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Small switch-mode PSUs - Poor PF or something else?

AncientMariner

Have noticed that small switch-mode PSUs appear to have a shocking PF.

Two examples, both CE marked. 1) a Motorola phone charger. Label states Input 100 - 240V 0.2A  Output 5.0V  550mA

2) A psu for a string of decorative LED lights. Label states Pri: 220 - 240V 0.12A  Sec: Urated 12V  Prated 2.4W

In both cases the input VA is much, much higher than the output wattage measured the input currents.

I am assuming that because they are low power and in the UK at least domestic consumers pay for Watts rather than VA, designers have no incentive to improve the PF.

Clive

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  • 0

    Below 60 watts there is no requirement to consider power factor in the requirements for CE marking. So For LEDs a series capacitor to drop the volts and then a diode bridge with the LEDs as the load.yes it  is a poor PF but at least it is more or less a sine wave and will probably be balanced out by the fridge motor to a degree.

    For small switchers USB chargers etc direct rectification feeds a capacitor to provide a DC bus for the oscillator that drives the ferrite transformer. Direct rectification means no current flows except at the top 10-20 degrees of each half cycle. Adding PFC to turn short pulses into a longer more sinusoidal splurge is hard, and needs is more components, so in turn more unreliability and more heat to be got rid of so why bother if it is not mandated ?

    Mike.

  • 0 in reply to mapj1

    That certainly makes sense.  There cannot be much inside the LED string one since it is featherweight and that includes an optional 6 hours on 18 hours off option via the push-button switch!

    In comparison, PF wise, how do the now unavailable linear PSUs compare? I am thinking of those for routers etc - the linear PSU that my DrayTek ADSL router - circa 2006 - had, weighed a "ton".

    Clive

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  • 0 in reply to mapj1

    That certainly makes sense.  There cannot be much inside the LED string one since it is featherweight and that includes an optional 6 hours on 18 hours off option via the push-button switch!

    In comparison, PF wise, how do the now unavailable linear PSUs compare? I am thinking of those for routers etc - the linear PSU that my DrayTek ADSL router - circa 2006 - had, weighed a "ton".

    Clive

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  • 0 in reply to AncientMariner

    Quite variable - most iron core designs had a 50hz transformer and then a rectifier on the secondary, If the rectifier fed directly into a smoothing cap the same 'conduct at the crest of the waves' applies but the series L & R of the transformer conspire to limit the peak current and to spread the conduction angle, especially the rather thinly optimized transformers found in 'wall wart' adapters,   so the conduction angle is longer 30-40 degs perhaps, and the diode switch off is less snappy and RFI prone.
    so PF better than some,  but not great.

    Mike

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