Power Factor Correction (PFC) in AC-DC Supplies

Last Updated: August 5, 2026 · Reviewed by the Sunshine Electronics engineering team

This is an engineering guide to power factor correction inside AC-DC power supplies. For the full product reference, see our 12V Power Supply Buyer’s Guide.

Quick answer

In an AC-DC supply, power factor correction (PFC) shapes the input current to look resistive so the supply draws cleanly from the mains — a power factor above 0.9 instead of ~0.6 — and cuts harmonic distortion. It is mandatory above 75 W in the EU. Active PFC (a boost stage) does it electronically; passive PFC uses a bulky inductor. This is separate from facility power factor correction, as the MPS PFC primer explains.

Ask two engineers about “power factor correction” and you can get two unrelated answers — one about capacitor banks on a factory switchboard, the other about a boost stage inside a phone charger. This guide is about the second: PFC inside an AC-DC power supply, the front-end that decides how cleanly your product draws current from the wall — and, above a certain wattage, whether it is even legal to sell.

It is written by the engineering team at Sunshine Electronics — 22+ years building AC-DC supplies, many with active PFC front ends, across our dual-factory operations in China and Vietnam, under UL 62368-1, FCC Part 15-B, CE, and DoE Level VI. PFC is a stage in the switching supplies we cover in our switch mode power supply guide; this post is the deep-dive on that stage.

Contents

01What PFC Means in a Power Supply
02Why It Matters: The IEC 61000-3-2 Harmonics Rule
03Active vs Passive PFC
04PFC vs Efficiency: What It Does and Doesn’t Do
05When Does Your Supply Need PFC?
06Frequently Asked Questions

What PFC Means in a Power Supply

Every AC-DC supply draws current from the mains, but not every supply draws it nicely. A basic switching supply pulls current in short, tall spikes near the peak of each mains cycle, which means it takes far more apparent power (volt-amps) from the grid than the real power (watts) it actually uses. The ratio of the two is the power factor; an uncorrected switcher sits around 0.5–0.65.

Power factor correction is a front-end stage that fixes this by forcing the input current to follow the input voltage — to look resistive — so the supply draws a smooth, in-phase current and its power factor rises above 0.9. The CUI AC-DC primer covers where this stage sits in the overall supply. One clarification up front, because the term is overloaded: this is not the facility-level “power factor correction” done with capacitor banks to cut a plant’s utility bill. Same words, different problem — here PFC is a circuit inside the product.

Why It Matters: The IEC 61000-3-2 Harmonics Rule

PFC is not optional above a certain size. In the EU and most markets that follow it, IEC 61000-3-2 limits the harmonic current a device may inject back onto the mains, and for most equipment drawing more than about 75 W the only practical way to meet those limits is an active PFC front end. Below ~75 W the limits are looser and many supplies pass without it; above it, PFC is effectively mandatory.

The reason is harmonics. The spiky current an uncorrected supply draws is rich in odd harmonics (3rd, 5th, 7th…) that flow back into the building wiring, distort the mains voltage, overload neutral conductors, and interfere with other equipment. When one charger does it the grid shrugs; when millions do, it is a real problem, which is why the limits exist. For a B2B buyer the takeaway is blunt: if your product exceeds 75 W and ships into Europe, its supply needs PFC to be CE-compliant — there is no design-around.

From our compliance team

the first question we ask on any supply above ~75W bound for the EU is whether the customer has budgeted for active PFC, because IEC 61000-3-2 is a pass/fail gate at CE, not a nice-to-have. Catching it at the RFQ stage is a line item; catching it at the pre-compliance test lab is a redesign and a slipped launch.

Active vs Passive PFC

There are two ways to correct power factor in a supply, and they are not equivalent. Passive PFC adds a bulky low-frequency inductor (and sometimes a filter network) in the input to smooth the current draw. It is simple and rugged, but heavy, and it only lifts the power factor to roughly 0.7–0.85 — enough for some lower-power cases, not for demanding ones.

Active PFC is the modern default. It adds a boost converter ahead of the main converter, switching at high frequency under a controller that actively forces the input current to track the voltage waveform, reaching power factors of 0.95–0.99. It is the approach the controller ICs in the Texas Instruments power-management portfolio are built to drive.

The wide-bandgap devices in the Infineon power-supply portfolio push its efficiency and density higher. Active PFC costs a few more components and a little board space, but it is the only route to compliant, high-power-factor operation above ~75 W — which is why nearly every laptop brick, server supply, and higher-wattage USB-C PD charger uses it.

From our engineering team

our rule of thumb: passive PFC only survives in a few low-power, cost-driven designs that still need to nudge the power factor up; everywhere above roughly 75W we design in active PFC from the start. Retrofitting PFC onto a supply that was not planned for it is far more expensive than specifying it on day one.

PFC vs Efficiency: What It Does and Doesn’t Do

The single most common misconception is that PFC makes a supply more efficient. It does not, at least not in the way buyers assume. Efficiency is how much of the power drawn is delivered to the load versus lost as heat; power factor is how cleanly that power is drawn from the mains. A supply can be 94% efficient with a terrible power factor, or have excellent power factor and mediocre efficiency — they are different numbers measuring different things, and rules like DoE Level VI govern efficiency, not power factor.

What good power factor does do is let a given circuit and building wiring deliver more real watts, reduce losses in the distribution system, and — the part that matters commercially — keep the product legal to sell above 75 W. So spec them independently: an efficiency tier (DoE Level VI) and, where required, a power-factor and harmonics target (IEC 61000-3-2). Conflating the two is how a supply ends up compliant on one axis and failing on the other.

When Does Your Supply Need PFC?

For a product decision, the threshold does most of the work. If your supply draws under ~75 W and ships only where IEC 61000-3-2 is loose at that level, you likely do not need PFC, and adding it is wasted cost. If it draws more than ~75 W and ships into the EU or a market that follows it, you need active PFC to be compliant — full stop. Between those, the answer depends on the exact wattage, the target markets, and any customer power-factor spec (data-center and telecom buyers often require >0.95 regardless of the legal minimum).

Treat it as a spec line, not an afterthought: name the target markets and the wattage at the RFQ stage, and let the manufacturer confirm whether PFC is required and design it in. For the higher-wattage form factors where PFC lives, our wall-mount adapter and desktop ranges cover the builds, and our switching vs linear comparison covers the architecture the PFC stage sits in front of.

Frequently Asked Questions

What is power factor correction in a power supply?

A front-end stage that forces the supply’s input current to follow the input voltage, so it draws power cleanly from the mains (power factor above 0.9) and injects less harmonic distortion. It is separate from facility-level power factor correction done with capacitor banks.

When is PFC required?

Under IEC 61000-3-2, most equipment drawing more than about 75 W must limit its mains harmonics, and active PFC is the practical way to meet those limits. Above 75 W into the EU, a supply effectively needs PFC to be CE-compliant; below it, many supplies pass without.

What is the difference between active and passive PFC?

Passive PFC uses a bulky low-frequency inductor and reaches a power factor of about 0.7-0.85. Active PFC adds a high-frequency boost converter under a controller and reaches 0.95-0.99. Active is the modern default above ~75 W.

Does PFC improve efficiency?

Not directly. Power factor and efficiency are different measurements — power factor is how cleanly power is drawn from the mains, efficiency is how little is lost as heat. Good power factor reduces distribution losses and is required for compliance, but a high power factor does not by itself mean a more efficient supply.

What power factor should an AC-DC supply have?

Above 75 W, typically 0.9 or higher to meet harmonics rules, with active PFC designs reaching 0.95-0.99. Data-center and telecom buyers often specify >0.95 regardless of the legal minimum. Below 75 W a corrected value is often not required.

Conclusion

Inside an AC-DC supply, power factor correction is the front-end stage that makes the product draw current cleanly from the mains — raising power factor above 0.9 and cutting the harmonics that IEC 61000-3-2 limits above about 75 W. Active PFC (a boost converter) is the modern default; passive PFC survives only in a few low-power designs.

For an OEM the decision reduces to wattage and market: above ~75 W into the EU, PFC is a compliance gate, not an option. Specify the target markets and wattage early, keep power factor and efficiency as separate line items, and the supply clears both without a late redesign.

Ready to Source a PFC-Compliant Power Supply?

Sunshine designs AC-DC supplies with active PFC front ends where the wattage and target market require them — to a controlled spec with UL 62368-1, FCC, CE (including IEC 61000-3-2), and DoE Level VI test reports, at our dual-factory operations in China and Vietnam. See our Class 2 power supply guide for another compliance angle, and our OEM program team returns a quoted spec sheet with the PFC and harmonics approach within 3 business days of an RFQ.

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