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Old 29th September 2006, 03:56 PM   #1
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Default Power Factor Correction

How many of you worry about power factor correction when you design your power supplies?


I searched, it looks like some people have included it, but not too many. But that could be skewed by who is posting designs and who is not.
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Old 29th September 2006, 04:43 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
are you referring to inductive loads on the mains?

I have previously tried to start a discussion on whether a transformer feeding capacitors is resistive or not.
Very little response from anyone. The two of us were just guessing.

I shall be listening to your responders.
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Old 29th September 2006, 04:55 PM   #3
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Yes, basically to compensate for that, and for either a linear supply or a switching supply.

If I was designing a power supply at work, I would probably include some sort of PFC. Part of that might just be because customers would expect it, whether it was really needed or not.

I guess for me at home, if my 300W load (amplifier) costs me the same as a purely resistive 330W load because I didn't include any sort of correction, I don't care (the circuit would have been simpler, parts cost lower, and really, the change in usage cost is minimal). I might care if my 300W load costs me the same as a 550W load.

I'm just curious what other people are doing, and why. Maybe someone has actually measured. Maybe someone will claim a PFC for the mains makes the amp sound better.
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Old 29th September 2006, 05:00 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I can gather from your reply that you are in the camp that says a transformer is a phase lag load on the mains, irrespective of whether it is 0degrees phase lag or not!
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Old 29th September 2006, 05:06 PM   #5
poobah is offline poobah  United States
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People avoid PFC because of cost and complexity.

Capacitors on the secondary reflect as inductance in the primary and vice versa. Is is counterintuitive at first, capacitance allows large secondary currents, these currents create the counteremf that appears inductive at the primary.

Complex impedances merely "flip" over the the real axis; so things that are primarily resistive stay that way. Understanding all this thoroughly is best done with continuous sines as well. It all holds true for short conduction angles etc... but quirky and will lead to headaches and crossed eyes.




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Old 29th September 2006, 05:09 PM   #6
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I don't believe there is such thing as "0 degree".

Once you get away from a schematic and into the real world, everything presents some sort of reactive load. And I have a hard time believing that an inductor (the primary winding on a transformer, in this case) can exist that does not behave like an inductor. But I could be wrong.

How clean would the AC supply need to be in order to accurately measure the phase shift, I wonder?
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Old 29th September 2006, 05:22 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
I am guessing that the transformer & PSU presents a substantially resistive load to the mains. I could well be wrong.

If the load is inductive, then what does the meter read?
What is the heating effect in the supply cables and generators?

If the consumers had to pay for the real consumption of inductive loads and the suppliers were paid for what we really consume, then why do we have to fit PFC to a lagging load? To reduce the income of the suppliers!
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Old 29th September 2006, 05:36 PM   #8
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I was also wondering about the power factor also. I "played"
a little with trying to correct the power factor of my refregirator.
Using my Fluke meter I was measuring about 10% less current
when the a 4uf cap was shunt across the line. As I stated I was
"playing" and there would probably be a far better method to do this. But back to say a large amp. If a large amp could benifit
from from some power supply correction by something simple like a cap across the primary of the transformer wouldn`t this also
kill some line noise at the same time?
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Old 29th September 2006, 05:41 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
a cap across the primary becomes a load.

Would it be a cap in series with the load?

Ahh, reminds me of a DC blocker.
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Old 29th September 2006, 05:45 PM   #10
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Adding a capacitor across the primary becomes a capacitive load in parallel with the inductive load, which counters the phase shift the inductive load places on the primary.
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