critical conduction mode PFC

I'm learning about PFC converters as a wish to make a 100W PFC front end as part of a home made power supply I'm building. The web has given me a some insight in building such a power stage. Now, I have found the preferred topology for my 100W PFC converter is the critical conduction mode. But what I don't understand is how do the authorities allow such high frequency high peak currents on the AC line originating from a critical conduction mode PFC converter ? Some analysis shows;

I_pk = 2*sqrt(2)*P / Vac_rms

where Vac_rms = RMS of line input voltage P = average input power I_pk = peak current through inductor (or line)

The current of the inductor (or line) is a high frequency triangle wave with a peak that follows the line voltage AC cycle and the RMS of this waveform is;

I_rms = I_pk / sqrt(6) { substituting for I_pk yields } = 1.1547 * P / Vac_rms

So I_rms is 1.15 times higher that it would be for a resistive load of the same power. Where is the PFC ?

Adam.

translation: filter the crap out of it. The advantage of a Discontinuous Conduction Mode boost converter is that it can be made to automatically give PFC behaviour (IIRC by keeping Ton constant). The disadvantage is the current is a hairy-arsed mess. for a low power level like 100W, its pretty easy to filter the DCM mess to the required level. At 1kW or so it gets a lot trickier, and above that level (ish) it tends to be done as Continuous Conduyction Mode, where the inductor current is sinusoidal with small triangular ripple (IOW a straight-line approximation, rather than a bunch of right-angled triangles whose average value is sinusoidal)

Cheers Terry

If done properly a PFC with CrCM (Critical Conduction Mode) can be easily made to ZVS (Zero Voltage Switch) and yield low EMI. The only problem with this topology is the large AC current in the boost inductor and the input filter due to the higher peak currents but not as bad as DCM. As mentioned, it is the best choice for PFC

Another thing I found is that DCM results in lots of AC flux in the boost inductor, which means a great deal more core losses and proximity effects compared to CCM. Is there a reason why a 100W PFC is not normally built using the CCM approach ? The costs appear to be similar between the two types.

How is such heavy line filtering implemented without compromising the PF ? I mean, if a large cap put after the rectifier then the that'll add harmonics in the line current.

I read in sci.electronics.design that Harry Dellamano wrote (in ) about 'critical conduction mode PFC', on Sat, 30 Apr 2005:

Use tape instead of round wire?

Hi John, Not sure it will help. Usually use Litz, does good job on Skin Effect but not Proximity Effect. Normally need about 300uH and 2.5A peak at 100KHz. I usually use a lot of flux density to keep the turns down and AC effects on windings. Takes about 40 turns with a gap. Does not foil (tape) still have Proximity effects? Regards, Harry

I read in sci.electronics.design that Harry Dellamano wrote (in ) about 'critical conduction mode PFC', on Sat, 30 Apr 2005:

I don't know that it's better than Litz, but the geometry of any proximity effect would be quite different. You could either search for a paper on the subject, or try it and see.

Since the proximity effect is due to the magnetic field, it might not be a bad idea to try reducing it. Presumably there is an optimum balance between ordinary resistance (goes up with the number of turns) and proximity effect (goes down with the number of turns because the induction goes down).

The problem with tape winding is the large number of effective layers formed by the turns count. Usually only practical at higher frequencies when turns count is low - approaching unity - or the current has mostly LF or DC components.

Critical conduction currents aren't right-angle triangles, nor is the current discontinuous. Critical conduction mode is, by neccessity, frequency-varying, which can make EMI control a little tricky. Lowest conversion frequencies correspond to maximum power.

RL

I read in sci.electronics.design that legg wrote (in ) about 'critical conduction mode PFC', on Sun, 1 May 2005:

How is the number of layers a problem?

The near-by current sheets in the conductors have the same immediate direction, so adjacent and buried layers will have current crowding towards the sheet edge (high Rac/Rdc ratio), resulting in both higher leakage fields and power loss, for AC. This is one reason the shape is not used often in power coupling transformers, if turns count strays far from unity.

Placing the sheets at right angles to the bobbin axis, as is formed in a flat spring coil, doesn;t correct for this effect, though helically formed parts are marketed as being superior in some way - their turns count is 'fortunately' limited by mechanical fabrication constraints.

Adjacent litz twisted conductor bundles have adjacent turn currents flowing effectively at right angles to each other, though the averaged direction is still parallel in the group.

Practical measurements were attempted in conjunction with research into the use of capacitor stock foil winding methods for the fabrication of inductors -

"Analysis of ac Resistance in Aluminium Foil, Air Cored Inductors" N.Williams abd C.Pollock Proc. of 8th European Conference on Power Electronics and Applications, 7-9 September 1999

Another article deals with the same structure for superconducting material -

"Air-core transformer for high frequency power conversion" Cheng, Chan abd Sutanto. Same conference

I expect there are other references, but just the general discussion of transformer layering and sectioning will show the principles involved (admittedly buried within a host of others), even in uncoupled structures.

Past my bed-time.

RL