Re: Switch-mode power supply design

Eeyore wrote:

Came across this

http://i.cmpnet.com/planetanalog/2009/03/C0368_to_post.pdf

Don't know how long the link will last.

The article gets almost as far as an isolated, mains
powered
40W HV supply. The data*** even has a PCB layout that
can
be adapted.

Challenges to overcome would be transformer specification

" The transformer (T1), part number 75031040, is available
off-the-shelf
from Wurth Electronics. "

That's for a low voltage input, I think. There are plenty of
cheap inverters available for 240V AC output that shouldn't
be too hard to modify if you want to run your valve stuff
from a battery. However, considering the power output and
frequency is the same, the off-the shelf part is a welcome
clue, as is the 3:1 winding ratio.

It's the combination of mains in and HV out that caught my
eye, together with the ability to operate directly into a
large capacitor. The article gives a ratio for the
transformer suitable for a mains input design, and it might
be reasonable to assume that the same core and secondary
winding could be used as for the data***'s low voltage
input design but with, pro rata, a longer and thinner
primary winding that could fit into the same window.

and arranging for secondary-side feedback via
opto-isolators
whilst maintaining stability.

" The circuit in Figure 1 shows the LT3751 operating with
the output
voltage being sensed via the primary-side winding of the
transformer.
This method of primary-side output-voltage sensing
maintains isolation
with only one part, namely the power transformer crossing
the isolation
barrier, and is a very simple circuit. The output voltage
is sensed
through the RVOUT pin and is programmed by the selection
of R8, R9, and
the transformer-turns ratio."

No opto required. It's not an uncommon configuration for
flyback
supplies. It won't regulate quite as well as one with opto
feedback but
that's unlikely to be critical.

You may be right. Certainly for a class A amp. However, the
data*** uses secondary-side feedback wherever it
illustrates a design intended for continuous operation,
rather than as a capacitor-charger. It could be that
regulation would be *really* poor without it. I guess it's
not just the extent of the sag that might result from
powering class AB operation, but also its shape. A sudden
voltage drop followed by a slow recovery wouldn't do my
extended crescendos much good, maybe. I would be happier
knowing that, if it turned out to be desirable, I could use
opto-isolators in an output-derived feedback loop without
threatening stabiltity.

Just as I'm thinking about it, it occurs to me that class B
operation isn't what an SMPS is most naturally happy with
anyway. If you design for the maximum current demand, then
for much of the time the supply will be operating with a
short duty cycle or in burst mode, where it is less
efficient and possibly more noisy.

All the same, this is the most complete, simple,
regulated
HV SMPS design I've seen. Worth experiment, perhaps?

Why not ? Beware of the radiated field from the
transformer though.

How would you screen against hf magnetic fields? My
umbilical cords are quite long, so a metal box and plenty of
distance should suffice. SMPS transformers are designed to
minimise leakage when used as intended, I suppose. It seems
common to enclose the winding almost entirely in the core.

Ian


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