Introduction to Push-Pull and Cascaded Power Converter Topologies

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Introduction to Push-Pull and Cascaded Power
Converter Topologies

• Presented by Bob Bell

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About the Presenter
The author, Bob Bell, has been involved in the
power conversion industry for 20 years, currently
a Principal Applications Engineer for the
National Semiconductor Phoenix Design Center. The
Phoenix Design Center is developing next
generation power conversion solutions for the
telecommunications market. Education BSEE
Fairleigh Dickinson University, Teaneck, NJ
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OutlineBuck Regulator Family LinesPush-Pull
Topology IntroductionPush-Pull Controller
Cascaded Push-Pull TopologiesCascaded
ControllerCascaded Half-Bridge Topology
Introduction
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Common One-Switch Power Converter Topologies
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Common Two-Switch Power Converter Topologies
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Buck Regulator Basics
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Buck Converter Characteristics

• Non-Isolated Grounds
• Voltage Step-down Only
• Single Output Only
• Very High Efficiency
• Low Output Ripple Current
• High Input Ripple Current
• High Side (Isolated) Gate Drive Required
• Large Achievable Duty Cycle Range
• Wide Regulation Range (due to above)

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Forward Converter
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Forward Diode Currents
Forward Diode D1 Current
Freewheel Diode D2 Current
Vin 48V Vout 3.3V Iout 5A
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Forward Converter Characteristics

• A Forward Converter is a Buck type converter
  with an added isolation transformer
• Grounds are isolated
• Voltage Step-down or Step-up
• Multiple Outputs Possible
• Low Output Ripple Current
• High Input Ripple Current
• Simple Gate Drive
• Limited Achievable Duty Cycle Range

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Push-Pull Topology
Q1
Q2
D
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Push-Pull Switching Waveforms
Vin 48V Vout 3.3V Iout 5A
Output Inductor Current I(L1)
Push Primary Switch VDS(Q1)
Pull Primary Switch VDS(Q2)
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Push-Pull Diode Currents
Vin 48V Vout 3.3V Iout 5A
Output Diode Current I(D1)
Output Diode Current I(D2)
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Core Utilization Forward Push-Pull Converters
Operation in Quadrant 1 only
Operation in Quadrants 1 3
Forward Converter B-H Operating Area
Push-Pull Converter B-H Operating Area
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Push-Pull Characteristics

• A Push-Pull Converter is a Buck type converter
  with a dual drive winding isolation transformer
• Push-Pull transformers and filters are much
  smaller than standard Forward converter filters
• Voltage Stress of the Primary Switches is Vin
  2
• Voltage Step-down or Step-up
• Multiple Outputs Possible
• Low Output Ripple Current
• Lower Input Ripple Current
• Simple Gate Drive (dual)
• Large Achievable Duty Cycle Range

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LM5030 Push-Pull Controller

• Features
• Internal 15-100V start-up regulator
• CM control, internal slope comp.
• Set frequency with single resistor
• 100k 600kHz
• Synchronizable Oscillator
• Error amp
• Precision 1.25V reference
• Programmable soft-start
• Dual mode over-current protection
• Direct opto-coupler interface
• Integrated 1.5A gate drivers
• Fixed output driver deadtime
• Thermal shutdown
• Packages MSOP10, LLP10 (4mm x 4mm)

 
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LM5030 Push-Pull Demo Board
Performance Input Range 36 to 75V Output
Voltage 3.3V Output Current 0 to 10A Board
Size 2.3 x 2.3 x 0.45 Load Regulation 1 Line
Regulation 0.1 Current Limit Measured
Efficiency 84.5 _at_ 5A 82.5 _at_10A
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LM5030 Push-Pull Demo Board
36V-75Vin to 3.3V _at_ 10A
Input 36 75V
Output 3.3V _at_ 10A
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LM5030 3G Base Station RF Power Supply
Performance Input Range 36 to 75V Output
Voltage 27V Output Current 0 to 30A Board Size
6 x 4 x 2 Load Regulation 1 Line Regulation
0.1 Line UVLO, Current Limit Output OV
Protection Measured Efficiency 91 _at_ 30A
(810W)
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LM5030 3G Base Station RF Supply
-48Vin to 27V _at_ 30A
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Cascaded Buck Push-PullPower Converter
(Voltage Fed)
Buck Stage
Push-Pull Stage
Buck Stage Vpp Vin D Push-Pull Stage Vout
Vpp / N Overall Vout Vin x D/N
Buck Control Output is pulse-width modulated to
regulate Vout
Push-Pull Outputs operate continuously,
alternating at 50 duty cycle
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Cascaded Voltage-Fed Converter Benefits

• A Voltage-Fed Push-Pull Converter is a Buck type
  converter consisting of a Buck Regulation stage
  followed by (cascaded by) a Push-Pull Isolation
  Stage
• The Push-Pull Stage FET voltage stresses are
  reduced to Vout x N x 2 over all line conditions
• The output rectification can be easily optimized
  due to reduced and fixed voltage stresses
• The output rectification is further optimized
  since the power is equally shared between the
  rectifiers over all load and line conditions
• Favorable topology for wide input ranges

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Current Fed Push-Pull Concept
OUTPUT INDUCTOR REMOVED
Buck Stage
Push-Pull Stage
BUCK OUT CAP REMOVED

• Push and Pull outputs operate continuously,
  alternating with a slight overlap.
• Output voltage is controlled by the Buck stage
  which operates at 2X the Push-Pull frequency.
• Continuous output current from the Push-Pull
  stage requires minimal filtering.
• High Efficiency achieved with low Push-Pull
  switching losses and matched Sync rectifier
  loading

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Cascaded Current-Fed Converter Benefits

• A Current-Fed Push-Pull Converter is a Buck type
  converter consisting of a Buck Regulation stage
  followed by (cascaded by) a Push-Pull Isolation
  Stage
• There is no high current output inductor!
• Reduced switching loss in Push-Pull stage
• Favorable topology for multiple outputs since all
  outputs are tightly coupled
• Favorable topology for wide input ranges, since
  the Buck stage pre-regulates while the Push-Pull
  and Secondary operate independently of the input
  voltage level

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Current-Fed Switching Voltages
Trace 1 Push_Pull SWPUSHVDS   Trace 2
Push_Pull SWPULL VDS   Trace 3 Buck Stage
Switching Node 
Vin 60V Vout 2.5V Iout 20A
Note There is an overlap time where both the
Push and the Pull switches are ON. This is
required to maintain the inductor current path.
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Current-Fed Push-Pull Switches
Ch 1,2 Push-Pull VDS Ch 3,4 Push-Pull IDS
Vin 48V Vout 2.5V Iout 20A
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Current-Fed Switch WaveformsExpanded Scale
Ch 1,2 Push-Pull VDS Ch 3,4 Push-Pull IDS
Note Each switch carries ½ the current, during
the overlap time
Vin 48V Vout 2.5V Iout 20A
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Why is it important to reduce secondary
rectification losses?
Estimate for typical 3.3V Output, 35 80V Input
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Comparison of Rectifier Stresses
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Sync Rectifier Waveforms
Ch 1 Sync1 VDS Ch 2 Sync2 VDS
Vin 48V Vout 2.5V Iout 20A
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LM5041 Cascaded PWM Controller

• Features
• Internal 100V Capable Start-up Bias Regulator
• Programmable Line Under Voltage Lockout with
  Adjustable Hysteresis
• Current Mode Control
• Internal Error Amplifier with Reference
• Dual Mode Over-Current Protection
• Internal Push-Pull Gate Drivers with Programmable
  Overlap or Deadtime
• Programmable Soft-Start
• Programmable Oscillator with Sync Capability
• Precision Reference
• Thermal Shutdown (165?C)
• Packages TSSOP16 and LLP16 (5 x 5 mm)

 
 
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LM5041 Block Diagram
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LM5041 Current Fed Push-Pull Demo Board
Performance Input Range 36 to 75V Output
Voltage 2.5V Output Current 0 to 50A Board
Size 2.3 x 3.0 x 0.5 Load Regulation 1 Line
Regulation 0.1 Line UVLO, Current
Limit Measured Efficiency 89 _at_ 50A
91 _at_20A
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LM5041 / LM5100 Demo Board2.5V _at_ 50A Cascaded
DC-DC Converter
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Cascaded Half-BridgeConcept
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Cascaded Half-Bridge Characteristics

• A Cascaded Half-Bridge Converter is a Buck type
  converter consisting of a Buck Regulation stage
  followed by (cascaded by) a Half-Bridge Isolation
  Stage.
• The isolation stage is Voltage-Fed.
• Voltage splitter capacitors and a small output
  stage inductor are required.
• Dead time is required for Half-Bridge switches
• The Half-Bridge Stage FET stresses are reduced,
  to Vout N. (2x less than the Push-Pull)

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Cascaded Full-Bridge Concept
Full-Bridge Stage
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Cascaded Full-Bridge Characteristics

• A Cascaded Full-Bridge Converter is a Buck type
  converter consisting of a Buck Regulation stage
  followed by (cascaded by) a Full-Bridge Isolation
  Stage
• The isolation stage is Current-Fed
• No voltage splitter capacitors or output stage
  inductor are required as in the Cascaded
  Half-Bridge
• Overlap time is required for Isolation Stage
  switches
• The Full-Bridge Stage voltage stresses are Vout
  x N, similar to the half-bridge
• Full-Bridge Stage current levels are half that of
  a Half-Bridge.

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High Side Gate Driver Operation

• Initially Q1 is activated by Low Side control
• Cboot is charged from Vcc through D1, Q1
• Cboot is charged to (Vcc-Vdiode)

• Floating Vcc, referenced to Q2 source, is
  available for upper gate driver
• Q2 Gate drive voltage is provided by Cboot

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LM5100, LM5101 High Voltage Buck Stage Gate Driver
 

• Features
• 2-Amp Driver for High and Low Side N-Channel
  MOSFETs
• Independent inputs (TTL-LM5101, CMOS-LM5100)
• Bootstraps supply voltage to 116VDC
• Short Propagation Delay (45ns)
• Fast Rise, Fall times (10ns into 1nF)
• Unaffected by supply glitching, HS ringing
• VDD Supply under-voltage lock-out (6.7V)
• Low power consumption (1.5mA _at_ 0.5MHz)
• Pin for pin compatible with HIP2100 / 2101
• Package SOIC-8, LLP-10 (4x4mm)

• Typical Applications
• Cascaded Power Converters
• Half Bridge Power Converters
• Full Bridge Power Converters
• Two Switch Forward Power Converters
• Active Clamp Forward Power Converters

 
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LM5102 Driver with Adjustable Leading Edge Delay
 

• Features
• 2-Amp Driver for High and Low Side MOSFETs
• Independently Adjustable Leading Edge Delays
• Bootstraps drive high side gate to 116VDC
• Short Propagation Delay (45ns)
• Fast Rise and Fall times (10ns into 1nF)
• VDD Supply under-voltage lock-out (6.7V)
• Low power consumption (1.5mA _at_ 0.5MHz)
• Packages MSOP-10, LLP-10 (4 x 4mm)

• Typical Applications
• Cascaded Power Converters
• Half and Full Bridge Power Converters
• Two Switch Forward Power Converters
• Active Clamp Forward Power Converters

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LM5102 Timing Diagram
Adjustable Leading Edge Delay
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LM5104 Driver with Adaptive Deadtime,
Programmable Delay

• Features
• 2Amp Driver for Complementary High and Low Side
  FETs
• Adaptive Deadtime with programmable additional
  delay
• Single TTL-Level logic input
• Bootstraps drive high side gate to 116VDC
• Short propagation delay (45ns)
• Fast rise and fall times (10ns into 1nF)
• VDD supply under-voltage lock-out (6.7V)
• Low power consumption (1.5mA _at_ 0.5MHz)
• Packages SOIC-8, LLP-10

• Typical Applications
• Cascaded Power Converters
• High Voltage Buck Regulators
• Active Clamp Forward Power Converters

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SummaryNew 100V controllers and drivers enable
higher performance power converters with a
minimum of external components LM5030 Push
Pull Controller LM5041 Cascade
Controller LM510X Gate DriversQuestions or
Comments?
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