# Thread: How Does APFC Work?

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## How Does APFC Work?

A bit of a stupid question but i've always wondered how an APFC circuit works and since the only devices i have that have APFC are either potted or are under a warranty (so if i was to open it up it would void said warranty) i decided to ask here.
Last edited by Magic Smoke; 08-20-2018 at 01:11 AM.

2. Originally Posted by Magic Smoke
A bit of a stupid question but i've always wondered how an APFC circuit works and since the only devices i have that have APFC are either potted or are under a warranty (so if i was to open it up it would void said warranty) i decided to ask here.

Edit: Just realized i misspelled the title.
This document from On-Semi might help, depending on your level of technical expertise.

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4. Hard to answer in a short forum post. If you can give me some time, I can try to type something up.

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Originally Posted by Magic Smoke
A bit of a stupid question but i've always wondered how an APFC circuit works and since the only devices i have that have APFC are either potted or are under a warranty (so if i was to open it up it would void said warranty) i decided to ask here.

Edit: Just realized i misspelled the title.
The common single phase APFC uses a boost converter to shape the input current waveform in a way that current follows the (rectified) input voltage in shape, which is a half-sine wave theoretically. An ideal sine wave current perfectly in phase with the voltage has a PF of 1.
A boost converter has an inductor (aka PFC choke) on the input side, a bulk capacitor on the output side, a diode which connects the inductor to the capacitor, and a MOSFET (aka PFC switcher) whose drain pin is connected to the joint of inductor and diode, and source pin is connected to the ground (which is primary ground since we discuss the APFC stage in PSU here). When the switcher is on, the input inductor stores the energy and inductor current rises gradually since the inductor prevents current from changing suddenly. When the switcher is off, the choke discharges its stored energy to the bulk capacitor and the load, which is the primary switching stage of the PSU, and the current falls gradually.
So by switching the PFC MOSFET on and off, we get a zigzag current waveform which approches the half-sine wave of the rectified input voltage. Then, we add an X-cap on the input side of APFC to filter the input zigzag current, making it a smooth, half-sine wave in shape. The PFC controller chip monitors the input voltage, inductor current and output voltage at the same time, and controls the PFC MOSFET accordingly using an analog or digital feedback circuitry, thus the input current is shaped and the output voltage is loosely regulated to 385V DC.
There are several methods to control the APFC which can be found in most SMPS textbooks.

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8. That's high level, but quite accurate. I was going to go into a bit more detail, but we'll see if Magic Smoke is satisfied with this.

9. Originally Posted by Travis
So by switching the PFC MOSFET on and off, we get a zigzag current waveform which approches the half-sine wave of the rectified input voltage.
Not sure I'd use the phrase 'zig-zag', but rather 'pulsed width modulation', which is then smoothed.

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Thank you for the explanation Travis (i'm not sure what else to write down here).

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A dip in Power Factor can attract operational losses and a penalty from electricity board, responsible for electricity supply. APFC Panels can effectively and automatically manage quickly changing and scattered loads along with the retention of high Power Factor.

12. Originally Posted by Mritunjay Gautam
A dip in Power Factor can attract operational losses and a penalty from electricity board, responsible for electricity supply. APFC Panels can effectively and automatically manage quickly changing and scattered loads along with the retention of high Power Factor.
The question was HOW does it work, not what does it do.

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In other words:

Unlike the rectifier circuit, the APFC circuit draws current and charges the bulk caps under active control. The APFC circuit can adjust it's current draw according to what, in addition to the rectifier current, is required to achieve a smooth sinusoidal input current waveform in phase with the input voltage. This also reduces power draw through the diodes or synchronous rectification mosfets thereby reducing the amount of distortion which needs to be corrected.

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