Single stage (PFC) boost converter

[kathmandu]

Hello,

I need a power supply for a LED projector (19V/5A) and I thought of using a 230V/12V AC toroidal transformer (for isolation and low voltage supply) followed by a bridge rectifier and a classic PFC (boost) converter.

Here comes my question: could I get a good enough output stability (and voltage ripple) using this single boost stage? I don't know the actual voltage ripple requirements but I think the LED projector already has some further DC-DC converters inside thus it could manage some decent voltage input variations (ripple).

Thanks in advance for any oppinions.

 

[kathmandu]

Could anyone suggest a good PFC controller IC (DIP socket) for this application?

I guess I'll choose the CCM (average current) topology but there are so many ICs available and I don't know what's the best choice.

 

[brushhead]

Well I have used the Fairchild FAN6982 with decent results. Easy to use and will drive a reasonable sized MOSFET directly. Take care on layout though as this can screw up the control loop.

 

[kathmandu]

It looks like a good option, indeed. But I've just discovered a featured IC (Fairchild ML4800) which contains an extra PWM module to further regulate the PFC output.

Do you have any experience with it?

 

[treez]

i wouldnt bother with the 50hz transformer, just use a pfc chip after a mains rectifier. Its only 60W so for that i woudl use a boundary conduction mode pfc chip.....texas.com do them. Also fairchild and many others probably.

 

[kathmandu]

Actually, the rated power is 90-100W (19V/5A) but I want to make a PSU as "clean" as possible (power factor, EMI). My entire home runs on solar/wind hence I want to protect the inverters from any avoidable surge currents.

Of course, 100W it's not that much but it counts. That's why I've decided to go with CCM/average current topology (lowest peak/rms input currents).

Btw, I thought of using a LF transformer as I have a lot of low voltage (< 200V) Mosfets available, toroidal LF transformers have a pretty good efficiency (the extra space not being a problem) and.. I don't like working with (or debugging) high voltage circuits. .

Anyway, seems like there's no chance to get a decent output voltage stability using a single PFC stage. I'll surely need a further stage (buck/boost/flyback) to get a small ripple output voltage.

 

[kathmandu]

I have another question though: what could be the influence of the LF transformer on PFC circuit??

Do I have to get rid or make use of? (its mutual inductance, by example)

 

[FvM]

Transformer leakage acts as additional PFC inductor.

 

[kathmandu]

What about the transformers' primary/secondary current being 90 degree out of phase with the corresponding voltage?

A PFC circuit is supposed to simulate a load current (transformer primary) in phase with the mains voltage.

Do I have to tweak the PFC driver (by changing its input voltage reference or something) to get the right behaviour?

 

[kathmandu]

Any input on how the LF transformer could affect the PFC circuit operation?

Thank you very much for your time.

 

[FvM]

What about the transformers' primary/secondary current being 90 degree out of phase with the corresponding voltage?

It isn't. Voltage and current with a real load are basically in phase, on both sides of the transformer.

Presumed you have a real load on the transformer secondary, the primary power factor will be slightly inductive by working of the leakage inductance. That's not specific to driving a PFC.

As said, leakage inductance adds to the PFC inductance, at least if you don't have a filter between transformer and PFC. Increased inductance may be unwanted in some cases, depending on the PFC controller's operation mode.

 

[kathmandu]

Thanks a lot for your detailed answer. I did not find any other online informations about this subject.

What about the transformers' primary/secondary current being 90 degree out of phase with the corresponding voltage?

It isn't. Voltage and current with a real load are basically in phase, on both sides of the transformer.

I was trying to say that secondary current is being 90 degree out of phase with secondary voltage.

Primary and secondary voltages being in phase (as you just mentioned) that means the secondary current is supposed to be 90 degree out of phase with the primary voltage, too. That's why I was unsure if the PFC circuit should work.

 

[FvM]

I was trying to say that secondary current is being 90 degree out of phase with secondary voltage.

I don't see why this should be the case.

Simplified equivalent circuit of a real transformer is comprised of magnetizing inductance (shunt element), leakage inductance (series element) and ideal voltage transformation (doesn't affect phase). Current through magnetizing inductance is 90 degree out of phase, but it's only small compared to load current. It can be ignored for this analysis in a first order. Leakage inductance causes a load dependent voltage drop between primary and secondary voltage which has also 90 degree phase shift. But it's small compared to input voltage (e.g. 2 - 10 % at rated current), effect on secondary voltage is even smaller by phasor arithmetic.

As a result, transformer currents are almost in phase with voltage for resistive (real power) load.

 

[Easy peasy]

Using a boost stage to get UPF in your transformer can give you a reasonably flat and stable output, just need to use good size electro's on the output, using a separate boost choke is a good idea as it reduces losses in the Tx, stick to 20-35kHz, you'll be fine... UC3854 is an old chip but works well - easy to set up...

 

[kathmandu]

Thanks a lot for your input, I'm going to give it a try this weekend.

Btw, I saw that "real" PFC drivers (like UC3854) use the input voltage reference to shape the waveform of the input current.

What if I am using a regular boost driver (UC3843)? It should take care of voltage output and Mosfet peak current; could it be of any help to achieve a reasonable PFC factor?

(I'm asking this because I have no PFC driver available for this weekend)

 

[Easy peasy]

the 3843 is a complete waste of time, may as well go sailing or something...

 

[kathmandu]

Then I'll go sailing.. as I don't have a positive rail current sensor neither (or should I use a second winding on the PFC inductor?). I have a spare MCU though, I could simulate a PFC driver (found some Microchip app notes on this regard).