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Old 16th April 2007, 09:05 PM   #1
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Default Is PFC needed for higher power amps?

I am in the process to build my own 350W@4Ohms FETamp.
The supply uses a 500VA torroid, 2x55Vsec and 2x10.00uF caps.

I simulated the powersupply to check what I am up against.
I loaded the supply to draw a continuous 10Amps to simulate worst case conditions at the primary side of the torroid.
It turned out that inrushcurrents can be controlled by switching the torroid ON during zerocrossing of the mains and switching a 22Ohms resistor in series with the torroid for 0.5seconds.
This asumes that the poweramp is connected to its supply later than 0.5 seconds after power-up.
The 10.000uF caps are then charged at the peaks of the secundary sinewaves and also at the primary, causing momentary current peaks up to 30Amps.
Duration of these current surges is short (1-2mS at the peaks of the period).
This type of mains current loading is I beleive no longer permitted without any form of current shaping (phase correction PFC is not required in this case)

Passive PFC literature only mentions phase correction (V-I).
There is no literature that I could find that mentions if you need to correct this and how this can be done.
SMPS designs make this easier with ready to use components but asume a mains-AC to DC, then switch and transform/isolate down to the required lower DC voltage.
I want to stick to the traditional torroid in my powersupply design for several reasons.

Any hints whether current shaping is mandatory or suggestions how to implement this are welcome.
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Old 16th April 2007, 10:47 PM   #2
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Much equipment makes heavy loading to mains when switched on/off, yet no one thinks of PFC. Why bother?
If inrush current is a problem use a low resistance NTC thermistor.
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Old 19th April 2007, 07:51 PM   #3
godzich is offline godzich  Finland
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Hi,

Inrush current surge at power on has very little to do with power factor correction. Practically EVERY toroidal mains transformer above 300VA requires some sort of peak inrush limit to ensure that you do not blow the fuses, or even destroy your mains switch. This is true, independent of what size of caps you have on the output of the rectifiers.

An efficient way of doing this is to apply the mains wia a suitable rated (separately fused) resistor, that is bypassed with a relay that gets its coil power from the secondary, preferrably BEFORE the rectifiers. Use a large relay with some mass to ensure durable contacts AND a small delay of some cycles before the resistor is bypassed. The resistor MUST be fused separately with a fuse that will blow before the resistor, if for example the seconday of the transformer is shorted, by accident or by some other problem.

A suitable resistor is a ceramic power (wire-wound) resistor of some tens of watts, and with a resistance that places the power across the resistor at about 10 times higher than its power rating, at full mains voltage across it.

I know that a professional amp manufaturer once replaced a huge quantity of mains switches, claiming that they were defect, they never heard of such a thing than inrush current...

Best regards,

Christian
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Old 19th April 2007, 09:38 PM   #4
Spirtos is offline Spirtos  Greece
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Hi,
with a 500VA transformer you certainly need a soft start circuit, as Christian already said, otherwise the mains fuses will be blown up everytime you'll try to switch on the amp. There are a lot of them in the forum and a lot of information. Also you can try these:
http://sound.westhost.com/project39.htm
http://sjostromaudio.com/joomla/inde...d=63&Itemid=27

The PFC circuits are necessary in electronic equipments with lot of power consumption like large UPS units, in order to correct the power factor (cosφ). This minimizes the reactive power. You cannot limit the inrush current of a toroid with PFC technique.

Regards
Spiros
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Old 20th April 2007, 04:09 AM   #5
d3imlay is offline d3imlay  United States
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The implication here seems to be that given two xfmrs of the same VA rating, one a toroid and one with steel laminations, the toroid has a higher inrush current. At the same time it is acknowledged that filter caps are responsible for high in rush currents. This would seem to conclude that the toroid does not limit the current to the capacitors as much as the laminated core. Is this true? If not then the toroid must be saturating at turn on.
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Old 20th April 2007, 05:21 AM   #6
CBS240 is offline CBS240  United States
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Large solid toroid cores can have magnetic 'memory' in where the transformer has been cut off at peak flux swing, and turned back on at different cycle(phase), core saturation can cause large currents in the primary. If a resistor is placed in series with the primary for a time, it can limit current and re-set the core.
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Old 20th April 2007, 09:08 PM   #7
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Default inrushcurrent versus capacitor charge replenish current

I think everybody agrees on inrush current limitation, in whatever form that is established.

What I mena with PFC is that the caps at the secondary DC side (after the recitfier) will only replenish after when the secundary sinewave value is higher than the momentary voltage overe the cap plus the voltage drop over the rectifier.
Result is that a large charge replenish current flows for a short moment of time at the peak of the sinewave and consequently also at the primary side.
PFC is not only about phase correction but also about loading the mains with a sinewave shaped current, whilst charge replenishment shows momentary current peaks in excess of 30Amps momentarely.
A "switching"PFC can do this only because the output of a PFC stage is at a DC voltage that is higher than the rectified mains voltage. It is also in boost mode and can switch ON and OFF to have the replenish currents follow closely the voltage waveform.
In a "non switching"system this works only by placing in series an element that increases current gradually to spread out this current peak in time.
The element that does this is an inductance.
The value of this inductance however is at least as large as the torroid itselves.
So in practice, everybody forgets about it but these replenish current spikes are there all right.
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Old 20th April 2007, 10:49 PM   #8
Gordy is offline Gordy  United Kingdom
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Power Factor legislation varies from country to country, but usually applies only to professonal organisations using substantial power (such as factories), or to equipment manufactured in large quantities (such as Pioneer, Sony, etc.).
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Old 21st April 2007, 08:52 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
You are getting me confused. So I summarise, correct me where I have it wrong.
PFC in this context is not power factor correction of non zero phase angle loads on the mains.

We are talking about charging pulses drawn from the mains of capacitor input filters (rC or RC depending on the value of the source resistance before the smoothing caps).

It appears that our thread starter is wanting to investigate a choke regulated filter (LC).

In my opinion it will not catch on.
Too many of us are on solid state electronics that generally operate at lower voltages and this necessarily requires larger currents. When this is done the size of the inductor is so large it probably becomes the biggest and heaviest part of the whole equipment package. It becomes uneconomic and technically difficult to achieve for widely varying currents.

Looking briefly at existing switch mode supplies installed in lieu of a mains transformer (to save weight and space and cost), these currently charge a capacitor input filter and then the switch mode uses this stored DC to run the down stream equipment. Again it is afflicted with charging pulses and what happens in the switch mode cannot alleviate the charging pulses that try to keep the DC supply up near Vpk from the mains. All our computers, most of our small electronics and much of the retail equipment available have gone this route. We will never get them to reverse those commercial decisons.

I know of one British manufacturer that uses LC supplies for part of their power amplifier range. They are the dearest items they sell and extremely heavy.
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Old 21st April 2007, 11:23 AM   #10
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Default PFC

Andrew,

Yes, this is indeed the case.
This is also the reason why in most higher power torroid based
powersupplies (and large caps at the secondary side) only inrush currents are limited (mainly to protect switches and prevent fuses from blowing out too often).
We know the replenish currens can be high and not "PFC legislation compliant" but we simply ignore this.

For me this discussion learned me that the subject is known but aparently it isn't much of an issue.
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