The Active Diode A Current Driven Synchronous Rectifier

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The Active Diode A Current Driven Synchronous Rectifier. W2-tech Inc. Demand for Low Voltage High Current Power Converters. Modern Microprocessor operates at low voltage and high current The future demand will go for less than 1 V and more than 150A.
The Active DiodeA Current Driven Synchronous Rectifier W2-tech Inc.Demand for Low Voltage High Current Power Converters
  • Modern Microprocessor operates at low voltage and high current
  • The future demand will go for less than 1 V and more than 150A
  • Reasons for low voltage & high current
  • Switching time is shorter between close voltage levels
  • Less loss due to capacitance
  • IC sub micron technology requires low operating voltage
  • Large number of devices need high current
  • Distributed Power StructureSynchronous Rectification is needed
  • In order to handle high current at low voltages, SR is needed
  • Low Rdson MOSFET greatly reduces losses at the output rectifier
  • There are many problems with conventional SR
  • Different topologies need different drive circuit design
  • Active clamp on the primary side is often needed
  • Problems with conventional SR …cont’d
  • MOSFET driving voltage is directly coupled to the input voltage
  • Gate voltage limits input voltage range
  • Gate drive voltage not optimized
  • Input voltage120V to 380VBody diode conductionT1T1 I/P voltageM1M2 gateT1 O/P voltageM2M1 gate0.6VM1M2 conduction voltage0.1VM1, M2 power lossM1 currentM2 currentProblems with conventional SR …cont’d
  • Leakage inductance produces long body diode conduction period
  • This increases dissipation and greatly reduces efficiency
  • Bad at high frequencies
  • VoVo-Vo+Problems with conventional SR …cont’d
  • MOSFET is a bi-directional switch
  • Converters with SR cannot be connected in parallel, as reverse current will flow between converters
  • Problems with conventional SR …cont’diL
  • MOSFET is a bi-directional switch
  • No discontinuous mode
  • Poor light load efficiency because of current peaks
  • Problems with conventional SR …cont’d
  • The gate drive will be lost after the transformer is reset in a forward converter
  • Active clamp on the primary side is often needed
  • More components and violation of patents
  • SR1SR1SR2SR2Vgs(SR1)Vgs(SR2)Desired Solution
  • The SR should turn on and off according to current flow
  • This makes the SR behaves like a diode
  • Solves all aforementioned problems
  • The Active Diode – Basic configurationCurrent sense circuitM1N1N2AmplifierN3N4D2D1
  • N1 is the current sense winding
  • N2 amplify voltage at N1
  • N1 N3 & D1 form energy recovery circuit
  • N4 & D2 form reset circuit
  • Energy recovery circuitReset circuitBasic Operation of the Active DiodeVoltage drop Vcs across current sense winding N1 is depend on the winding ratio of N1 to N3 and voltage source VoIiVoltage source Vo can be any voltage source in a converter, e.g. output voltageM1Voltage across winding N2 or gate drive voltage Von of SR depends on ratio of N2 to N3and voltage VoN1N2T1N3N4D2D1VoToffTon_dIiVonVN2VoVN3VoVN4Toff_dWaveformsIiM1N1N2T1N3N4D2D1VoKAAKADKAActive Diode – the way to a perfect diodeIt's a Diodea perfect DiodeADADIt is better than Synchronous RectifierSync RectActive Diode
  • Complicated primary circuit
  • Converter cannot be paralleled – Reverse current
  • Poor efficiency at low load
  • Special driving circuits SR are needed for different topologies
  • Sensitive to transformer leakage inductance
  • Limited input voltage range
  • Simple primary circuit
  • Discontinuous mode is allowed
  • Good low load efficiency
  • Converter can be paralleled
  • Works just like a diode
  • KKAAKAADKAIt is far better than Schottky diode
  • Inherent forward Volt. drop
  • Low reverse voltage
  • No or little avalanche rated
  • No inherent Volt. drop
  • high reverse voltage
  • 100% avalanche guaranteed
  • KA0.24 V7.8 mKA9 mSCK Diode or Active Diode?Average 30V SO8 MOSFETState of the Art 30V SCKSCK 2.8W losses @ 10AAD 0.7W losses @ 10AADSCK Diode VS Active DiodeComparisons of Sync-Rect, SCK and Active DiodeADIR1176ADDesign engineer’s considerationThe Active Diode is
  • 5 times lower losses than state of the art Schottky diode
  • 50 times lower losses is also possible
  • 100% avalanche guaranteed
  • Only MOSFET solution can ensure important no load power <0.3W
  • Cheapest solution compared with other Sync-Rect solution
  • Replace diode on all old and new converter designs
  • Much higher operating temperature than Schottky diode
  • Wide frequency & voltage range from 50 Hz to 500kHz and 12V to 1000V
  • The Active Diode works in all topologiesFlybackForwardIt works just like a low loss diodeHalf Bridge centre tapCurrent Doubler……. in different topologiesand many others….Resonant converterConclusions
  • A new “Active Diode” technology is presented
  • A kind of current driven synchronous rectifier which solves many problems of the conventional Sync Rect
  • Well proven by many converter designs
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