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I’m reading the user guide for a Microchip reference design of a 300W AC/DC converter. Section 1.2 starts with these requirements:

A conventional SMPS must implement PFC if it draws more than 75 watts from the AC Mains. The PFC circuitry draws input current in phase with the input voltage, and the Total Harmonic Distortion (THD) of the input current should be less than 5% at full load. [emphasis by me]

What drives these requirements (in addition to efficiency)? Are these requirements driven by a standard or a regulation? If so, what's the driver behind the regulation?

Any insight or reference is appreciated!

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I think this is a CE requirement. – Olin Lathrop Nov 1 '12 at 20:54

2 Answers

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EN61000-3-2 is a European standard which dictates PFC requirements. Most power supply manufacturers design in PFC so that there aren't any problems marketing the product worldwide.

PFC is also helpful if you want to operate with "universal" AC input (85-264VAC) as the down converter will see a constant input voltage (usually 400VDC) regardless of the input line voltage.

EN61000-3-2 also cites harmonic content limits in four broad categories of power supplies with explicit limits and test criteria. The 5% figure cited by Microchip is a rule of thumb that "should" allow you to pass harmonics testing, but doesn't replace a proper test with an accurate power meter and properly-controlled input AC.

Properly-working PFC makes the power supply look resistive to the mains (power factor as close to 1 as possible) which is why PFC requirements and line harmonic requirements go hand-in-hand.

In a nutshell, if the supply is greater than 75W, and has universal input, PFC is for all intents and purposes "required" and is a good feature to have even if you don't intend to sell in Europe.

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I'm from SL, I hope these regulations will soon invoke to selling products in SL too. – sandun dhammika Nov 2 '12 at 16:34
@Madmanguruman I did some searches about PFC requirements in the US. Found only some references that PFC is in the Energy Star requirements. But I'm not after Energy Star certification. The gadget which I'm incubating outputs 250W, but it's on only for short periods of time with 30% duty cycle. Population of my gadget will be relatively small. On the other hand, TV and PC power supplies, LED lighting seem like the main culprits behind PFC. There are lots of them, and they are on all the time. What's your take on PFC requirements for the US? – Nick Alexeev Nov 30 '12 at 5:38
Just do it. No need to limit yourself geographically to the USA, you may well want that Energy Star approval to help with marketing the product, etc. – Madmanguruman Nov 30 '12 at 17:08
@Madmanguruman I've got constraints on mechanical size. Other than that, I'd be delighted to design my thing with PFC. – Nick Alexeev Dec 3 '12 at 5:24
There are some more exotic approaches, like combining PFC with the DC/DC converter itself to give you correction without the preregulation, but most of the off-the-shelf stuff is based around CCM or boundary-mode boost PFC. (TV power supplies and cheap PC supplies rarely have PFC, at least for the NA market.) YMMV. – Madmanguruman Dec 3 '12 at 13:52
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The requirement is placed by the company that supplies your mains electricity and will be backed up by your relevant regulatory authority.

The actual requirement will possibly vary from country to country, but as most countries adopt international standards that have been produced by major participants odds are that the requirement will be similar in most places.

An offline switching power supply that uses simple front end rectification of the mains to DC tends to draw most of its power at voltage peaks and so has a very poor power factor. If such supplies were allowed to form a significant proportion of the grid-load they would lead to significant distortion of the sinusoidal waveform and would lower the net power factor. Lowered power factor means that voltage and current become out of phase with each other so that to convey a given amount of power more current is required. This leads to increased losses and the need to dimension cable etc to carry a higher amperage than is being paid for.

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