November 2012

Doc ID 023578 Rev 2

1/21

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:

overload/short-circuit conditions

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