PFC (power factor correction) · topology: how it works

A PFC is really a boost converter; what's different is that it makes the current drawn from the mains the same shape as the mains voltage (a sine). Its output is the ≈390 V bulk capacitor.

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

The IC sets each pulse's peak current to the mains sine's present value, so the peaks trace out a sine.

Measuring points: tap the numbers on the diagram

Phase 1/2: Q on

Q on: the inductor current rises. The IC drives Q so that the inductor current follows the shape of the mains voltage (a sine).

1. Bridge output

Humped DC, since there's no capacitor here: between 0 and 311 V.

On the scope

Multimeterhumped DC

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Real circuit: circuit tour and fault scenarios

The same topology with every part you'd find on a real board. Follow how it works step by step with the "Circuit tour", or pick a fault scenario below to see what to measure, in order, on the schematic.

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

GD HIGH: the inductor current rises. When the CS ramp reaches MULT's present value, GD goes LOW. When the inductor current reaches zero, the ZCD signal falls and the IC drives GD HIGH again at once, with no gap between pulses.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/2: Q on

Q ties the inductor to ground; its current rises linearly. The IC compares the voltage on Rs (CS) with a threshold set by the instantaneous mains voltage.

L6562 GD pin: HIGH (≈12 V, switch on)

1. Inductor current

A triangle on every pulse; the triangle peaks follow the mains sine. The current drawn from the mains is its average.

On the scope

Multimeter≈1.3 A (average)

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

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🔍 How to recognise it on a board

✓A large inductor (the PFC choke) between the bridge and the bulk capacitor
✓A MOSFET and a fast diode (the PFC diode) on a heatsink
✓A 400–450 V bulk capacitor
✓≈390 V, not 310 V, on the bulk capacitor while the device is on

Where it's used

Supplies above 75 W: TVs, PC power supplies, LED drivers, air conditioners

Common ICs

L6562 →ICE3PCSNCP1608

Go further

How it works: TV power board (PFC block)The boost topology

Other topologies

Flyback Half-bridge (LLC)Forward Buck (step-down)Boost (step-up)Full bridge (H-bridge) and SPWM Push-pull MPPT solar charge controller