Need help Understanding PFC in specific SMPS diagram

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moretti1993

Joined Sep 11, 2016
6
I need help understanding how the PFC component works in the attached diagram (HRP-75_PSU.pdf)
The SMPS diagram is build up around a chip called TEA1751, containing flyback controller together with PFC regulation.
I've read the datasheet multiple times and i still dont get the concept.

The PFC component is made up using the following pins

VIN_Sense
PFC_AUX
PFC_DRIVER
VO_Sense

Here is what i understand and please correct me and fill in my gaps:

VIN_Sense acts as a reference, it senses the pulsating DC input. It makes up the voltage reference which the current has to phase allign with.

PFC_AUX senses when the input current is at a valley, in which case the inductor will be demagnetized. Whenever the inductor is demagnetized the PFC_driver will start sourcing in order to turn on the PFC mosfet.

Im not sure how the VO_Sense works

As you can tell i have a major gap in my understanding of this concept. and it would a tremendous help if somebody could explain to me how PFC works in this particular setup
 

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ebp

Joined Feb 8, 2018
2,332
The first thing to understand about this type of power factor correction is that it isn't making phase angle correction as such. Instead, it is making the input current to the power supply sinusoidal so that the load on the AC mains looks like a resistance. The current will, in fact, be in phase with the voltage. In a normal rectifier circuit, there will be current "spikes" at the peaks of the AC voltage and zero input current through the rest of the half cycle - current only flows when the instantaneous input voltage is higher than that on the filter capacitor.

There are two conflicting requirements that arise because a switch mode power converter has a "negative resistance" input characteristic, that is, the input current must fall if the input voltage rises - the slope of a plot of this is negative. The requirement to keep the input current sinusoidal and in phase with the voltage means that the instantaneous input current must be directly proportional to the instantaneous input voltage, but the negative input resistance characteristic of a switcher means that the average input current must be inversely proportional to the average input voltage.

The power factor correction circuit will produce a reasonably accurately regulated DC voltage at its output, which is what the VO_Sense is for. It is the input to the "error amplifier" that controls regulation of that voltage. The voltage is "sampled" by a resistor voltage divider, often with one or more "frequency compensation" capacitors in the network. The PFC converter is essentially an elaborate boost converter - the output voltage is always higher than the instantaneous AC line voltage. Typically the DC output voltage would be between about 380 and 400 volts for a design intended to operate with AC mains voltages of nominally 100 to 240 V RMS. Most PFC converters will also work perfectly well with DC input voltage within the equivalent range.

I don't know if any of that helps.

Without spending a lot of time reading the datasheet, it sounds from your description like the PFC stage operates in "discontinuous current" mode - that is, the inductor current is allowed (forced) to drop to zero with each switching cycle. This has merit in terms of efficiency because the FET current starts at zero and ramps up, so switching losses both in the FET and the output diode of the PFC converter are minimized. It is also usually advantageous in terms of radio frequency interference reduction.
 

ian field

Joined Oct 27, 2012
6,539
I need help understanding how the PFC component works in the attached diagram (HRP-75_PSU.pdf)
The SMPS diagram is build up around a chip called TEA1751, containing flyback controller together with PFC regulation.
I've read the datasheet multiple times and i still dont get the concept.

The PFC component is made up using the following pins

VIN_Sense
PFC_AUX
PFC_DRIVER
VO_Sense

Here is what i understand and please correct me and fill in my gaps:

VIN_Sense acts as a reference, it senses the pulsating DC input. It makes up the voltage reference which the current has to phase allign with.

PFC_AUX senses when the input current is at a valley, in which case the inductor will be demagnetized. Whenever the inductor is demagnetized the PFC_driver will start sourcing in order to turn on the PFC mosfet.

Im not sure how the VO_Sense works

As you can tell i have a major gap in my understanding of this concept. and it would a tremendous help if somebody could explain to me how PFC works in this particular setup
The most common type of active PFC front end is an input circuit with no reservoir cap after the rectifier. previously - the main chopper draws current from the reservoir, which has to be topped up on every sinusoid peak. From a power factor point of view - those blips of current are pretty nasty. The reservoir cap is actually a noise filter and usually under about 1uF. the rough DC is fed to a flyback boost converter - the pulses of current it draws from the rectifier vary and are more or less proportional to the sinusoidal voltage value.

A dedicated chip usually means you can get data sheets and application notes that explain it all.
 
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