On 08/08/12 12:24, Gordon Scott wrote:
> 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.
The original URL should a 12v to 240v inverter which *ALSO* had a USB
output socket on it.
I rather feel that gives the best of both worlds. USB devices can be
powered and/or charge from the (more efficient) USB socket, but random
other, non-USB devices, can still be powered/charged using their regular
mains power supplies, using the 240V output.
I'd rather have that than for every non-USB device have to carry around
two power supplies (a mains and a 12V one).
Chris
>
>>> 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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