AN4248 Datasheet, PDF (1/46 Pages) STMicroelectronics – 60 W PSU design details for water purifier system

AN4248
Application note
60 W PSU design details for water purifier system
Harjeet Singh
Introduction
This power supply is based on quasi-resonant mode of operation using ST multi-mode
controller L6566B. The L6566B is an extremely versatile current-mode primary controller IC,
specifically designed for high-performance offline flyback converters. Both fixed-frequency
(FF) and quasi-resonant (QR) operation are supported. The user can pick either of the two,
depending on application needs.
The device features an externally programmable oscillator: it defines the converter's
switching frequency in FF mode and the maximum allowed switching frequency in QR
mode. When FF operation is selected, the IC works like a standard current-mode controller
with a maximum duty cycle limited at 70% min. The oscillator frequency can be modulated
to mitigate EMI emissions. QR operation, when selected, occurs at heavy load and is
achieved through a transformer demagnetization sensing input that triggers MOSFET turn-
on. Under some conditions, ZVS (zero-voltage switching) can be achieved. The converter's
power capability increasing with the mains voltage is compensated by line voltage feed-
forward. At medium and light load, as the QR operating frequency equals the oscillator
frequency, a function (valley skipping) is activated to prevent further frequency rise and
keep the operation as close to ZVS as possible. With either FF or QR operation, at very light
load the IC enters a controlled burst-mode operation, to help keep consumption from the
mains low and meet energy saving recommendations.
The protection functions included in this device are: not-latched input undervoltage
(brownout), output OVP (auto-restart or latch-mode selectable), a first-level OCP with
delayed shutdown to protect the system during overload or short-circuit conditions (auto
restart or latch-mode selectable) and a second-level OCP that is invoked when the
transformer saturates or the secondary diode fails short. A dedicated pin to latch the
controller in any abnormal condition is also provided.
The main features of the controller are listed here:
• Selectable multi-mode operation: fixed frequency or quasi-resonant
• On-board 700 V high-voltage startup
• Advanced light load management
• Low quiescent current (< 3 mA)
• Adaptive UVLO
• Line feed-forward for constant power capability vs. mains voltage
• Pulse-by-pulse OCP, shutdown on overload (latched or auto restart)
• Transformer saturation detection
• Programmable frequency modulation for EMI reduction
• Latched or auto restart OVP
• Brownout protection
May 2014
DocID024214 Rev 1
1/46
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