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AN1621 APPLICATION NOTE 300W SECONDARY CONTROLLED TWO-SWITCH FORWARD CONVERTER WITH L5991A 1 INTRODUCTION A typical off-line isolated switch-mode power supply has the controller located on the primary side of the transformer, whereas the output voltages to be controlled and the housekeeping functions are located on the secondary 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 secondary 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. rectified mains voltage (thanks to the two clamping diodes), with a proper Rdson to reach the target efficiency. 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 floating 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. AN1621/1104 Rev. 2 1/14 T1 STW14NK50 D19 1N4148 10T 50T 1T R30 180 D13 STTH1L06 D14 STTH1L06 D15 STTH1L06 D2 IN4148 32T R8 430 D9 18V C16 390pF ISEN R15 10 Tsf2 10T PGND C14 470nF C30 2.2nF Y1 L6 9T D5 1N4148 D3 IN4148 R1 3.6 R2 10 R14 22K C3 100pF D16 1N4148 T5 BC327 10T D17 1N4148 T4 BC327 T2 STW14NK50 R16 360K DIAC DB3 VC 13 OUT 9 VCC 8 5 FB 6 COMP 2/14 BYV52-200 10T 1T 77930 C4 1000µF C3 1000µF C2 1000µF C5 1µF P1 10K D18 1N4148 Tsf1 C6 1nF 50T R24 30K D1 L1 24T 220VAC EMI AN1621 APPLICATION NOTE C17 100nF 24V 13A Vo C26-27 220µF 400V