A Secondary Side Switch For Synchronous Rectification

The traditional converter always use common diodes or Schottky diode rectifier.While the former’s voltage drop is 1.0V~1.2V,and the latter’s is 0.4V~0.8V,which result in rectifier loss increased,power efficiency degraded.Due to power MOSFET’s much more lower on-resistance,the switching loss is greatly reduced and the conversion efficiency is significantly improved.Therefore,we can use power MOSFET to rectify,namely as Synchronous Rectification(SR)Technology.The flyback converter has the advantages of simple circuit topology,electric isolation of input and output,wide range of voltage rise / fall,multi-output solutions and so on.Based on CSMC 1 μm/25 V CMOS process,founded on flyback synchronous rectifier structure and used voltage self-oscillating mode,a secondary side MOSFET driver for synchronous rectification in CCM(Continuous Conduction Mode)operation and DCM(Discontinuous Conduction Mode)operation was designed.It can effectively reduce the power dissipation of the secondary side rectifier.The chip integrates the output voltage detection function for primary side control system.By sensing SR’s drain-to-source voltage,the chip can output ideal drive signal with fewer external components.It can provide high performance solution for 5V~20V application.Tested after tapeouted,the power efficiency of the chip is as high as about 87% with lowest voltage and biggest current.The main modules of the chip include the power module,the reference source module,the logic control module,the driver module and the protection circuit module.The advantages of the chip are mainly reflected in the following aspects:(I)Superior control theory.The traditional synchronous rectification control chip’s gate turn-on threshold and turn-off threshold is (1= 0.In this paper,by setting dynamic reference voltages,the chip can work in both CCM mode and DCM mode.(II)An LDO contained in the power module.On the one hand,it supplies power to the internal low-voltage circuit.On the other hand,it drives the synchronous rectifier MOSFET.(III)Shorter delay time,higher efficiency.The turn-on delay time and turn-off delay time of the chip are less than 30 9),which are much lower than the corresponding parameters of APR34309 C,the similar kind of rectifier.(IV)Wider power supply voltages.The power supply voltage range of the chip is 5V~20V,while APR34309C’s is only 5V.(V)Fewest external components.