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

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Introduction to Push-Pull and Cascaded

Power Converter Topologies

Presented by Bob Bell

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© 2003 National Semiconductor Corporation

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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Outline:Buck Regulator Family Lines

Push-Pull Topology Introduction

Push-Pull Controller

Cascaded Push-Pull Topologies

Cascaded Controller

Cascaded Half-Bridge Topology Introduction

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Common One-Switch Power Converter Topologies

Ns

L

Vo

Vi n

Na ux

Np

Forward Converter

L

VoVi n

Buck Converter

VoVi n

Boost Converter

Np Vo

Vi n

Ns

Flyback Converter

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Common Two-Switch Power Converter Topologies

LVo

Vin

Ns

Np Ns

Np

Push-Pull Converter

Np

Ns

Ns

L

Vo

Vin

Half Bridge Converter

Np

Ns

Ns

L

Vo

Vin

Full Bridge Converter

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Buck Regulator Basics

VOUT = D * VIN

D*Ts

Ts

I(Q1)

I(D1)

IL

Q1

D1

L1

C1

VIN

VOUT

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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

Vout = Vin x D x Ns

Np

Same transfer function as a Buck converter with an added turns ratio term

Q1

D1

RC1

+

-

+

Ns D2

L1

Np Nr

D3

Vin

Vout

I(D1) =I(Q1) x Np/Ns

I(D2)

I(L1)

1 2 3 4

1 2 3 4

1 2 3 4

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Forward Diode Currents

Forward Diode D1Current

Freewheel Diode D2Current

Vin =48VVout =3.3VIout = 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

Vout = Vin x D x Ns x 2Np

Q1

Q2

D

D1

RC

+

-

+

Q1

ns

ns

D2Vg

Q2

np

np

L

Vin

Vout

PUSH PULL

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Push-Pull Switching Waveforms

Vin = 48VVout =3.3VIout = 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 = 48VVout =3.3VIout = 5AOutput Diode

Current I(D1)

Output DiodeCurrent I(D2)

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Core Utilization: Forward & Push-Pull Converters

FLUX DENSITYB (GAUSS)

MAGNETIC FIELDINTENSITY

H (OERSTED)

BSAT

BR

FLUX DENSITYB (GAUSS)

MAGNETIC FIELDINTENSITY

H (OERSTED)

BSAT

Forward ConverterB-H Operating Area

Push-Pull ConverterB-H Operating Area

Operation in Quadrant 1 only

Operation in Quadrants 1 & 3

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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)

S

R

OSC

VFB

CS

0.5V

0.625V

LOGIC

OUT1

OUT2

J

K

SLOPECOMPRAMP

GENERATOR

PWM5K

5V

Rt / SYNC

SS

45uA

0

1.4V

100K

50K

CLK

SS / SD

COMP

1.25V

10uA

Vcc

Vcc

SS

0.45V SHUTDOWNCOMPARATOR

CLK

7.7V REGENABLEVin Vcc

2K

RTN

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LM5030 Push-Pull Demo Board

Performance:Input Range: 36 to 75VOutput Voltage: 3.3VOutput Current: 0 to 10ABoard Size: 2.3 x 2.3 x 0.45Load Regulation: 1%Line Regulation: 0.1%Current Limit

Measured Efficiency: 84.5% @ 5A 82.5% @10A

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LM5030 Push-Pull Demo Board 36V-75Vin to +3.3V @ 10A

Output:3.3V @ 10A

Input:36 – 75V

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Performance:Input Range: 36 to 75VOutput Voltage: 27VOutput Current: 0 to 30ABoard Size: 6 x 4 x 2Load Regulation: 1%Line Regulation: 0.1%Line UVLO, Current LimitOutput OV Protection

Measured Efficiency: 91% @ 30A (810W)

LM5030 3G Base Station RF Power Supply

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LM5030 3G Base Station RF Supply

-48Vin to +27V @ 30A

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Cascaded Buck & Push-PullPower Converter (Voltage Fed)

OSCILLATOR

PUSH

PULL

Vin VoutVppN : N : 1 : 1

BUCKCONTROL

CONTROLLER

FEEDBACK

BuckStage

Push-PullStage

Buck Stage: Vpp = Vin * DPush-Pull Stage: Vout = Vpp / NOverall: 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

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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

• 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

FEEDBACK

LM5101

PUSH

PULL

33 - 76V Vout

HIVin

LM5041

Vcc

HD

LD

FB

LI

Vcc

HB

HO

HS

LO

Vss

OUTPUT INDUCTOR REMOVED

BUCK OUT CAP REMOVED

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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


Note: There is an overlaptime where both the Pushand the Pull switches are ON.This is required tomaintain 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


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Current-Fed Switch WaveformsExpanded Scale

Note: Each switch carries ½ the current,

during the overlap time


Ch 1,2 Push-Pull VDS

Ch 3,4 Push-Pull IDS

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Why is it important to reduce secondary rectification losses?

Estimate for typical 3.3V Output, 35 – 80V Input

Primary

Switching

15%

Filter Inductor

15%

Transformer

20%

Control

10%Secondary

Rectifiers

40%

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Comparison of Rectifier Stresses

Topology Rectifier Voltage Stresses

Example: 3.3V Out, 35 - 80V Input Example: Assumptions

Forward Vin * (Ns/Np) 20V High Line with XFR Ratio 4:1Push-Pull Vin * (Ns/Np) * 2 26.7V High Line with XFR Ratio 6:1Cascaded PP Vout * 2 6.6V All Line conditions XFR Ratio 6:1

Topology Rectifier Current Ratios

Example: 3.3V Out, 35 - 80V Input Example: Assumptions

Forward Iout * D and Iout * (1-D) 16 / 84% Ratio at High LinePush-Pull 50% * Iout 50% All line conditionsCascaded PP 50% * Iout 50% All line conditions

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Sync Rectifier Waveforms


Ch 1 Sync1 VDS

Ch 2 Sync2 VDS

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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 (165C)Packages: TSSOP16 and LLP16 (5 x 5 mm)

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LM5041 Block Diagram

TIMERt / SYNC

Vcc

Vref

LOGIC

5V REF

FB

PWM5K

5V

1.4V

100K

50K

COMP

0.75V

SS

VccUVLO

CLK

LD

R

S

Q

Q

HD

OFF TIMEGENERATOR

LM5041-1 ONLY

CS

0.5V

0.6VCLK + LEB

2K

SS

10uASS

0.45VSHUTDOWN

COMPARATOR

ENABLE

SLOPECOMPRAMP

GENERATOR

45uA

0

Vin

OSCILLATORDIVIDE

BY 2

PUSH

PULL

CLK

DRIVER

DRIVER

Vcc

Vcc

OSC

LOGIC

UVLO

2.5V

UVLOHYSTERESIS

(20uA)

9V REGENABLE

DEADTIMEOR

OVERLAPCONTROL

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Performance:Input Range: 36 to 75VOutput Voltage: 2.5VOutput Current: 0 to 50ABoard Size: 2.3 x 3.0 x 0.5Load Regulation: 1%Line Regulation: 0.1%Line UVLO, Current Limit

Measured Efficiency: 89% @ 50A 91% @20A

LM5041 Current Fed Push-Pull Demo Board

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LM5041 / LM5100 Demo Board2.5V @ 50A Cascaded DC-DC Converter

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Cascaded Half-BridgeConcept

PULL

Vin33 - 76V

Vout

LM5041

Vcc

HD

LD

FB

VDD

T1

L1

T1

LM5102

PUSH

Vin

VDD

LM5100

FEEDBACK

BuckStage

Half-BridgeStage

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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

LM5102

PUSH

PULL

Vin33 - 76V

Vout

Vin

LM5041

Vcc

HD

LD

COMP

VDD

LM5100

VDD

T1

L1

T1

LM5100

VDD

FEEDBACK

BuckStage

Full-BridgeStage

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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

Vcc

HI

LI

Vcc

LEVELSHIFT

VIN

Q1

Q2

• Initially Q1 is activated by Low Side control

• Cboot is charged from Vcc through D1, Q1

• Cboot is charged to (Vcc-Vdiode)

Vcc

HI

LI

Vcc

LEVELSHIFT

VIN

Q1

Q2

• 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 @ 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

LOUVLO

HOUVLO

HB

HSHI

Vcc

LI

Vss

LEVELSHIFT

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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 @

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

VDD

HB

HO

HS

LO

DLYLogic

LI

HI

RT1 RT2

DLYLogic

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HI

HO

LI

LO

LM5102

K x RT1

K x RT2

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 @

0.5MHz) Packages: SOIC-8, LLP-10

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

LM5104

VDD

HB

HO

HS

LO

DLYLogic

IN

RT

AdaptLogic

AdaptLogic

DLYLogic

K x RT

K x RT

IN

HO

LO

TPROP

TPROP

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Summary:New 100V controllers and drivers enable higher performance power converters with a minimum of external components:

LM5030 Push Pull ControllerLM5041 Cascade ControllerLM510X Gate Drivers

Questions or Comments?http://www.national.com/appinfo/power/hv.html

http://power.national.com

Documents

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