X-Mailer: Mozilla 4.79 [en] (Win98; U)
Subject: Re: switching dc-dc: DIY or off-the-shelf
NNTP-Posting-Date: Sat, 07 Dec 2002 19:53:26 GMT
Organization: AT&T Broadband
Date: Sat, 07 Dec 2002 19:53:26 GMT
Mike, Mike Harrison, Mike wrote:
>>> Hi, I'm wondering if anyone has any opinions regarding the use of
>>> off-the-shelf DC-DC converter modules (around $25 for 10 watts) or
>>> doing it yourself (with e.g. the National Semi 2592HV) (about $10
If production cost is the primary concern then even the NSemi solution
will be several times more expensive than a totally custom design done
from scratch without specialized ICs ($2 to $5).
>>> Note that this is for 48 volt in, so it's a little more complicated.
>>> Are there any drawbacks to the DIY approach, and if not, why doesn't
>>> everyone do it instead of using the modules?
[analog rant] I guess even cook book recipes are too intimidating for
the bit heads who pass for electrical engineers these days.
>>> In the little testing I've done, it seems that the DIY aproach has
>>> a lot higher EMI, but I don't know if that's the only problem. Any
Yes. With the knowledge of the proper principles and techniques, DIY,
unlike off-the-shelf, should have just the *right* amount of EMI.
Sometimes off-the-shelf includes EMI suppression components one doesn't
need and would rather not pay for. Or sometimes, just because the norms
don't require it, not enough EMI suppression is included where it is
needed. For example, there may be more common-mode noise on the outputs
than your whiz-bang application's low noise umpty-ump bit, super
sensitive A/D converters can tolerate.
>> Depends if it's a 1-off, a small production run or large volume.
>> DIY is more design effort, but has the advantage that it can be
>> tailored to your application, and usually fitted on the PCB with
>> everything else.
Very true, although where small size is paramount, modules may be best
since they often use non-standard, ultra-dense technologies.
>> EMI is down to good PCB layout. It's unlikely to be hard to get EMI
>> good enough for approvals - if you have particularly sensitive stuff
>> near it it may be a little harder, but using an off-the-shelf unit
>> may be no better in this respect.
Yep. Also, with a DIY design it is possible to exactly taylor for the
application the trade-off between switching speed and EMI.
>> With off-the-shelf unit, you also run the risk that the manufacturer
>> will change the design without telling you. here may also be a higher
>> risk that they may discontinue it - this is less likely to happen
>> with a DIY solution.
>> The bottom line is to balance the extra cost of buying ready-made
>> units against your time & effort designing a DIY solution.
>> Switchers using the NS parts are not especially hard to get to work
>> well, and personally I'd only ever consider a ready-made unit for
>> 1-offs or where an isolated output is needed, which usually needs
>> custom magnetics.
> Hi, Thanks for your thoughts. That's the direction I'm heading -- we
> initially used off-the-shelf bricks, but then they got discontinued
> and we had to scramble to find a replacement. And they cost more.
> I was worried, however, that since they have so many more components
> than the "simple switcher" approach, they must be better in some way :)
> But with the NS webbench design software, it's quite easy indeed to do
> a design, I just wanted to make sure there wasn't some catch.
Catch? There is none - unless you consider a very basic knowledge of
electronics and the ability to follow the app note cookbook recipe a
catch. -- analog