But the current drawn by a /small/ SMPS will be too small to mess up the waveform, especially when off-load. Hence appliances of under 50 W consumption are exempted from the harmonic current emission limits of EN
61000-3-2. Larger SMPSs require power factor correction these days.The magnetising current waveforms of typical small mains transformers isn't particularly sinusoidal either.
CE marking is a form of self approval. Plenty of smallish companies manage it.
Dave Plowman (News) presented the following explanation :
Which can and do cause power factor problems. Whether the correction is applied locally at the motor, or as an overall single correction for the entire factory depends upon the type of use the motors are subjected to.
If all the motors normally run and the factory has a predictable power factor, then correction can be done in one go at the mains. If motor use is irregular and not predictable it needs to be done at each individual motor.
Ballasted fluorescent lights present the same problem and sometimes will have a built in PF correction capacitor.
SMPSU's are no more efficient off load than are transformer type PSU's, but on load they are much more efficient.
|Dave Plowman (News) presented the following explanation : |>> This consideration was sufficiently important for factories with |>> numerous motors to install capacitor rooms where large capacitors were |>> wired across the supply to neutralise the inductive effect and present a |>> resistive load. | |> Most factories would use three phase motors. | |Which can and do cause power factor problems. Whether the correction is |applied locally at the motor, or as an overall single correction for |the entire factory depends upon the type of use the motors are |subjected to.
Our department had a *huge* three phase synchronous motor which did the power factor correction for the whole factory.
Off-load the efficiency is surely always zero (zero power out divided be finite power in). Thus it's only meaningful to quote a standby power consumption (power input with no load).
That's not necessarily the case at all.
What..not more efficient than zero? :-)
Agreed SMPS are not used for efficiency, because they will not match a good transformer, which is up there at the 9-98% sort of areas..they are used because they are CHEAP.
Quite. But for wallwarts it is, theyre too small to get decent efficiency.
NT
Switched mode?
They don't seem to get particularly warm in most cases.
Yes. I meant mains frequency transformers, in warts theyre just too small to be efficient. Fairly high copper losses plus magnetising current with minimal iron isn't really a formula for high efficiency.
NT
Another sweeping statement? SMPS will be smaller and lighter for the same current output. Have the ability to work properly on a wide range of input voltages, which a transformer supply can't.
As regards being cheaper, how do you price such things? In all practical costings they're more expensive.
Isn't that a rather sweeping statement? They are smaller and lighter, and that's why they are/were used in VCR'S, etc... Or so I believe.
Sylvain.
Transformers are not safe. They are robots in disguise. Better warn your old mum.
That is very dependent on the required output power level though. Where only a few milliwatts DC (or secondary AC) output is required it's usually possible to keep the standing losses in a 50 Hz transformer to under half a watt or so. TBH I don't really know how that compares with SM designs as I don't have so much experience of those as with 50 Hz trannies.
Magnetising current, /per se/ is not a cause of loss - it's the in-phase component of the primary current due to hysteresis and eddy current losses that's relevant. Indeed when designing for higher power output levels the transformer efficiency on-load will be improved by keeping the core flux density as high as practicable. This increases the iron loss but reduces the turns per volt figure for the windings, leading to lower winding resistances and therefore less copper loss (and better regulation). IOW there's a design trade-off here where achieving high efficiency at full load leads to increased standing loss off-load, and vice-versa. In an ideal world one might be able to optimise a design for minimum energy consumption, taking the usage duty cycle into account. In practice this approach might be frustrated because the lower full-load efficiency leads to some combination of excessive temperature rise on-load, size or cost.
For very small transformers though (say 3 VA or less) the copper loss in the primary winding due to the flow of magnetising current can be a significant contribution to the standing loss. You will often find that transformers of this size have unequal primary and secondary winding window areas (primary has >50% of the net window area) in an attempt to mitigate this effect.
No, they are not.
That's why they are used. At some point - about 20-50W..the iron costs and the copper costs of a decent 50Hz transformer outweigh the minimal costs of a couple of chips and some high voltage transistors and a small ferrite transformer running at 10Khz. The point at which you DON'T see a wall wart, and you DO see an SMPS.
Apart from maybe HIFI units, you will be pushed to find any equipment using an iron cored 50/60Hz transformer at over 50W..OTOH you will be hard pushed to find anything NOT using one under that power level.
The exception being the National Grid of course ;-) Where efficiency really matters, and lifetime is long enough to render massive transformers the optimal solution over many years of amortization.
Smaller and lighter=cheaper in this context..seen copper prices recently? Or magnetic materials..
I can assure you that the driving force of MOST electronic design is cost. Not smaller/lighter..who cares if a VCR is 6 oz heavier?
Mobile phone chargers?
Do you think you could learn to use google groups correctly and reply in thread ?
Dave
In article , Andy Wade wrote: [snip]
It can be the other way round. Some years ago I bought an RS 1KVA 240:115 isolating transformer. The surface temperature was very high on no-load and actually decreased as it was loaded.
Hello Gorty, looks alright to me.
Hmmm I suppose your looking at it in google groups, nuff said.
Dave
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