L6561

The L6561 is a transition-mode (also known as boundary conduction mode or BCM) power factor correction controller from STMicroelectronics. It is designed to drive a boost converter stage in AC-DC power supplies to achieve near-unity power factor (>0.99) and low total harmonic distortion (THD), enabling compliance with IEC 61000-3-2 and other international harmonic current standards. In transition mode, the inductor current always returns to zero before the next switching cycle begins, and the IC automatically adjusts the switching frequency to maintain this boundary condition. This mode of operation offers several advantages: zero-current switching at turn-on (reducing switching losses and EMI), simpler control compared to continuous conduction mode (CCM), and no need for a current sense transformer. The L6561 features very low startup current (50µA) and low operating current (3mA), minimizing power loss and enabling efficient standby operation. The integrated gate driver can source/sink up to 400mA, capable of driving external power MOSFETs directly. Protection features include output overvoltage protection (OVP), cycle-by-cycle current limiting, and thermal shutdown. The device can be disabled via the MULT pin for power management or fault conditions. Available in both 8-pin DIP (through-hole) and SO-8 (surface-mount) packages, the L6561 is widely used in switch-mode power supplies for consumer electronics, industrial equipment, LED lighting, and telecom systems requiring power factor correction.

The L6561 implements a transition-mode (TM) power factor correction controller that shapes the input current to follow the AC line voltage waveform, achieving near-unity power factor. The core principle is a boost converter where the switch is turned on when the inductor current reaches zero (zero-current detection) and turned off when it reaches a peak value set by the instantaneous line voltage. This results in a triangular inductor current waveform whose average value follows the input voltage envelope, creating a resistive input impedance. The control loop works as follows: (1) The error amplifier compares the output voltage (via a resistive divider on the INV pin) with an internal 2.5V reference, generating an error signal. (2) This error signal is multiplied by a signal proportional to the instantaneous AC line voltage (from the MULT pin, which monitors the rectified input voltage). (3) The multiplier output sets the current reference level for each switching cycle — higher at the line voltage peak, lower near zero crossing. (4) When the switch turns on, the inductor current ramps up. When this current (sensed via a resistor on the CS pin) reaches the reference level, the switch turns off. (5) When the switch is off, the inductor current ramps down, delivering energy to the output. When the current reaches zero (detected by the ZCD pin monitoring the auxiliary winding), the switch turns on again and the cycle repeats. The switching frequency thus varies with line voltage and load — highest near the line zero crossings and lowest at the peaks. This variable-frequency operation is inherent to TM/BCM PFC. The zero-current switching at turn-on reduces switching losses and EMI emissions. The overall result is an input current that closely tracks the input voltage waveform, achieving high power factor and low harmonic distortion.

• AC-DC SMPS PFC pre-regulator stages
• LCD/LED TV and monitor power supplies
• Notebook adapter and charger front-ends
• LED street lighting driver PFC
• Server and telecom power modules
• Industrial control power supplies
• Home appliance power converters
• Fluorescent and HID lighting ballasts