Power Factor Correction Help

[JimboJones21]

Hi All,

I'm confused with pf. I have a 50W switching power supply that I measured a pf of 0.5.

Does that mean that mean if I connect a load to it it will require double the amount of current to run the device, so it will will be operating at 100Watts?

This doesn't seem right. Someone set me straight on this please!

 

[DHambley]

Jimbo,
Can you tell us a little about the circuit? Is the front-end a standard boost PFC or a transformer-rectifier?
DH

 

[JimboJones21]

This is what I did:

I connected a switching power supply from digikey (part number 102-1337-ND) to a bank of LEDs drawing about 36Watts. I then connected it to an HP universal power supply and it gave me a pf of 0.5.

I'm trying to understand what the pf factor means. Does it mean my LED light is drawing double the amount of power to get 36Watts, so its actually drawing 72Watts?

 

[Skogsgurra]

It just means that your current has a bad (distorted) waveform. So that its RMS value is higher than its AVG value.

The reason is that the front end rectifier charges the smooting capacitor in the power supply during peak voltage and not during the whole AC cycle. So the diodes conduct current only a fraction of the cycle. That makes "fast delivery" necessary, so that the peak current is a lot higher than it would be if current were sine formed.

This results in a high VA number and a low W number. Since PF is defined as PF = W/VA, you get this low number. If you, instead, had been looking at cos(phi1) - where phi1 stands for phase angle of fundamental (50 or 60 Hz) you would probably get close to unity.

Gunnar Englund

http://www.gke.org

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