smps design

[Steve]

Hi all,
I study smps design and I have the following question:
When designing continous mode half-bridge and full-bridge designs; has
the transformer core magnetization current (in primary) anything to do
with maximum throughput power? I mean, must the magnetizing current
have a minimum magnetude to achieve a certain output power level? I
guess this would be a question for DNA / Genome, anyone seen him lately
by the way?
Regards,
Stefan

 

[Eeyore]

I guess this would be a question for DNA / Genome, anyone seen him lately
by the way?

He's been growing encrustations from his bottom apparently. I wouldn't advise
meeting him at all !

Graham

 

[Tim Williams]

I would suspect so. I think you have it backwards; the maximum inductive
current is equal to saturation current, which depends on turns and core
material. The current is, of course, recycled each cycle, being purely
inductive, so you don't lose anything*.

*Minus resistance, diode drops, and extra capacitor stress of course.

You can, of course, reduce magnetizing current as much as you want, by using
a hugely oversized transformer, so that resistance is low and inductance is
high.

In flyback or buck (half wave I suppose) designs, the peak transformer
amp-turns must, of course, be less than saturation. There is no minimum
limit, though.

Tim

 

[John Popelish]

Hi all,
I study smps design and I have the following question:
When designing continous mode half-bridge and full-bridge designs; has
the transformer core magnetization current (in primary) anything to do
with maximum throughput power? I mean, must the magnetizing current
have a minimum magnetude to achieve a certain output power level?

(snip)

No direct connection. The magnetizing current is an artifact of
having a non infinite permeability in the core.

If you don't drive the primary through a capacitor or otherwise force
the average current to zero, you may have to gap the core and that
lowers its effective permeability more, raising the magnetizing current.

If you can afford a larger core, you can raise the winding inductance
and lower the magnetizing current.

So the magnetizing current is one factor among many that you
compromise when designing a transformer.

 

[Steve]

John Popelish skrev:

(snip)

No direct connection. The magnetizing current is an artifact of
having a non infinite permeability in the core.

If you don't drive the primary through a capacitor or otherwise force
the average current to zero, you may have to gap the core and that
lowers its effective permeability more, raising the magnetizing current.

If you can afford a larger core, you can raise the winding inductance
and lower the magnetizing current.

So the magnetizing current is one factor among many that you
compromise when designing a transformer.

Ok, so the best thing would be to have a very high inductance primary
for extremely low magnetizing current to lower dissipation in switches.
This will appearently have no effect upon maximum output power level?
May I ask you another thing:
In buck derived converters, the primary current is a result of
secondary loading + the magnetizing current. To keep secondary voltages
constant you need a certain duty cycle, but that has nothing to do with
the primary current??
If this doesn't make sense, it's probably because of my bad
explanation. I try this one also:
Imagine a full-bridge design outputting 100VDC on secondary. Load is
very low. This results in a 50% duty-cycle. Primary current is low.
Suddenly load increases sharply. Duty-cycle goes up to 75% to
compensate for drops. Primary current is high.
What I want to ask is there are no direct relationship between primary
current and output voltage regulation, so how would a current mode full
bridge work??
Best regards,
Stefan

 

[John Popelish]

Ok, so the best thing would be to have a very high inductance primary
for extremely low magnetizing current to lower dissipation in switches.
This will appearently have no effect upon maximum output power level?

That is the general idea. In fact, if you lower the magnetizing
current by using a larger core, the power output capability will go up
while the magnetizing current goes down. The magnetizing current is
essentially a process going on in parallel to energy transfer between
windings. It is just necessary to produce volt seconds across each
turn (even at zero power transfer).

May I ask you another thing:
In buck derived converters, the primary current is a result of
secondary loading + the magnetizing current. To keep secondary voltages
constant you need a certain duty cycle, but that has nothing to do with
the primary current??

That's right. In a buck derived converter, the load current
determines the primary current except for magnetizing current) and the
primary voltage and the duty cycle determine the average voltage per turn.

If this doesn't make sense, it's probably because of my bad
explanation. I try this one also:
Imagine a full-bridge design outputting 100VDC on secondary. Load is
very low. This results in a 50% duty-cycle.

By this, I assume that you mean that the primary switches are
supplying voltage to the winding only half of each half cycle, and
zero the other half.

Primary current is low.
Suddenly load increases sharply. Duty-cycle goes up to 75% to
compensate for drops. Primary current is high.

That is a lot of drops, but okay. Usually, the drops need a much
smaller correction, assuming the output filter has continuous current.
Most of the duty cycle variation is usually reserved for changes in
primary voltage.

What I want to ask is there are no direct relationship between primary
current and output voltage regulation, so how would a current mode full
bridge work??

There has to be a monotonic relation between primary current and
output voltage or current mode control will not work. But they don't
have to be proportional under all conditions, with a fixed
proportionality factor. But at any output load current, increasing
the primary current must increase the output voltage.

 

[Genome]

Hi all,
I study smps design and I have the following question:
When designing continous mode half-bridge and full-bridge designs; has
the transformer core magnetization current (in primary) anything to do
with maximum throughput power? I mean, must the magnetizing current
have a minimum magnetude to achieve a certain output power level? I
guess this would be a question for DNA / Genome, anyone seen him lately
by the way?
Regards,
Stefan

You promised me a good face sitting session with Agnetha.

I don't know what went wrong but perhaps you might write back to her and
explain that it involved her buttocks and my face rather than the other way
around. Obviously I shall buy some more Haddock and Chips with an extra
portion of scraps along with mushy peas and curry sauce. I have learnt how
to do gravy as well, ask her what flavour she wants from the Bisto range, I
might be able to stretch to a bit of best if she wants to be picky.

Thanks

DNA

 

[Steve]

Genome skrev:

You promised me a good face sitting session with Agnetha.

I don't know what went wrong but perhaps you might write back to her and
explain that it involved her buttocks and my face rather than the other way
around. Obviously I shall buy some more Haddock and Chips with an extra
portion of scraps along with mushy peas and curry sauce. I have learnt how
to do gravy as well, ask her what flavour she wants from the Bisto range, I
might be able to stretch to a bit of best if she wants to be picky.

Thanks

DNA

Ok...things seems to have collapsed for you...
I remember you as a competent smps designer but obviously you've gone
to other business.
Good luck with Agnetha!

--- Stefan

 

[Genome]

Genome skrev:



Ok...things seems to have collapsed for you...
I remember you as a competent smps designer but obviously you've gone
to other business.
Good luck with Agnetha!

--- Stefan

Sigh, I suppose you are right.

It's a trade off between core losses and winding losses. The number of turns
on your primary, and hence secondary, are set by the peak (or peak to peak)
flux excursion you can accept.

N = VIN.TON/Bp.Ae

The greater the flux excursion the fewer turns you need so the lower the
winding losses. Unfortunately the flux excursion causes core losses and the
greater the flux excursion the greater the core losses. Of course operating
frequency also affects core and winding losses.

With fewer turns the primary magnetising inductance is smaller so the
magnetising current is larger so that implies a higher magnetising current
is associated with a higher throughput power. That's really a secondary
effect though. The higher magnetising current also results in greater switch
losses but the relative levels mean it's not so important.

You can work out core losses by referring to the graphs given by
manufacturers. Winding losses are a little bit harder but here's a program
that works out what the AC impedance of your wires might be......

http://www.genomerics.org/software/roundwires.html

Not gauranteed to give the right answer......

So, yes, higher magnetising current implies greater power throughput.
However it's all a trade off with other things.

DNA

 

[Steve]

Genome skrev:

Sigh, I suppose you are right.

It's a trade off between core losses and winding losses. The number of turns
on your primary, and hence secondary, are set by the peak (or peak to peak)
flux excursion you can accept.

N = VIN.TON/Bp.Ae

The greater the flux excursion the fewer turns you need so the lower the
winding losses. Unfortunately the flux excursion causes core losses and the
greater the flux excursion the greater the core losses. Of course operating
frequency also affects core and winding losses.

With fewer turns the primary magnetising inductance is smaller so the
magnetising current is larger so that implies a higher magnetising current
is associated with a higher throughput power. That's really a secondary
effect though. The higher magnetising current also results in greater switch
losses but the relative levels mean it's not so important.

You can work out core losses by referring to the graphs given by
manufacturers. Winding losses are a little bit harder but here's a program
that works out what the AC impedance of your wires might be......

http://www.genomerics.org/software/roundwires.html

Not gauranteed to give the right answer......

So, yes, higher magnetising current implies greater power throughput.
However it's all a trade off with other things.

DNA

Hi!
That's the 'DNA' I know of...
Nice to hear from you again, I'm still reading your docs about slope
matching and error amp compensation, subharmonic oscillation and how
big waste the voltage mode topology is.
Thanks for your reply. It indicates that I'm not that far out in
space....

Regards,
Stefan