L6562 pins: what to measure on a PFC controller

The brain of the PFC in TV and PC power supplies. If the PFC isn't running (the bulk capacitor sits at 310 V instead of 390 V), measure the pins in this order. Tap a pin.

Where you'll meet it LED TV power boards, PC power supplies, adapters over 75 W (equivalents: L6561, NCP1608, FAN7930)

Control / signalAnalogClock (crystal)GroundDrive outputSupply

Pin 1 · Control / signal

INV

The error amplifier's inverting input, fed from the PFC output (≈390 V) by a high-value divider; the IC tries to hold this pin at 2.5 V.

🔧 How to measure

DC voltage against hot-side ground, with the PFC running.

Expected on a working board2.5 V

On the scope

⚠ If the reading is wrongAbove 2.5 V with the PFC not running: the divider's bottom resistor is open or the top ones have drifted. Around 2.7 V the over-voltage protection stops the pulses.

📋 What order to measure in?

Unplugged: any short between GD–GND (7–6) or VCC–GND (8–6)?
With the device on, is VCC (8) 12–18 V?
MULT (3): is there a humped signal?
INV (1): is it 2.5 V? If not, measure the output divider.
ZCD (5): is there a signal in step with the pulses?
GD (7): is there a square wave? If so, the bulk capacitor should be at ≈390 V.

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How it works in a circuit and drives the MOSFET

A real circuit using this IC. The line lighting up yellow is the IC's gate output: when the IC drives it HIGH, the MOSFET turns on. Follow the IC's output alongside the circuit's currents and voltages in the slow-motion strip below.

⏱ 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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Go further

The PFC topology and real circuit How it works: TV power board Repair Workshop: TV power board