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Old 9th July 2009, 03:54 PM   #1
luka is offline luka  Slovenia
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Default Power Factor Correction - PFC with MC34262

Hello again,

so I have just finished building it, I used circuit from datasheet, values are not the same, but close enough I think.

It works great, I have only tested it on 234Vac [measured it]
The way I have resistor network set to set output voltage, I get 387Vdc.

I did a bit more quick testing and this is what I got:
Vin: 233.5Vac
Iin: 2.046Aac

Vout:387Vdc
Iout:1.178Adc

Eff: 456w/477.74w = 95.5% [Good? I think it is]


And this voltage holds up as far as 760 measured watts on output. I didn't go for more because several reasons, like rectifier diods for input is probably only 4A, and for output I used two Mur4100, coz I couldn't get mur460's. This heat up pretty fast at max power tested, say within 10-15s you can't hold them[I guess about 55C].

For current sense I have 0.22R resistor and I get peak drop on it 1.9V @ 760w.

Now here is what I don't really understang, this voltage is on PIN4 of IC[didn't check this yet, I looked on resistor directly]. But as I understood (or not) that voltage on that pin should be max 1.5V. So can you all explain what is going on here, or what does that text in PDF mean , since I get more then I should [again I will have to check this right on PIN4 of IC]

So Steve[I just found yours, with 33262 ], this is right up your ally, also anybody else, please explain if you can

Edit: So only "big" diffrence I have from datasheet is, I have 1.2M and 12k resistor network to "see" input, and it should be 1.3M and 12k if you look at last schematic, 450w version
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Old 9th July 2009, 09:32 PM   #2
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On the Datasheet I'm looking at (From ON semi) It says that the absolute max voltage on pins 1,3,and 4 is -1.0V to +10V.

Lower down it states that the current comparator range is 1.3-1.9V,with a note that it is measured with Vfb=0 and Vpin3=3.0V

They use 0.1ohms on the smaller designs,and 0.05ohms on the larger 450W design.

Swap out your 0.22ohm resistor for something smaller.
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Old 9th July 2009, 09:41 PM   #3
luka is offline luka  Slovenia
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hey

Yea, but Vfb can't be 0, since there is output voltage and it is not 0...if this is right thinking...

Vpin3=3v... probably close, since my line is 233Vac loaded..

But I don't see anything, that I would get from going down to 0.11 [2x 0.22] or lower
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Old 10th July 2009, 10:24 AM   #4
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To my understanding, since you have limited the peak current, you may expect higher switching frequency, which is useful when you decide for relatively low core saturation inductor and can afford some more switching losses and/or better mosfet.
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Old 10th July 2009, 12:28 PM   #5
luka is offline luka  Slovenia
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but how is peak limited if it is not, not in a way if I load more and more the pfc. Yes, current does track input voltage, but I can't say it is limited fully, it is limited so that average looks like voltage waveform...maybe this is it, but it sure is not fully limites, as I could make short or way overloaded and something would go to hell
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Old 10th July 2009, 01:30 PM   #6
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When drop on current sense resistor reaches some value, the current sense comparator switches off the mosfet, isn't it cycle-by-cycle peak current limitation? The inductor current swings between temporary peak and zero.
The peak current limit is a function of input and output voltage of course, so there may be no long-term peak current limit.
Since the peak current is limited further, the result must be higher duty cycle (for the same on-time) and therefore higher switching frequency.

That's my understanding of critical conduction mode PFC, however datasheet says:

Quote:
Set the current sense threshold VCS to 1.0 V for universal input (85 Vac to 265 Vac) operation and to 0.5 V for fixed input (92 Vac to 138 Vac, or 184 Vac to 276 Vac) operation. Note that VCS must be <1.4 V.
Which is unclear to me, because it would indicate, that peak current never exceeds 1.4V/Rsense.
I may be wrong on some things here, maybe others could exlain more. Markus? Eva?
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Old 10th July 2009, 02:10 PM   #7
luka is offline luka  Slovenia
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Quote:
Originally posted by darkfenriz
Which is unclear to me, because it would indicate, that peak current never exceeds 1.4V/Rsense.
I may be wrong on some things here, maybe others could exlain more. Markus? Eva? [/B]
Same here, but:

Quote:
Abnormal operating conditions occur during preconverter
startup at extremely high line or if output voltage sensing is
lost. Under these conditions, the Multiplier output and Current
Sense threshold will be internally clamped to 1.5 V.
Therefore, the maximum peak switch current is limited to:
Ipk(max) =1.5 V/R7
That means to me, that only if sense is lost [don't see how that would happen, unless FB network would die on me[again, don't see how]] OR extremely high line [I would guess 268Vac][also don't see this happening], but by then I wouldn't care about this PFC

So I guess, current is not limited in sense of short circuit[well maybe if short, but not in 150% overload]. But its all good, I just don't know what is upper limit for it, since I am about 800w limited by input rectifier, only 2 diods in boost section, and then fet would come into play[but I can have 2 of them ]
Core is only unknow at this point, when would it had enough.

Its 2.2cm^2 cross section, and my input will not be below 230v, so I guess this PFC will go in front of my smps, to power ClassD amps. Should be all way too much power for one small room
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Old 9th October 2009, 03:26 PM   #8
luka is offline luka  Slovenia
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Default Update

While testing my smps, that I will use to power amps, I used PFC in front of smps. The way it was set, it was current limiting at 800w @ 362Vdc (instead of 388Vdc) with 0.22R resistor for current sense.

This was reason I couldn't get more then 770w @ 163Vdc from smps (+/-96V idle= 110w)

So I added another 0.22R as planed before, now I have no problem getting 1150w out of PFC, 387.7Vdc on 130R dummy load, this should be enough
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Old 9th October 2009, 06:14 PM   #9
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Luka-

Pulled my original MC33262 PFC off the shelf last night and found out why it belw a 10A input fuse- I had the MUR 8100 output diode put in backward, creating a mess. Will correct that soon and retry it in near future. I expect much better results. Will still have to go with 250V output caps, for some safety headroom.

Steve
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Old 9th October 2009, 07:02 PM   #10
luka is offline luka  Slovenia
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I used 2x mur4100 and they can't take the power, they are good for few seconds, but get hoooooot -> high power sucks, if you can't get some good elements, I hope I will be able to get 4x mur460 instead, I would like to stay free air cooled...

maybe there are some better diodes I could use? or just more of them?
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