[RFI] "Efficient" replacements for linear voltage regulators

[Dale]

Jeff,

You bring forth some very good issues - I would add that just as important as the 300 kHz ripple may be (or not) to devices being powered is the issue of how much extra radiated noise is contributed by the new type of regulator.

"Good EMC design" is the subject of many text books and extensive courses.  It is way beyond the intent of this posting.  However, let me say that using a simple one-sided circuit card is NOT going to work when using a switcher power supply.  In fact, even 2-sided boards are not very adequate.  Modern best practices use multiple layer boards (typically between 4 to 8 layers, often more) to isolate power layers, digital grounds, analog grounds, clocks and data busses, etc.  You want to have a power trace run directly above or below its associated return trace, so that fields cancel and the capacitance between the traces (from the pc board dielectric material) helps to reduce high frequency ripple and/or transient currents.  The list goes on and on.  

Finally, after all of the work of making a good layout, you then have to manage common mode coupling into or out of the interconnect cables (power, signal, data, etc.), and control radiated emissions from the housing of the end device.  However, all that being said, I see no reason to attack some older piece of equipment that is working, do a retrofit with a switcher for the analog on-board regulator, and see what you get.  Just make certain you benchmark not only the data issues that are important, but also note any RFI to your radios, audio equipment, or TVs both prior to and after making the mod.  I suspect you may have a real learning experience, but the more you prep in advance by studying the appropriate text books and on-lime sources, the better your results may be.   Don't be afraid to have some fun, but yes, you are wise to be cautious.  In the meantime, don't get singed off the heatsink of an LM7805. 

73, Dale
WA9ENA      
     


-----Original Message-----
>From: Jeff Stevens <jeff at mossycup.com>
>Sent: Oct 1, 2013 2:56 PM
>To: Dale <svetanoff at earthlink.net>
>Cc: Rfi List <rfi at contesting.com>
>Subject: Re: [RFI] "Efficient" replacements for linear voltage regulators
>
>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 at 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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