3phase PFC

Circuits with the word "fuzzy" in them rarely work as well as ones without it.

IR may have one.

This may be a case where "use a PIC" really is the right answer. In a converter of this size, the cost of a slightly more complex controller section may be worth it. If you have a part that can remember what happened a full cycle ago, you can do a much better job on bringing the out of phase and harmonic content down to zero.

Hi,

My three phase source (a brushless generator) has a wide output voltage range, rectified from 60VDC all the way up to 360VDC depending on the RPM, the power output scales fairly linearly over this range, from only

60watts at 60VDC, all the way up to about 1300watts at 360VDC.

I would like the output of the 3phase PFC boost to be about 400VDC or so over the whole input voltage range. Is it possible to maintain a high efficiency (96%+ ideally) over this whole range using the same boost inductors, or would it be better to switch boost inductors depending on the input voltage.

I am trying to size the PFC boost inductors for this using this tool:

For the same output voltages, it suggests a high inductance inductor at the low voltage input, low power output, and then a smaller inductance inductor at higher input voltage, higher power output.

I'm not sure how to size this inductor for the whole voltage range.

cheers, Jamie

already has a

PFC at all? A

Hi,

It is more efficient to use a PFC stage rather than a boost stage as the generators RMS current is lower with PFC. This means more energy can be extracted from the generator for the same work input and the heat in the generator will be lower for the same output power level.

cheers, Jamie

Hi,

Since only small output current is required at low input voltages, but this requires a large inductance, and then large output current is required at higher input voltages, which requires a smaller inductance, can these two inductors be put in series, and the high inductance one will saturate when the input voltage is high, or would it require a bypass current path (ie. relay) to short out this inductor?

Here's a boost circuit with what I am thinking of:

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If the second inductor doesn't saturate then the inductance will be too large for high input voltages, so maybe the switching frequency could be decreased.

Here's another circuit I was thinking of, it uses two switches, S2 runs at low input voltages to get the large inductance, and then S1 runs at higher input voltages with only the 300uH inductor.

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Here's one more using two mosfets to short out the large inductor for decreasing the inductance at high input voltages:

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cheers, Jamie

MooseFET wrote:

already has a

PFC at all? A

You might want to crunch some numbers on this for the 3ph case, while examining other methods of power factor correction that are available.

What you should really be considering is deliverable power vs your requirement and permissible temperature rise.

PFC requirements are primarily driven by legislation based on infrastructure requirements, not by end-use economics or concerns about efficiency, as such. Your source is free from that major influence.

RL

This is intuitively backwards from your real application requirement. For a constant power drain, current will increase as voltage reduces. This argues for an inductor that handles higher current at low voltage input, while still having an inductance that is still high enough to be practically functional (not discontinuous) at higher input voltages.

For the same power transfer, the low voltage inductor would have a lower inductance, if you were actually switching between optimum part types. The optimum parts have energy storage requirements that increase at lower input, but this energy storage is related to the square the current.

If you are considering altering RPM to adjust for power drain (suggested by your lower current lower voltage reference), you might just as well do so to crudely regulate the generator output voltage instead, considerably reducing the work of downstream regulators.

RL

Being able to work over that range pushes the cost of the inductors up a lot. It seems to me that you really don't need a good PFC so much as a good booster design.

You may be better off with a two range design. With low input voltages you are beyond where the simple booster works well and into where a transformer looks better. Perhaps you could share the inductive elements. I'm thinking of something like this:

======================== -----))))))----+-----))))))-----+------>!--- To load ! ! !!- V ----!! Q1 --- D1 !!- ! ! !!- GND ---!! Q2 !!- ! GND

Q1 is only used when the voltage is low. At high voltages Q2 is used. D1 is needed to keep reverse current out of Q2 when Q1 is on.

Try changing the frequency with the input voltage. This reduces the range of the inductors.

- All the controller outputs were labeled for ph1 - effectively shorted together

- All modulator references had the same phase angle. (After correction, your model will not work with alt solver, so you'll have to switch back to the normal solver.)

- There was no current feedback. This chip expects to see negative-going current sensing signals on the Mout pin of less than

1V1 peak.

- Once running, you have to load it to get current waveforms.

- Don't expect to see attempted regulation prior to the 6.5mSec timestamp, as the inrush and overshoot doesn't clear prior to that time, even with a 1KW load. Also don't expect 60Hz performance at

600Hz.

The asc file is reworked (and still needs much more attention to run properly as a simulation) at:

With simulations that take as long to run as this one does, doublechecking your work, your ideas and your aims becomes important, prior to pressing the GUI's 'run' button. Make sure you've got plenty of drive space. The raw files mount up at the rate of about 150Meg per millisecond of active converter runtime.I'm always surprised that they run at all.

The Vienna rectifier is about the most expensive,complicated and lossy version of PFC correction that a body could have picked.

RL

Thank you for modifying the sim to work! :)

What circuits would be better than the vienna rectifier for 3phase PFC? I found two other ones:

I would like to generate +220V and -220V rails, and use them to generate splitphase 120VAC sine waves using halfbridges and two LC filters. What would be the best 3phase PFC circuit to generate these two rails, or if not using 3phase PFC, could a boost circuit off the 3phase 6diode rectifier generate a positive and negative rail?

cheers, Jamie

Hi,

Can the inductors also be on separate cores? For a 60VDC to 380VDC input voltage range, with linear power increase with voltage up to

2kW at 380VDC, and 60watts at 60VDC input, what inductances would be a good starting point for these two inductors?

cheers, Jamie

I'm using an FPGA, so can control the PWM frequency (increase it for lower input voltage), With this method and the linear graph of input voltage over output power (required output power is low with low input voltage) would it be possible to use a single stage boost design and still have high efficiency over the whole range? Or is it a good idea to use the two stage boost as well as variable frequency? Easy to do in an FPGA, so the only real variable is finding good inductances to use I think!

cheers, Jamie

already has a

PFC at all? A

Yes and no. Energy efficiency improvements do have economic payback.

I guess this is like an autotransformer?

cheers, Jamie

already has a

PFC at all? A

Depends on whether you're designing hardware to cook for ten thousand, or just two.... Or maybe even just planning a menu.

RL

Yes that is the winding polarity I was suggesting. You still need a physically larger inductor to be able to do it.

Not combining the inductors may be better from the point of view of needing two smaller cores in place of one large one.

LOL. too true. Im sure they could have added a few more diodes if they had tried just a little bit harder

It sure can. use a pair of boost converters, with a half-bridge as the switches, the center-tap of which becomes 0V. So its:

---nnnnnn---+--->|-----+--- V+ | |

3ph X [Cbig] BR | | +--0V------+ | | X [Cbig] | | ---nnnnnn---+---|

Hi Terry,

Thanks for the dual rail boost! ;)

Is this an alternate circuit to the vienna rectifier for 3phase PFC? Do you have a schematic of it?

Here's an ltspice sim from Larry Carroll on the ltspice yahoo group, it implements 3phase PFC with one cycle PFC control!

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backup.zip"

cheers, Jamie