreadsheet and work out what SHOULD
be there

There wont be any point in the control that will be referenced to anything
but earth ground except the gate drivers.. using those hall effect current
sensors gets me COMPLETELY isolated no matter if I'm sensing high side or
low side.. very important since with ground shared between 2 boosts, I cant
get in the low side at all, nor do I want to !! (I'm using low side with a
bit of a difference from common usage in SMPS design -- to me -- low side is
always ground, high side is anything that is away from ground, no matter the
polarity. )

Spikes are common in simulation. Sometimes it's because you've done
something stupid. Sometimes it's because there are two answers, the one
you want and the one that has a big spike. Some series and or parallel
resistance in inductors can help. However, in this case, it is likely
that......

and sometimes because its really going to generate spikes

Yes, the node you mention can and will cause stress. Try adding a
capacitor from that end of the boost inductor to circuit ground. You'll
need one in real life as well. By all means, send me a copy of what you
have got. There is an e-mail icon on the front page of my webshite....

Dropped in the cap and spikes go BYE BYE !!

http://www.genomerics.org


Ideal switches are nice..... (ideal everything) stick with them until it's
working the way you expect. Then you might start adding 'real' stuff.

Thats the point I'm at -- getting it all "real" so I can subckt this and
start the control electronics.

<a coupla hours later>

welp, everything is real except the switches now --

ASC file available if anyone else wants to look -- just ask

let me know what you think