1/14

AN1621

APPLICATION NOTE

1

INTRODUCTION

A typical off-line isolated switch-mode power supply has the controller located on the primary side of the trans-

former, whereas the output voltages to be controlled and the housekeeping functions are located on the sec-

ondary side. Usually the voltage feedback signal is transferred to the primary controller by using an optocoupler

or a transformer.

We want to propose here an asymmetrical half bridge forward converter with the controller located on the sec-

ondary side. This solution offers some advantages:

– direct use of the on-board voltage reference

– no need of the opto feedback, with its temperature and ageing gain dependence

– available on-board housekeeping functions eliminates the need of an additional dedicated device

– negligible extra cost on the gate drive transformer to satisfy safety requirements

The controller operating on the secondary side requires a specific concept for the start-up sequence, realised

here with a very simple, low consumption and cost effective solution.

2 POWER SUPPLY DESCRIPTION

2.1

Topology

Considering the 300W of output power delivered to the load, the most appropriate topology is the asymmetrical

half bridge converter.

This topology requires two power MOS transistors with a voltage breakdown equal or little higher than the max.

Fig. 1 shows the complete schematic diagram of the 300W Power Supply.

2.2

Start-up circuit

As already mentioned in the introduction section, the controller is located on the secondary side of the power

transformer, and for this reason, a start-up circuit has to be provided for a correct system activation.

The start-up circuit is based on a diac sending a train of controlled pulses to the low side drive section; the float-

ing drive section is energised by the second secondary gate drive transformer winding .

As soon as the L5991a wakes-up, it generates a pwm signal enabling the start-up circuit.

2.3

Gate driver

The two power mosfets, T1 and T2, are driven by a small transformer designed to satisfy the isolation safety

requirements and fast switching times.

For optimum magnetic coupling (required by the high switching frequency operation) and minimum number of

turns, a high permeability core has been selected.

300W SECONDARY CONTROLLED TWO-SWITCH

FORWARD CONVERTER WITH L5991A

AN1621/1104

Rev. 2