Analysis of the Status Quo and Development of Converter Technology in DCDC module power supply.Analysis of the Status Quo and Development of Converter Technology in DCDC module power supplyIn the DC/DC industry,it should be said that the development and testing of soft-switching technology,until it is used in engineering practice,is not too small,but the effect is not too great.The energy and money spent in this area is really not as good as the improvement of MOSFET technology in the semiconductor industry.After several generations of MOSFET design industrial technology advances,from the first generation to the eighth generation.The lithography process progressed from 5μM to 0.5μM.The epitaxial layer of the perfect crystal lattice allows us to drastically reduce the resistivity chosen for the material.Plus further thinned wafers.Excellent chip bonding soldering technology reduces the on-resistance of today's MOSFETs(such as 80V40A)to less than 5mΩ,switching time is less than 20ns,gate charge is only 20nc,and it can be driven at logic level.Under such conditions,synchronous rectification technology has achieved excellent results,almost increasing the efficiency of DC/DC by nearly ten percentage points.Efficiency indicators have generally entered the range of>90%.At present,self-biased synchronous rectification has been commonly used for low voltage and low power output below 5V.The self-biased synchronous rectification is simple and easy to use.Choosing a good MOSFET is successful.It is not mentioned here.
For the synchronous rectification of 12V or more to 20V or so,the control drive IC is often used,so that good results can be obtained.ST's STSR2 and STSR3 are well suited for flyback converter circuits and forward converter circuits.We give its reference circuit.Linear Technology's LTC3900 and LTC3901 are better synchronous rectification control ICs introduced last year.In IC-driven synchronous rectification circuits,it should be said that the best ZVS,ZCS synchronous rectification circuit is only officially used in the industry in 2002.It brings the efficiency of DC/DC converters to a historic level of 95%.
ZVS,ZCS synchronous rectification is only applicable to the case where the primary side is a symmetrical type circuit topology and the magnetic core can work in both directions.That is,the push-pull,half-bridge,and full-bridge hard-switching circuits.When the secondary side output voltage is 24 V or less and the output current is large,the best effect can be obtained at this time.We know that the loss of a hard switch that transmits the same power and high voltage and small current is much lower than that of a low voltage and large current hard switch.We use this performance to transmit the PWM output signal to the secondary side through a transformer or high-speed optocoupler,and properly process the pulse width before driving the synchronous rectified MOSFET.The synchronous rectified MOSFET has no voltage between its source and drain,and turns on and off when no current flows.As long as the on-resistance of the synchronously rectified MOSFET is sufficiently small and the gate drive charge is sufficiently small,the conversion efficiency can be greatly improved.The highest 95%conversion efficiency is obtained in this way.The industry calls it CoolSet,a cold device,which eliminates the need for heat sinks and fans.
The output voltage of this circuit topology is the most efficient at 12V,15V output,the voltage is reduced or increased,and the efficiency is reduced.When the output voltage exceeds 28V,it will be equivalent to the rectification of the Schottky diode.When the output voltage is lower than 5V,the use of double current rectification will make the transformer more fully utilized and the conversion efficiency will be higher.
After the ZVS and ZCS synchronous rectification technology was applied successfully in engineering,people also conducted soft-switch synchronous rectification control experiments in the asymmetrical circuit topology.For example,the synchronous rectification drive(NCP1560)of the active box forward circuit and the synchronous rectification drive of the two-transistor forward circuit(LTC1681 and LTC1698)have not achieved ZVS,ZCS synchronous rectification,such as symmetrical circuit topology.Excellent results.
Recently,TI engineers have used the method of measuring the switching state of the synchronous rectifier MOSFET,and then using digital technology to adjust the switching time of the MOSFET to make a breakthrough synchronous switching of ZVS,thereby solving the soft switching synchronous rectification of the asymmetric circuit.