1 Introduction to Push-Pull and Cascaded Power Converter Topologies Presented by Bob Bell

© 2003 National Semiconductor Corporation 2 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

3 Outline: Buck Regulator Family Lines Push-Pull Topology Introduction Push-Pull Controller Cascaded Push-Pull Topologies Cascaded Controller Cascaded Half-Bridge Topology Introduction

© 2003 National Semiconductor Corporation 4 Common One-Switch Power Converter Topologies

© 2003 National Semiconductor Corporation 5 Common Two-Switch Power Converter Topologies

© 2003 National Semiconductor Corporation 6 Buck Regulator Basics

© 2003 National Semiconductor Corporation 7 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)

© 2003 National Semiconductor Corporation 8 Forward Converter Vout = Vin x D x Ns Np Same transfer function as a Buck converter with an added turns ratio term Q1 D1 R C Ns D2 L1 Np Nr D3 Vin Vout

© 2003 National Semiconductor Corporation 9 Forward Diode Currents Forward Diode D1 Current Freewheel Diode D2 Current Vin =48V Vout =3.3V Iout = 5A

© 2003 National Semiconductor Corporation 10 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

© 2003 National Semiconductor Corporation 11 Push-Pull Topology Vout = Vin x D x Ns x 2 Np Q1 Q2 D Vin Vout PUSH PULL

© 2003 National Semiconductor Corporation 12 Push-Pull Switching Waveforms Vin = 48V Vout =3.3V Iout = 5A Output Inductor Current I (L1) Push Primary Switch V DS(Q1) Pull Primary Switch V DS(Q2)

© 2003 National Semiconductor Corporation 13 Push-Pull Diode Currents Vin = 48V Vout =3.3V Iout = 5A Output Diode Current I (D1) Output Diode Current I (D2)

© 2003 National Semiconductor Corporation 14 Core Utilization: Forward & Push-Pull Converters Forward Converter B-H Operating Area Push-Pull Converter B-H Operating Area Operation in Quadrant 1 only Operation in Quadrants 1 & 3

© 2003 National Semiconductor Corporation 15 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

© 2003 National Semiconductor Corporation 16 LM5030 Push-Pull Controller Features Internal V 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)

© 2003 National Semiconductor Corporation 17 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: 5A

© 2003 National Semiconductor Corporation 18 LM5030 Push-Pull Demo Board 36V-75Vin to 10A Output: 10A Input: 36 – 75V

© 2003 National Semiconductor Corporation 19 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: 30A (810W) LM5030 3G Base Station RF Power Supply

© 2003 National Semiconductor Corporation 20 LM5030 3G Base Station RF Supply -48Vin to 30A

© 2003 National Semiconductor Corporation 21 Cascaded Buck & Push-Pull Power 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 Push-Pull Outputs operate continuously, alternating at 50% duty cycle Buck Control Output is pulse-width modulated to regulate Vout

© 2003 National Semiconductor Corporation 22 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

© 2003 National Semiconductor Corporation 23 Current Fed Push-Pull Concept 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 Buck Stage Push-Pull Stage OUTPUT INDUCTOR REMOVED BUCK OUT CAP REMOVED

© 2003 National Semiconductor Corporation 24 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

© 2003 National Semiconductor Corporation 25 Current-Fed Switching Voltages Trace 1: Push_Pull SWPUSHV DS Trace 2: Push_Pull SWPULL V DS 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.

© 2003 National Semiconductor Corporation 26 Current-Fed Push-Pull Switches Ch 1,2 Push-Pull V DS Ch 3,4 Push-Pull I DS Vin = 48V Vout =2.5V Iout = 20A

© 2003 National Semiconductor Corporation 27 Current-Fed Switch Waveforms Expanded Scale Note: Each switch carries ½ the current, during the overlap time Vin = 48V Vout =2.5V Iout = 20A Ch 1,2 Push-Pull V DS Ch 3,4 Push-Pull I DS

© 2003 National Semiconductor Corporation 28 Why is it important to reduce secondary rectification losses? Estimate for typical 3.3V Output, 35 – 80V Input

© 2003 National Semiconductor Corporation 29 Comparison of Rectifier Stresses

© 2003 National Semiconductor Corporation 30 Sync Rectifier Waveforms Vin = 48V Vout =2.5V Iout = 20A Ch 1 Sync1 V DS Ch 2 Sync2 V DS

© 2003 National Semiconductor Corporation 31 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)

© 2003 National Semiconductor Corporation 32 LM5041 Block Diagram

© 2003 National Semiconductor Corporation 33 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: 50A LM5041 Current Fed Push-Pull Demo Board

© 2003 National Semiconductor Corporation 34 LM5041 / LM5100 Demo Board 50A Cascaded DC-DC Converter

© 2003 National Semiconductor Corporation 35 Cascaded Half-Bridge Concept

© 2003 National Semiconductor Corporation 36 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)

© 2003 National Semiconductor Corporation 37 Cascaded Full-Bridge Concept Full-Bridge Stage

© 2003 National Semiconductor Corporation 38 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.

© 2003 National Semiconductor Corporation 39 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

© 2003 National Semiconductor Corporation 40 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 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

© 2003 National Semiconductor Corporation 41 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 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

© 2003 National Semiconductor Corporation 42 LM5102 Timing Diagram Adjustable Leading Edge Delay

© 2003 National Semiconductor Corporation 43 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) V DD supply under-voltage lock-out (6.7V) Low power consumption 0.5MHz) Packages: SOIC-8, LLP-10 Typical Applications Cascaded Power Converters High Voltage Buck Regulators Active Clamp Forward Power Converters

44 Summary: New 100V controllers and drivers enable higher performance power converters with a minimum of external components: LM5030 Push Pull Controller LM5041 Cascade Controller LM510X Gate Drivers Questions or Comments?