On 08/08/2012 09:23, Vic wrote:
<snip>
> So you don't use a 240V intermediate stage. You have a single
> conversion from Vin to Vout. That only gets complicated if you haev to
> cope with variable conditions, such that you can't say for all
> situations whether Vin > Vout or Vin < Vout; that might involve a
> two-stage conversion.
We agree here in principle but have a misunderstanding, I think.
The original url showed a 12V to 240V converter. If that goes before a
further power supply than there are at least two conversions, possibly
three if the (e.g.)laptop power supply also has power-factor correction,
though I think that's unlikely.
>> it unlikely to be better than 90% efficient
> Got any stats for that? Or is that a made-up number?
Erm, lots, many from LTC.
>> Add to that that the best efficiency will
>> be at a high load and in the real world you may well be nearer 60%.
> LTC's datasheets say something rather different. But I'm sure your data is
> more accurate than the device manufacturers.
All SMPSU datasheets show essentially the same curves. Efficiency starts
fairly poor at low load because of standing losses, increases to a peak
efficiency at maybe 50% load, then starts to fall again. Peak efficiency
is usually between 90 and 95% for a single conversion around the 12V
area, sometimes lower, rarely higher.
Low-cost 240V->Laptop PSUs are almost certainly less efficient, though
probably still 80% or so at peak .. and that _is_ a guestimate on my
part, working on the presumption that selling price is more important to
the manufacturer than efficiency.
>> One single conversion from 12V to wanted voltage will almost certainly
>> be more efficient.
> ...Which is exactly what I am proposing. But that does not make the system
> more efficient running from 12V compared to running the same converter
> from 240V.
>
Indeed not, which is why we agree.
The difference in our interpretation is because I included the likely
inefficiency due to the 12V->240V converter of the device in the
original url.
It's most likely that the USB output is single conversion, but a laptop
or similar would need powering via the 240V, which consequently would be
less efficient for possibly the highest and most-used load.
I think we both have the same advice. Unless you really want 240V, go
straight from 12V to device voltage, as it's more efficient.
FWIW, there's another calculation also to consider.
A car battery is, say 60Ah capacity.
A 240V converter when delivering 150W is consuming around 11A (assuming
a probably optimistic 90% efficiency).
So at that rate you'll use half your car's battery capacity in a little
under 3 hours, and you really don't want to take a car battery much
below that.
A laptop should draw rather less than 150W, I think.
My (old) Turion-based laptor draws <= 3A on my 12V->Laptop SMPSU,
depending on whether it's charging the internal Li-On battery.
ATB,
Gordon.
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