AN4154 Datasheet, PDF (1/21 Pages) STMicroelectronics – Power supply for energy meter and power line modem

AN4154
Application note
Power supply for energy meter and power line modem
By Harjeet Singh, Mohammed Shakir
Introduction
The power supply is based on the quasi resonant mode of operation using the ST primary
side flyback switch ALTAIR04-900. The ALTAIR04-900 is a high-voltage all-primary sensing
switcher intended for operating directly from the rectified mains with minimum external parts.
It combines a high-performance low-voltage PWM controller chip and a 900 V avalanche-
rugged power section in the same package.
The controller is a current-mode specifically designed for offline quasi-resonant flyback
converters. The device is capable of providing constant output voltage using all primary
sensing feedback. This eliminates the need for the optocoupler, the secondary voltage
reference, and the current sensor, while still maintaining quite accurate regulation. It is also
possible to set the maximum deliverable output current, therefore increasing end product
safety and reliability during fault events. Quasi-resonant operation is guaranteed by means
of a transformer demagnetization sensing input that turns on the power section. The same
input serves also the output voltage monitor, to perform CV regulation, and the input voltage
monitor, to achieve mains-independent maximum deliverable output current (line voltage
feedforward).
The maximum switching frequency is top-limited below 166 kHz, so that at medium-light
load a special function automatically lowers the operating frequency still maintaining the
valley switching operation. At very light load, the device enters a controlled burst-mode
operation that, along with the built-in high voltage startup circuit and the low operating
current, helps minimize the standby power. Although an auxiliary winding is required in the
transformer to correctly perform CV/CC regulation, the chip is able to power itself directly
from the rectified mains. This is especially useful during CC regulation, where the flyback
voltage generated by the winding drops below the UVLO threshold. However, if ultra-low no-
load input consumption is required to comply with the most stringent energy-saving
recommendations, then the device needs to be powered via the auxiliary winding.
In addition to these functions that optimize power handling under different operating
conditions, the device offers protection features that considerably increase end product
safety and reliability: auxiliary winding disconnection - or brownout - detection and shorted
secondary rectifier - or transformer saturation - detection. All of these are in auto restart
mode.
The main features of the controller and internal block diagram are shown below:
■ Primary side constant voltage operations with no optocoupler
■ Adjustable and mains-independent maximum output current for safe operation during
overload/short-circuit conditions
■ 900 V avalanche rugged internal power section
■ Quasi-resonant valley switching operation
■ Low standby consumption
■ Overcurrent protection against transformer saturation and secondary diode short-circuit
■ SO-16 package
November 2012
Doc ID 023578 Rev 2
1/21
www.st.com