What Is Power Factor and Why It Matters
Power factor (PF) is the ratio of active power (W) — the useful work done by the load — to apparent power (VA) drawn from the mains. An ideal load has PF = 1.0. A typical switching power supply without PFC draws current in narrow pulses near the voltage peak, giving PF ≈ 0.55–0.65 and generating significant harmonic currents.
The practical consequence: a 240 W power supply with PF 0.6 draws 400 VA from the mains. If a panel contains ten such supplies, the feed cable and meter see 4,000 VA for just 2,400 W of useful work. On large sites this means higher energy costs and the need for a larger supply cable.
Active PFC: How It Works
Active PFC is an additional conversion stage between the input rectifier and the main PWM controller. It shapes the input current to follow the mains voltage sinusoid, achieving PF 0.95–0.99. As a side effect it holds the intermediate DC bus at a stable ~400 V DC regardless of input voltage, which improves output regulation during voltage sags and generator operation.
Passive PFC is a simpler approach: a large input choke partially shapes the current waveform and raises PF to ≈ 0.7–0.8 without active switching. The improvement is real but much weaker than active PFC.
In a DIN-rail power supply datasheet, "Active PFC" or PF ≥ 0.95 confirms active PFC. A PF of 0.7–0.8 or no PF spec indicates passive correction or none at all.
EN 61000-3-2: When PFC Is a Requirement, Not a Choice
Standard EN 61000-3-2 limits the harmonic currents that equipment can draw from a 230 V single-phase supply. For single devices above 75 W, the limits are tight enough that active PFC is the only practical way to comply. This is why Mean Well introduces active PFC starting with the MDR-100 (100 W) and NDR-240 (240 W), not at every wattage.
In practice: if your installation must be certified or connected to a utility with harmonic requirements, power supplies above ~75 W should carry active PFC.
When Active PFC Is Critical in Industrial Applications
There are four scenarios where active PFC moves from "nice to have" to "engineering requirement":
- Diesel generator supply. A generator is rated in VA. Without PFC, a panel full of switching supplies overloads the generator before its protective relay trips. With PFC the reactive draw is minimal — the generator's capacity is used for real work.
- UPS supply. Same principle: a UPS is VA-limited. PFC-corrected supplies reduce wasted VA and extend backup runtime.
- Many supplies on one phase. Harmonic currents from uncorrected SMPS add up. In large distribution panels with dozens of supplies, the combined harmonics cause neutral overheating, nuisance relay trips, and interference with sensitive automation.
- Sites with power quality requirements. Medical, telecom, or green-certified buildings where power quality metrics (EN 50160) are monitored.
Passive PFC or None: When It Is Sufficient
For small supplies up to 60–75 W (e.g., HDR-60-24, DR-60-24), active PFC is unnecessary — EN 61000-3-2 grants these devices relaxed harmonic limits. In a typical automation panel with two to five small supplies, passive PFC or no PFC works perfectly well.
A simple selection rule: below 75 W per supply — PFC is not critical. At 100 W and above on demanding sites (generator, UPS, many supplies, certification) — choose series with active PFC.
Mean Well and Delta Series With Active PFC
Active PFC series available in our power supply catalogue:
| Series | Brand | Power | Mounting | PF (typical) |
|---|---|---|---|---|
| MDR-100 | Mean Well | 100 W | DIN rail | ≥ 0.95 |
| NDR-240 | Mean Well | 240 W | DIN rail | ≥ 0.95 |
| NDR-480 | Mean Well | 480 W | DIN rail | ≥ 0.95 |
| RSP series | Mean Well | up to 504 W | Enclosed chassis | ≥ 0.95 |
| DRM | Delta | 120–960 W | DIN rail | ≥ 0.95 |
| DRP | Delta | 30–960 W | DIN rail | ≥ 0.95 |
Series without active PFC (fine for typical low-power applications): HDR, DR, LRS, MDR-20/40/60, NDR-75/120, Delta DRL, PMT, PMC, PMU.
Catalogue Examples: Supplies With PFC
Mean Well MDR-100-24 — 100 W, DIN Rail, PFC
Compact DIN-rail supply, 100 W, 24 V / 4.17 A with active PFC. Well suited to mid-size panels that already carry several small supplies and need one higher-power unit with a good power factor.
Mean Well NDR-240-24 — 240 W, DIN Rail, PFC
One of the most widely used options for medium-to-large automation panels: 240 W, 24 V / 10 A, active PFC ≥ 0.95, efficiency ≥ 90%. Maintains rated output when fed from a generator. If your installation uses backup power, this is the leading DIN-rail candidate.
Delta DRP Series — 30–960 W, DIN Rail, PFC
The Delta DRP series covers a wide power range with active PFC. Three-phase input is available on higher-rated models — relevant for industrial panels where load balancing across phases matters.
Summary: Choosing a Supply With PFC in Mind
If the panel is fed from the utility grid, total load is modest (three or four small supplies up to 150 W combined) and there is no backup generator — PFC is not required. For everything else — 100 W or more per supply, generator or UPS feed, more than five supplies on one phase, or a certified installation — choose MDR-100, NDR-240, NDR-480, RSP, or Delta DRM/DRP.
Have an unusual specification? Send us your load list — we will recommend the right power supply within one business day.
