I've thought quite a bit about incorporating switch mode supplies into
my projects for quite a while. It becomes a very attractive option
when I look at running 3.3V devices on the 'standard' 13.8V supplies
we all have in our shacks. The linear supplies are just so
inefficient with all that voltage drop they have to eat up. In
particular, I've looked at lots of circuits which use the MC34063. It
seems to be the switch mode equivalent of the 78xx linear regulators
we see all over the place -- That is, it's been used 'everywhere' for
a very long time and I could probably find one in a junked electronic
device around the house.
What's stopped me is simply that I don't really know what 'good EMC
design' is when it comes to power supplies. I have no idea what the
300kHz ripple is going to do when it hits something like a DDS chip,
MMIC amplifier, or mixer. The actual reference circuits are dead
simple. There are only a couple added external components over a
linear regulator. It's just not knowing what that ripple is going to
do when it gets into the rest of my circuit that keeps me from
investigating switchers.
If any radio amateurs have used switchmode circuits extensively, I'd
love to see what projects they have been used in. It would give me
some confidence that it *can* be done. In the meantime I just know
I'm not the person so experiment in that particular field of home
brewing. When I'm troubleshooting other parts of a project, it's just
nice to know that the stable DC power provided by a linear IC isn't
contributing to any issues I experience.
-Jeff
W7WWA
On Tue, Oct 1, 2013 at 1:47 PM, Dale <svetanoff@earthlink.net> wrote:
> All,
>
> I thought that the technically inclined on this list might want to be aware
> that at least one switching power supply manufacturer has now come out with
> "equivalent" replacements for the ubiquitous LM78-- and LM79-- series of IC
> voltage regulators that have been around for decades. Touting efficiencies
> in the 90% range and that no heat sink is required (in some applications),
> few external parts are needed.
>
> So they say ... well, the new devices do required the input and output side
> by-pass caps as per the linear originals. The switching frequency is not
> consistent, either, but seems to be around 300 kHz. Here is the link:
>
> http://www.cui.com/dc-switching-regulators
>
> To be fair, if a manufacturer (or serious hobbyist) does proper pc board
> layout and follows good EMC design practices, an end-user device that
> incorporates one or more of these new chips can be just as RF "clean" as the
> linear versions (when checked externally). However, design and manufacturing
> shortcuts to save a few pennies can easily result in more RFI screamers being
> foisted upon the market place.
>
> Please note that if a hobbyist ("ham") buys one of these and does a drop-in
> on a circuit board that had an LM78-- series device on it, the net result is
> not likely to be very RF clean. The good news will be less heat, but you had
> better like more hash with your lower temperature. Note that these devices
> are offered in pin-compatible (for TO-220) plastic packages. My scary
> thought offering for Halloween.
>
> 73, Dale
> WA9ENA
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