The Active Diode A Current Driven Synchronous Rectifier

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The Active DiodeA Current Driven Synchronous
Rectifier
W2-tech Inc.
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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

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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

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Distributed Power Structure
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Synchronous 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

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There are many problems with conventional SR

• Different topologies need different drive circuit
  design
• Active clamp on the primary side is often needed

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Problems with conventional SR contd
Input voltage 120V to 380V

• MOSFET driving voltage is directly coupled to the
  input voltage
• Gate voltage limits input voltage range
• Gate drive voltage not optimized

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Problems with conventional SR contd

• Leakage inductance produces long body diode
  conduction period
• This increases dissipation and greatly reduces
  efficiency
• Bad at high frequencies

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Problems with conventional SR contd

• MOSFET is a bi-directional switch
• Converters with SR cannot be connected in
  parallel, as reverse current will flow between
  converters

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Problems with conventional SR contd
iL

• MOSFET is a bi-directional switch
• No discontinuous mode
• Poor light load efficiency because of current
  peaks

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Problems with conventional SR contd

• 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

SR1
SR1
SR2
SR2
Vgs(SR1)
Vgs(SR2)
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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

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The Active Diode Basic configuration
Current sense circuit
M1
N1
N2
Amplifier
N3
N4
D2
D1

• 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 circuit
Reset circuit
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Basic Operation of the Active Diode
Voltage drop Vcs across current sense winding N1
is depend on the winding ratio of N1 to N3 and
voltage source Vo
Ii
Voltage source Vo can be any voltage source in
a converter, e.g. output voltage
M1
Voltage across winding N2 or gate drive voltage
Von of SR depends on ratio of N2 to N3 and
voltage Vo
N1
N2
T1
N3
N4
D2
D1
Vo
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Waveforms
Ii
M1
N1
N2
T1
N3
N4
D2
D1
Vo
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Active Diode the way to a perfect diode
It's a Diode
a perfect Diode
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It is better than Synchronous Rectifier
Sync Rect
Active 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

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It 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

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SCK Diode or Active Diode?
Average 30V SO8 MOSFET
State of the Art 30V SCK
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SCK Diode VS Active Diode
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Comparisons of Sync-Rect, SCK and Active Diode
Schottky Sync-Rect Active Diode
Losses Bad ? Good ? Good ?
Avalanche guaranteed No ? Yes ? Yes ?
Topologies Independent Yes ? No ? Yes ?
Low loading Eff. Good ? Bad ? Good ?
High reverse Volt. Bad ? Good ? Good ?
Operating Temp. Bad ? Good ? Good ?
Overall cost Good ? Bad ? Good ?
Commutation conduction Good ? Bad ? Good ?
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Design engineers consideration
Self voltage driven approach Self voltage driven approach
Detail Cost (USD)
Magnetic 1
Main Pri MOSFET 0.8
Aux. Pri MOSFET 0.4
Aux. Cap. 0.05
SR MOSFETs 1.0
O/P Cap. 0.5
Design Cost 0.3
Total 4.05
SR IC driven approach SR IC driven approach
Detail Cost (USD)
Magnetic 1
Main Pri MOSFET 0.8
IR1176 SR IC 1.2
SR IC aux. circuit 0.1
SR MOSFETs 1.0
O/P Cap. 0.6
Design Cost 0.1
Total 4.8
Active Diode approach Active Diode approach
Detail Cost (USD)
Magnetic 1
Main Pri MOSFET 0.8
Aux. MOSFET 0
Aux. Circuit 0
Active Diodes 1.72
O/P Cap. 0.6
Design Cost 0.1
Total 4.22
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The 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 lt0.3W
• 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

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The Active Diode works in all topologies
Flyback
Forward
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It works just like a low loss diode
Half Bridge centre tap
Current Doubler
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. in different topologies
and many others.
Resonant converter
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Conclusions

• 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