yuchween

I have a question on my thesis topic-Rectifier with unity power factor.
I am a student from studying in Australia and i have taken up a thesis topic regarding Rectifier with unity power factor.

My supervisor said that i am able to build a rectifier circuit using power factor correction ICs. I have surfed the internet about PFC ics but i still dont really understand how to use this IC.

Is there any sample circuit that i can refer to please? and also what is a rectifier circuit with unity power factor used for? is it for a stable DC supply?

I hope i can have a reference circuit on this rectifier with unity power factor.

thank you in advance.

Papabravo

It would help if you told us which IC you were talking about.

I'm confused about something. After you rectify an AC waveform isn't power factor an irrelevant and meaningless concept?

kchriste

Yes, but it is VERY meaningful at the input to the rectifier due to the way that a capacitive input rectifier circuit loads the AC line. Basically it's because the rectifiers only draw current at the peak of the AC waveform.
For more:
http://www.pleo.com/MICROMETALS/mmt7.htm

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ronsimpson

Papabravo, Power factor correction is not like 4 diodes and a cap. There are 4 diodes and no cap. The current and voltage waveforms make the power supply look much like a resistive load.

Yuchween, Every company that makes PFC ICs has application circuits, demo boards, examples circuits etc. Look for them!

Papabravo

I never said anything about 4 diodes and a cap. Give me some credit for knowing what power factor correction is. The power factor is just the cosine of the phase angle between the voltage and current waveforms. A cosine of 1 means the phase angle is zero. This happens in an AC circuit when the source impedance and the load impedance are both resistive. I believe there is another case where it happens and that is the case of the conjugate match.

The point of rectification is either to eliminate or invert the neagative half cycle. If you're not going to turn the rectified waveform into DC then what is the purpose of just a rectifier with power factor correction?

kchriste

If you follow the link I posted above, you will see this simplified circuit:

It is a pre-regulator that improves the power factor of a typical switching power supply. For simplicity, they don't show the relatively small capacitor that would be there just after the diodes to bypass the high frequency switching currents from the boost converter. I'm pretty sure this is what the OP was asking about.

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Papabravo

What is the purpose of the MOSFET with the floating gate?
Why would a schematic not show a component like a bypass capacitor?

The halfwave rectifier and the large(?) filter capacitor C1 I understand.

So the inductor in combination with the capacitance of the brigde rectifier provides the conjugate match for the AC source, and brings the the power factor closer to unity. Did I get that right?

What are the component values?

ronsimpson

Papabravo,
The gate of the FET is not floating, it goes to a IC that PWM the gate at about 100khz. “simplified circuit”

The DC output is higher that the peak AC from the power-line. Normally I set the DC out to 400 volts DC because that is higher than the maximum peak from 220 AC. This is a boost supply!

The IC monitors the line voltage and line current and forces the current to a sign wave in phase with the voltage.

Hero999

It's a bit more complex than that, non-linear loads can reduce the power factor because they draw the current in funny waveforms which can cause all sorts of problems in transformers and affect other loads like induction motors. In a non-linear load the current isn't proportional to the voltage, in the case with the rectifier it's only drawn when Vin>DC bus voltage which means that it's drawn in pulses.

As ronsimpson says, it's a boost converter that converts it to a voltage just a bit higher than the main's peak voltage. There's no filter capacitor on the input to the converter (or a very small one to filter the noise) and the switching regulator is carefully designed so as the voltage is increased the current drawn remains proportional to it.

Of course, nothing is perfect, the diodes will only turn on when the voltage gets above 1.4V and the boost converter probably won't work below a couple of volts so there will be still some harmonics but it won't be as bad as just a bridge rectifier plus capacitor.

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Papabravo

Thank you both for those clarifications. They were essential to understanding what you were trying to do. The thing I'm not quite clear on is if the AC input has the voltage and current in phase with no load. Placing the load on the AC source then causes the voltage or current to lead(lag) which you then can adjust, OR are the voltage and current intially out of phase, and by manipulating the MOSFET you bring the inductor in and out of the circuit in such a way as to BOTH adjust the power factor and capture the energy in the inductor's flyback. Have I got this right now?

BTW - where is the OP in all of this?

mneary

When harmonics are present in the current, the power factor is less than 1 regardless of phase. What the PFC chips do is shape the current drain back into a simple sinusoid matched to the input voltage.

To have a power factor of 1, the current must always be V/R (where R is the rms value of V divided by the rms value of I) at every point throughout the sine wave. You're probably thinking that a power factor of 1 can be achieved by just matching the peaks.

Papabravo

I wasn't actually thinking anything. I'm just trying to get my arms around what the gazintas and gazottas look like. How it actually does that is the next level problem.

The reproduced drawing from the link given by kchriste is very suspicious. Why can't the people with the wbsite just make an actual drawing. Why do things look like very old photcopies of original hand drawn schematics and graphs.

AND we still have no component values in sight, nor any clue to what is being sensed and how the MOSFET is controlled. I'm trying to learn this stuff as we go along and frankly the explanations for the initial conditions and the results are just a bit short of clear. If I were an investor and this was an elevator pitch, I'd be running for the door clutching my wallet with both hands in a death grip.

kchriste

A quotes from the site:
Sorry if it confused you. I thought the introduction summarized the principle of using a boost converter for PF correction quite well.

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Papabravo

It may be true for calculating losses, but that is putting the cart before the horse. I want to understand what the thing does and how it does it. I want a complete circuit with component values. I want to understand the properties of what goes in and what comes out.

Just because a claim is made in internet article doesn't mean that it is true. I don't think the paragraph that you quoted makes much of anything clear. In very general terms is makes two assertions neither one of which can be verified because we have no component values and we don't know what the rest of the circuit looks like.

I guess this is like ESP, either you get it or you don't. Complete schematic with component values, or complete loss of credability -- which is it?

Hero999

I've got a semiconductor sample kit for a power factor correction IC on my desk at work. I'll post the datasheets here next week.

I don't need the sample, if anyone wants it and are happy to pay shipping, I'll send it to them.

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